1991 5.0 - fires up every time, idles perfectly, but once it’s warm it won’t idle for more than 15 seconds

[7991LXnSHO]

I have a good analog Radio Shack meter, a decent digital meter with a boatload of features from HFT, and a cheap digital one from HFT. I usually use the cheap HFT one with longer leads when working on the car. If I drop it, NBD. If I blow the fuse, I will have to solder a new one or pick up a new meter on a trip where there is a HFT.

 

[LILCBRA]

Dumb question - what multimeter do you guys recommend?

I have an entry level Fluke that you can pick up off Amazon:

Amazon product ASIN B00HEAMLCOView: https://www.amazon.com/gp/product/B00HEAMLCO/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1


I've had the cheapies from HFT and different ones you can pick up at Home Depot and the like, and they work well - until they don't. My last cheapie was a Home Depot cheapie and it completely crapped out on me after a short time. After getting frustrated with it, I decided to not waste my time buying the cheapies and invest in a good brand. After having worked with industrial electricians and seeing that most all of them carry Fluke, this one seemed like it would be a good investment. I don't regret spending the money on this one.

 

[Mustang5L5]

If you can afford to splurge a little, find a DMM that can rear DC amps up to 10 amps.

You will use that feature when troubleshooting parasitic draws on a vehicle. A lot of DMM's only go up to 1Amp DC, and you'll find a lot of vehicle current draws are higher than that 3-6amps or so.

 

[Mustang5L5]

I should probably recommend one.

again, it’s a few more bucks (and this is a “lesser” brand than fluke/greenlee) but this one will do DC current to 10amps, which will come in handy.

Extech EX505 CAT IV-600V True RMS Industrial MultiMeter with Waterproof (IP67) Rugged Design for Heavy Duty Use Amazon product ASIN B001AGO9SWView: https://www.amazon.com/dp/B001AGO9SW/ref=cm_sw_r_cp_api_glt_fabc_RTKCJC288MY3JVXPG7YX?psc=1



I’ve actually been using an Extech 285 for my automotive wiring needs for years. I’ve had no issues with it.

 

[7991LXnSHO]

My Intermediate meter is a Fluke copy, snd the Radio Shack meter was picked out by a RCA TV and radio repair man that was a friend of the family. I wish I had his knowledge. He would be ancient by now, but was a really nice guy. Maybe getting shocked by electrolytic capacitors is like electroshock therapy?

Anyway, both better meters have a ten amp feature. If I was going to buy just one meter, that would be a consideration.

 

[General karthief]

I second Karthief's suggestion of the surging idle checklist thread. The TPS is the second item in his list as well, and he did a damned fine job with his writeup - as he always does!

Ok, before any newbees think I'm a methodical genius let me give credit to @jrichker whom posted this piece of automotive genius,
Oh, and he's involved with other automotive wizardry, you can find his and other member's tips and tech stuff in the 5.0 technical/how to thread.

 

[AmericanMuscle4.6GT]

Picking up a voltmeter this afternoon. Will test the TPS voltage.

The fine folks in the braintrust in the other forum are now telling me that it’s so easy and that I should have just turned the throttle screw and that would have fixed it. Im not sure that is productive in diagnosing the issue but what do you guys think?

 

[LILCBRA]

I think that any way you can take care of yourself, be it your person or your property, that's the way to go. People who are condescending to others are lacking something in their lives. If you don't know, you don't know, but at least you had the sense and the balls.... uh..... courage..... to ask for help. Speaking down to others and not really offering anything productive doesn't help anyone except one's own ego. That seems to be a growing problem in society.

Now, as far as turning the idle set screw..... if it works it works. As pointed out in the checklist, you may need to reset the TPS if you are to leave it that way though. For diagnosing, I don't see why you couldn't do that as long as you're able to take it back to where it was when you started. But as you said, it just shuts down. Most likely, turning the idle set screw will only mask the problem and that might only be a short term fix. I'd trust @jrichker 's list before I'd trust keyboard warriors.

Here's something else while I'm thinking of it. Years ago, in the mid 90s, I had a car that would just randomly shut down on me. Sometimes I would be able to keep it running by keeping the RPMs up, but not always. In that case it turned out to be junk spark plugs. Another time I had a car that would just shut down without warning. It was a loose battery ground that time. My latest escapade is with my Cobra, it would just randomly shut down. I found a barely exposed power wire going to my MSD that was grounding under just the right circumstance. What I'm getting at is that your problem sounds a lot like these and the common thread is that it was all electrical. Turning the idle screw won't fix that.

