Electrical New fox body owner little worried

Canadianfox

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Aug 18, 2019
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I hate being a first time poster with a tech issue but I’ve totally run out of ideas and need some help.Ive done a ton of reading and mechanical work based off the forums on this site trying to solve my issues .

I own a 1993 lx 5.0 convertible and am having idling issues ( sounds like its camed but isn’t) and predetonation ,and à first this last weekend the power lose in 5th,4th where my car just lost speed when I was floored on the highway and all I can watch was my speedo drop .No backfiring ,stalling just power loss until I got to a certain rpm .

Here is what I’ve done based off many posts in here so far.Keep in mind the car ran like this previous to these part replacements :
1: new tps and it’s calibrated A1
2: new act
3: new rotor ,coil ,cap ,plugs and wires
4: pulled the upper intake replaced injectors , redid most vacuum wires , and new pvc valve .
5: new coolant temp sensor
6:new MAF sensor
7:cleaned throttle body and replaced throttle cable
8: new O2 sensors and new O2 harness ( ecm side of harness was mint no burnt melted pins or corrosion). Still getting both O2 codes though so I’m lost on that one .I read there is a pin that melts on the ECM side pin 24 I think? Didn’t check that yet.

10: egr delete with block off plate .Removed air pump and plugged heads with bolts
11: BBK xpipe no cats anymore

Irrelevant mods are
1: SVE aluminum radiator and dual fan kit
2: tremec 500 trans with new crossmember and shifter .New clutch and cable
3: alternator kit
4:underpulley kit

What I think may be the issue(s)
1: ordered a TFI module
2: ordered a new pcv grommet old one seems lose and flabby
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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While you are replacing the pcv grommet, check/replace the screen underneath. (Not related to your problem)
Your problem sounds to me like a fuel pump issue, there is a diagnostic procedure to find your problem and I'm surprised you didn't come across it in your searches here.
But then it does say surging idle checklist so it could have been missed, do it step by step, don't skip around just because you have already replaced something. It shows how to check for codes, fuel pump pressure, all the things for idle,stall issues.
Thanks to jrichker and anybody else that contributed for their tireless efforts, sacrificing life and limb to bring us such an amazing piece of information.
Wow, you think I've had enough coffee already this morning?
 

Canadianfox

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Aug 18, 2019
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While you are replacing the pcv grommet, check/replace the screen underneath. (Not related to your problem)
Your problem sounds to me like a fuel pump issue, there is a diagnostic procedure to find your problem and I'm surprised you didn't come across it in your searches here.
But then it does say surging idle checklist so it could have been missed, do it step by step, don't skip around just because you have already replaced something. It shows how to check for codes, fuel pump pressure, all the things for idle,stall issues.
Thanks to jrichker and anybody else that contributed for their tireless efforts, sacrificing life and limb to bring us such an amazing piece of information.
Wow, you think I've had enough coffee already this morning?
Will do thanks for the info.I though because of surging idling issues it wasn’t the pump since she starts up fine but at this point I have nothing to lose and it’s only two hoses and two straps to lower the tank no big deal.
 

Mustang5L5

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Did you reset the computer and perform the idle reset procedure?

Afterwards, take car for quick ride and then pill codes and report all code numbers. You said you had o2 codes but which ones
 

jrichker

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Do the "Surging Idle Checklist" first (I am the guy who put it together) and then check the fuel pump...

Check fuel pressure:
The local auto parts store may rent or loan a fuel pressure test gauge if you don't have one.
Disconnect the vacuum line from the fuel pressure regulator. Check it for evidence of fuel present in the line by removing it and blowing air through it. If you find fuel, the fuel pressure regulator has failed. Reinstall the line; leave the fuel pressure regulator end of the vacuum line disconnected. Then cap or plug the open end of the vacuum line and stow it out of the way.
Connect the fuel pressure test gauge to the Schrader port located just behind the alternator.
Turn the ignition switch on & start the engine. Observe the pressure: you should see 38-41 PSI at idle.
Turn the ignition off; reconnect the vacuum line to the fuel pressure regulator. Then disconnect the fuel pressure test gauge. Watch out for squirting gas when you do this.