 

[Mustang5L5]

I made it to page 3 of that thread on Corral. I tend to check out of a thread once i see replies from you-know-who coming in.


I see you have a code 91. Here;s something simple to do. Take the two O2 sensors and swap them. Clear the codes, and rerun them. See if that code 91 turns into a code 41.

As for the TPS, usually when those have an issue a code 23/63 will be generated. This will mean they are set too high or low. It is still possible for them to be faulty in terms of dead spots, or failure to change voltage with increasing throttle blade position, so doesn't hurt to check.

If the car runs well on cold start (i forget if you said if it did or not) but poorly when warmed up, it's usually sensor related as open loop startup tends to ignore a few sensors. When the car is fully warmed up, it starts to take readings and if one is bad, you have issue.

I'd troubleshoot the 91 first since it is an issue that would cause poor running. By swapping the O2 sensors, you can see if it's a bad sensor, vs something like a vaccum leak causing the 91. If the code changes to a 41, replace the O2 sensor, wipe the codes, drive it and then pull them again.

There is probably more info in the corral thread, but i refuse to read it. You shouldn't need to put on a flame suit to ask a question. If someone doesn't understand, you need to take it down to a level that people can work with, not get increasingly condescending that people don't understand at your level.

 

[Wayne Waldrep]

I made it to page 3 of that thread on Corral. I tend to check out of a thread once i see replies from you-know-who coming in.


I see you have a code 91. Here;s something simple to do. Take the two O2 sensors and swap them. Clear the codes, and rerun them. See if that code 91 turns into a code 41.

As for the TPS, usually when those have an issue a code 23/63 will be generated. This will mean they are set too high or low. It is still possible for them to be faulty in terms of dead spots, or failure to change voltage with increasing throttle blade position, so doesn't hurt to check.

If the car runs well on cold start (i forget if you said if it did or not) but poorly when warmed up, it's usually sensor related as open loop startup tends to ignore a few sensors. When the car is fully warmed up, it starts to take readings and if one is bad, you have issue.

I'd troubleshoot the 91 first since it is an issue that would cause poor running. By swapping the O2 sensors, you can see if it's a bad sensor, vs something like a vaccum leak causing the 91. If the code changes to a 41, replace the O2 sensor, wipe the codes, drive it and then pull them again.

There is probably more info in the corral thread, but i refuse to read it. You shouldn't need to put on a flame suit to ask a question. If someone doesn't understand, you need to take it down to a level that people can work with, not get increasingly condescending that people don't understand at your level.

Or ... Just stick with the wealth of knowledge here. Why bother with the other?

 

[Mustang5L5]

Or ... Just stick with the wealth of knowledge here. Why bother with the other?

That's like telling a doctor that he should only treat patients at his own hospital.



I just like answering questions if i know the answer

 

[Wayne Waldrep]

That's like telling a doctor that he should only treat patients at his own hospital.



I just like answering questions if i know the answer

I didn't mean you... Lol. I meant the person that was asking the questions and was frustrated with the responses over there.

 

[Mustang5L5]

I didn't mean you... Lol. I meant the person that was asking the questions and was frustrated with the responses over there.

i mean they do have some smart guys over there. Ed Curtis and Woody from Ford Strokers are regulars along with Jack Hidley from MM.

They just need better (any) moderation as it’s a free-for-all in terms of attitude. I’m proud of this site in terms of welcoming newcomers and actually trying to help without too much condescending behavior. If you drive off the newbies, how do you expect to grow the forum?

I get it though. Some of those guys get paid for the sort of advice they give out, so they aren’t going to spend too much time hand-holding when they are feeling generous.

 

[Mstng93SSP]

It's funny, over on corral people ask for help and get crucified for it by the self proclaimed corral god. The reason people are asking for help is that they simply don't know the answer. We all have things we don't know the answer to...that's why we ask people who DO know the answer. I get so irritated seeing "I'm not going to GIVE you the answer...you need to figure it out yourself". My thought on this is...WHY? Why does he feel we need to figure it out for ourselves? If I know how to do something, and you don't I will gladly tell you how to do it. Or how to fix it. Or step by step how to troubleshoot it. We don't come on these forums to feel like we are taking university classes....we come on here because there are so many of us that have "been there and done that" that they can tell us what we need to know. If someone wants to just be condescending and make people who are asking for help feel bad about themselves there ae plenty of universities they can teach at.

 

[AmericanMuscle4.6GT]

I made it to page 3 of that thread on Corral. I tend to check out of a thread once i see replies from you-know-who coming in.