Fuel pump pressure test
Disconnect the larger of the two fuel lines up by the Schrader valve. It is the return line and does not have the Schrader valve on it. Find a piece of rubber fuel hose and clamp it on the return line coming from the regulator. Stick a bolt in the other end of the hose and make sure that all your connections are tight and leak proof as possible. When this powers up, you don't want fuel squirting everywhere. Hook up the fuel pressure test gauge. Turn the ignition switch on and watch for leaks. You may want to use a helper inside the car to cut the switch off quickly if you have a leak. To trick the fuel pump into running, find the EEC test connector and jump the connector in the Upper RH corner to ground.



Caution!!! You have blocked the return line for the fuel pump! Pressure will rise very quickly past safe levels with a good pump
If the pressure goes up past 55 PSI, the pump is good and the fuel pressure regulator is bad. If the fuel pressure does not hit 55 PSI or more in a few seconds, the pump is bad or you have electrical problems.
 

John Dirks Jr

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You say it sounds like it has a modified cam, but it does not. I think a compression check is in order. If you have a burnt or bent valve for instance, it might sound a bit like its cammed. A compression check is in order to test the overall health of the engine mechanical. You can throw all kind off accessory parts and sensors at it that you want. But, if it has a mechanical internal problem, you're not gonna solve the problem until you pull it apart.

Furthermore, poor combustion from burnt, bent or leaking valve or other low compression problems will throw the O2 sensors off. It might be reading rich and trying to compensate by leaning out the mixture until the point it starts shutting down like you described.
 
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Canadianfox

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You say it sounds like it has a modified cam, but it does not. I think a compression check is in order. If you have a burnt or bent valve for instance, it might sound a bit like its cammed. A compression check is in order to test the overall health of the engine mechanical. You can throw all kind off accessory parts and sensors at it that you want. But, if it has a mechanical internal problem, you're not gonna solve the problem until you pull it apart.

Furthermore, poor combustion from burnt, bent or leaking valve or other low compression problems will throw the O2 sensors off. It might be reading rich and trying to compensate by leaning out the mixture until the point it starts shutting down like you described.
Good point I never even thought of this and a compression test is pretty easy to do.Damn can you imagine all those parts for a valve issue I could have fixed for way cheaper .Ill do one this weekend fingers crossed
 
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Canadianfox

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Do the "Surging Idle Checklist" first (I am the guy who put it together) and then check the fuel pump...

Check fuel pressure:
The local auto parts store may rent or loan a fuel pressure test gauge if you don't have one.
Disconnect the vacuum line from the fuel pressure regulator. Check it for evidence of fuel present in the line by removing it and blowing air through it. If you find fuel, the fuel pressure regulator has failed. Reinstall the line; leave the fuel pressure regulator end of the vacuum line disconnected. Then cap or plug the open end of the vacuum line and stow it out of the way.
Connect the fuel pressure test gauge to the Schrader port located just behind the alternator.
Turn the ignition switch on & start the engine. Observe the pressure: you should see 38-41 PSI at idle.
Turn the ignition off; reconnect the vacuum line to the fuel pressure regulator. Then disconnect the fuel pressure test gauge. Watch out for squirting gas when you do this.

Fuel pump pressure test
Disconnect the larger of the two fuel lines up by the Schrader valve. It is the return line and does not have the Schrader valve on it. Find a piece of rubber fuel hose and clamp it on the return line coming from the regulator. Stick a bolt in the other end of the hose and make sure that all your connections are tight and leak proof as possible. When this powers up, you don't want fuel squirting everywhere. Hook up the fuel pressure test gauge. Turn the ignition switch on and watch for leaks. You may want to use a helper inside the car to cut the switch off quickly if you have a leak. To trick the fuel pump into running, find the EEC test connector and jump the connector in the Upper RH corner to ground.