I see you have a code 91. Here;s something simple to do. Take the two O2 sensors and swap them. Clear the codes, and rerun them. See if that code 91 turns into a code 41.

As for the TPS, usually when those have an issue a code 23/63 will be generated. This will mean they are set too high or low. It is still possible for them to be faulty in terms of dead spots, or failure to change voltage with increasing throttle blade position, so doesn't hurt to check.

If the car runs well on cold start (i forget if you said if it did or not) but poorly when warmed up, it's usually sensor related as open loop startup tends to ignore a few sensors. When the car is fully warmed up, it starts to take readings and if one is bad, you have issue.

I'd troubleshoot the 91 first since it is an issue that would cause poor running. By swapping the O2 sensors, you can see if it's a bad sensor, vs something like a vaccum leak causing the 91. If the code changes to a 41, replace the O2 sensor, wipe the codes, drive it and then pull them again.

There is probably more info in the corral thread, but i refuse to read it. You shouldn't need to put on a flame suit to ask a question. If someone doesn't understand, you need to take it down to a level that people can work with, not get increasingly condescending that people don't understand at your level.

This makes a ton of sense. I'm going to try this when I get a chance. Easy way to rule out O2 sensors. I'm also going to test the TPS voltage.

So I did think it was some kind of electrical issue like a bad sensor throwing everything off once the engine goes into closed loop. Basically, the car starts up fine and runs for about 3-5 minutes before shutting off. After that, it shuts off after about 20 seconds every time I start it. I'm not sure if the car goes into closed loop based on temperature, or if there's still some kind of "boot up" period where it runs in open loop for 15 second after a startup even if the car is already "warm." That would explain why it doesn't shut off immediately when I start it with the car warm, but I'm not sure if that's how it works.

 

[AmericanMuscle4.6GT]

It's funny, over on corral people ask for help and get crucified for it by the self proclaimed corral god. The reason people are asking for help is that they simply don't know the answer. We all have things we don't know the answer to...that's why we ask people who DO know the answer. I get so irritated seeing "I'm not going to GIVE you the answer...you need to figure it out yourself". My thought on this is...WHY? Why does he feel we need to figure it out for ourselves? If I know how to do something, and you don't I will gladly tell you how to do it. Or how to fix it. Or step by step how to troubleshoot it. We don't come on these forums to feel like we are taking university classes....we come on here because there are so many of us that have "been there and done that" that they can tell us what we need to know. If someone wants to just be condescending and make people who are asking for help feel bad about themselves there ae plenty of universities they can teach at.

Yea I was pretty taken aback by the responses I got on that thread. Of course my questions are beginner questions, but people seemed more interested in shaming me for my inexperience, than actually helping me.

I had theories about what was going on but I still had questions about how to do electrical tests. I looked up the wiring diagrams and how to test the O2 sensors, and I ran into a sticking point where the instructions said to connect one of the leads between one of the terminals on the connector, and the ECM itself. I wasn't sure if that meant to pierce the electrical insulation on the wire at any point between the connector and the ECM, or how that works exactly. I thought it was a reasonable question, but apparently that was the last straw for some folks on there.

I've never done ANYTHING on a car aside from extremely basic stuff like installing a cold air intake, or replacing axle back mufflers, or changing out suspension pieces with the help of a friend. I'm 100% green to this. I can only get so far myself, sometimes I do need to ask for help.

I've abandoned the Corral thread. Aside from a certain someone being extremely condescending, I had another user chiming in to repeatedly tell me that I shouldn't even by trying to work on cars. I see no reason to ever go back to that forum again.

I really appreciate the responses I've gotten on here and I'm very thankful people are being patient with me in helping me work through the issue. I'm going to learn one way or another, and i could use all the help I can get

 

[Mustang5L5]

i think a simpler thing to do rather than testing the sensors themselves is to just swap them.

by swapping the sensors, if that one sensor is bad the 91 will become a code 41. (Code 41 is the opposite side sensor)

if it doesn’t, there are other potential causes for the code 91, like an exhaust leak or even a vacuum leak in the engine. That’s why I wouldn’t necessarily replace the sensor blindly. (Unless you want to replace them due to age anyway)

If the sensor is bad, replace both


when you get your multimeter then we can work on testing the tps.

 

[manicmechanic007]

Any cheap multi will work even the harbor freight. Code 33 for the TP means it will die because the voltage is low. It needs to be about .8v-1.3v at closed throttle. You can back probe the tps connector to check the voltage. It should rise to about 4.5-5.0v at wot.

 

[knlx91]

So what did you ever find out with this?

 

[jrichker]

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.

Backside view of the computer wiring connector:

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