Caution!!! You have blocked the return line for the fuel pump! Pressure will rise very quickly past safe levels with a good pump
If the pressure goes up past 55 PSI, the pump is good and the fuel pressure regulator is bad. If the fuel pressure does not hit 55 PSI or more in a few seconds, the pump is bad or you have electrical problems.
 

Canadianfox

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Thanks for the procedure if my compression test turns out ok I’ll do this one as well,those fuel lines are a son of a b..... to get off even with the tools
 

Canadianfox

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Aug 18, 2019
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Montreal
You say it sounds like it has a modified cam, but it does not. I think a compression check is in order. If you have a burnt or bent valve for instance, it might sound a bit like its cammed. A compression check is in order to test the overall health of the engine mechanical. You can throw all kind off accessory parts and sensors at it that you want. But, if it has a mechanical internal problem, you're not gonna solve the problem until you pull it apart.

Furthermore, poor combustion from burnt, bent or leaking valve or other low compression problems will throw the O2 sensors off. It might be reading rich and trying to compensate by leaning out the mixture until the point it starts shutting down like you described.
Good news !! Here are my compression results

140,142,141,145,139,142,139,139
 

John Dirks Jr

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Spark plug coloring look normal and uniform?

You can check for vacuum leaks with a propane torch (not lit of course). Turn the gas on and move the nozzle along the intake runners, especially where the intake meets the heads. Any substantial leak would change engine idle if the propane gets sucked in
 

Canadianfox

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Aug 18, 2019
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Spark plug coloring look normal and uniform?

You can check for vacuum leaks with a propane torch (not lit of course). Turn the gas on and move the nozzle along the intake runners, especially where the intake meets the heads. Any substantial leak would change engine idle if the propane gets sucked in
Only one spark plug on the body not the top looked like it had a spot weld on the body really weird but they are a bit white cuz I’m running rich which is part of the issue but all worn well no oil bypass or anything.Next step is fuel pressure testing as per the post above. and another round of vaccum leak tests this weekend .Thanks for your time on this btw
 

jrichker

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Finding vacuum leaks

Revised 6 May 2018 to add carbon canister plumbing as a common leak area.

There is no easy way to find vacuum leaks. It is a time consuming job that requires close inspection of each and every hose and connection.

Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-725 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.

Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as starting fluid, propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

After you have done the simple visual checks and the check for vacuum leak on the underside of the intake manifold, consider doing a smoke test.
Some of the guys here have built smoke machines used to find automotive vacuum leaks. They seem to work quite well and are made mostly with parts you would have laying around in your garage. Check out smoke machine vacuum leak - YouTube and see if there is one that you could build.

The vacuum line plumbing is old and brittle on many of these cars, so replacing the lines with new hose is a good plan. The common 1/8” and ¼” vacuum hose works well and isn’t expensive.

The PCV grommet and the power brake booster check valve grommet are two places that often get overlooked when checking for vacuum leaks. The rubber grommets get hard and lose their ability to seal properly. The PVC grommet is difficult to see if it is correctly seated and fitting snugly.

The hoses and connections for the evaporative emissions (carbon canister and purge valve) are other common sources of vacuum leaks. The large vacuum outlets on the bottom side of the upper intake manifold are common hiding places for deteriorated vacuum lines and caps over unused vacuum ports.

Fuel injector O rings can get old and hard. When they do, they are prone to leaking once the engine warms up. This can be difficult to troubleshoot, since it is almost impossible to get to the injectors to squirt oil into the fuel injector mounting bosses. If the plastic caps on the fuel injectors (pintle caps) are missing, the O rings will slide off the injectors and fall into the intake manifold.

Fuel injector seal kits with 2 O rings and a pintle cap (Borg-Warner P/N 274081) are available at Pep Boys auto parts. Cost is about $3-$4 per kit. The following are listed at the Borg-Warner site ( http://www.borg-warner.com ) as being resellers of Borg-Warner parts:
http://www.partsplus.com/ or http://www.autovalue.com/ or http://www.pepboys.com/ or http://www.federatedautoparts.com/

Most of the links above have store locators for find a store in your area.

Use motor oil on the O rings when you re-assemble them & everything will slide into place. The gasoline will wash away any excess oil that gets in the wrong places and it will burn up in the combustion chamber. Heat the pintle caps in boiling water to soften them to make them easier to install.



Diagram courtesy of Tmoss & Stang&2birds
mustangFoxFordVacuumDiagram.jpg



Vacuum leak due to slipped lower intake manifold gasket...

Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.





Determining if you have a leak due to a slipped intake gasket as shown above. This test is only good if you can get the engine to run somewhere in the 1000-1700 RPM range
If your valve cover oil filler & PVC systems are still in the original configuration, try this:
Cap or plug the hose from the intake manifold to the PVC valve with a bolt.
Cap or plug the PVC valve with a piece of hose with a plug or bolt in it.
At that point the only vent for the crankcase is the tube from the oil filler neck to the throttle body.

Disconnect the tube that runs from the oil filler neck to the throttle body. Make sure the oil filler cap is on securely. Start the engine and put your thumb over the end of the tube that comes from the oil filler cap. If you feel suction, there is a leak. Another thing to do is to extend the tubing from the filler neck so that there is enough to stick the end in a jar or cup filled with motor oil. If it sucks up the oil, you definitely have a leak at the underside of intake manifold.

This isn't necessarily the definitive test, but it is the best thing I could come up with on short notice. If there is a lot of blowby, this obviously won't be of much help.

See the picture below to see the breather tube where in connects to the throttle body. It is close to the TPS and runs over the top of the IAC.

The following are diagrams courtesy of Tmoss & Stang&2birds



See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Ignition switch wiring

Fuel, alternator, A/C and ignition wiring

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

Vacuum diagram 89-93 Mustangs

HVAC vacuum diagram

TFI module differences & pinout

Fuse box layout
 

Canadianfox

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Aug 18, 2019
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Well I think I narrowed these issues down to electrical but sadly not by choice.I did a fuel pressure test at idle and it was fine , vaccum test at idle as well was normal.The car has been getting progressively worse and today she could barely get out the driveway .The minute I’d ask for fuel she would bog,predetonation etc etc .But as I’m sitting there looking at the dash in defeat I noticed no more chk engine? I pull out my odb1 reader follow the process of pulling codes and the car won’t go I go diagnostic mode ? Or even give me previous code history ? I did the paper clip method still nothing .

So with all that said I’m gonna follow the wire testing method and see what’s going on hopefully it’s just wiring and not a burnt circuit of the pin46 I’ve been reading about. I find it out it was getting worse and worse until this happened never really heard of a progressive wiring failure unless the eec relay was going on the blink on and off who knows .More work to do
 

Canadianfox

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Aug 18, 2019
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Well I checked the MAF all good, I checked the ACT ,TPS,all good .TFI module was replaced , wires ,plugs as well .I was able to pull codes after I fixed a loose connection on the STI wire and I got the following codes .Code 54 (my fault as I was testing the plug off with key on), code 33 (egr delete so no biggy) ,41/91 same damn o2 sensors as always even when I bought the car ,changed harness and o2 sensors still an issue), 66 MAF sensor but again only got that because I was testing harness with sensor unplugged and also plugged .Code 61 also
 

jrichker

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Well I checked the MAF all good, I checked the ACT ,TPS,all good .TFI module was replaced , wires ,plugs as well .I was able to pull codes after I fixed a loose connection on the STI wire and I got the following codes .Code 54 (my fault as I was testing the plug off with key on), code 33 (egr delete so no biggy) ,41/91 same damn o2 sensors as always even when I bought the car ,changed harness and o2 sensors still an issue), 66 MAF sensor but again only got that because I was testing harness with sensor unplugged and also plugged .Code 61 also
Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 01 Sep 2019 1.) To emphasize do not attempt to measure the O2 sensor resistance. Disconnect the O2 sensor from the wiring before doing any resistance checking of the sensor to computer wiring.

Code 41 is the passenger side sensor, as viewed from the driver's seat.
Code 91 is the driver side sensor, as viewed from the driver's seat.

Code 172 is the passenger side sensor as viewed from the driver's seat.
Code 176 is the driver side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:

"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.
When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.
Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the O2 sensor. Before checking the O2 sensor circuit wiring resistance, disconnect the O2 sensor from the rest of the circuit wiring. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage. End Quote

Testing the O2 sensors 87-93 5.0 Mustangs

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.


Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor, you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.
Computer wiring harness connector, computer side.
88243.gif


Backside view of the computer wiring connector:
71316.gif


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor

The computer pins are 29 (RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/lt green wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor

The computer pins are 29 (RH O2 with a Gray/Lt blue wire) and 43 (LH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


94-95 5.0 Mustangs; note that the 94-95 uses a 4 wire O2 sensor.
The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. . The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.



87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH O2 sensor harness and the Dark Green/Pink wire on the computer pin 29


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 27


There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged, and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Check the fuel pressure – the fuel pressure is 37-41 PSI with the vacuum disconnected and the engine idling. Fuel pressure out of range can cause the 41 & 91 codes together. It will not cause a single code, only both codes together.

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness. The exception is that the 94-95 uses a 4 wire O2 sensor.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports. This puts an excess amount of air in the passenger side exhaust and can set the code 41. Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

Complete computer, actuator & sensor wiring diagram for 94-95 Mustangs


Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs


Complete computer, actuator & sensor wiring diagram for 88-90 Mass Air Mustangs



Code 61 - Engine Coolant Temperature (ECT) sensor is or was too low. Failed sensor or bad wiring for the ECT.

Note that that if the outside air temp is below 50 degrees F that the test for the ECT can be in error.

Check the resistance of the black/white wire to battery ground. If it is less than 1 ohm, it is good. If it is more than 1 ohm, the black/white wire has bad connections or a broken wire. Always take resistance measurements with the circuit powered off. Make sure that you do not touch the metal probe tips when you you’re your measurements. If you do, your readings will be off. Check the resistance of the Lt green/yellow wire at the sensor connector and Pin 7 on the computer. It should be less than 1 ohm. If it is more than 1 ohm, the Lt green/yellow wire has bad connections or a broken wire.

If those tests pass, remove the passenger side kick panel. Disconnect the computer connector and disconnect the sensor from the wiring connector. Measure the resistance between the Lt green/yellow wire at the sensor connector and ground. You should see 1 meg ohm or an infinite open circuit. Less than 1 meg ohm means you have some bad wiring, or the Lt green/yellow wire is shorted to ground.

View attachment a9x-series-computer-connector-wire-side-view-gif.gif

The ECT sensor has absolutely nothing to do with the temperature gauge. They are
different animals. The ECT sensor is normally located it the RH front of the engine in
the water feed tubes for the heater.

See the graphic for the 10 pin connector circuit layout.
?temp_hash=3ef2497fff29a7a9daee955cf93e5805.jpg




The ACT & ECT have the same thermistor, so the table values are the same

ACT & ECT test data:

Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate
readings.

Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

Voltages may be measured across the ECT/ACT by probing the connector from
the rear. A pair of safety pins may be helpful in doing this. Use care in doing it
so that you don't damage the wiring or connector.

50 degrees F = 3.52 v
68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61
212 degrees F = .47 v
230 degrees F = .36 v
248 degrees F = .28 v

Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.

50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
212 degrees F = 2.07 K ohms
230 degrees F = 1.55 K ohms
248 degrees F = 1.18 k ohms

Diagram courtesy of Tmoss & Stang&2birds



See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

Ignition switch wiring

Fuel, alternator, A/C and ignition wiring

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

Vacuum diagram 89-93 Mustangs
 

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