Sputtering until warm

Barron91lx

Member
Dec 21, 2018
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48
Newton, KS
I finally have my car running great, when it's warm that is. It's very strange. It starts fine and idles fine, but when starting out it feels like it's missing. Most notably, driving at a constant speed in third, it sputters. Then like a switch, at around the same place on the road, it smooths out immediately every time.
It's a 91 LX former aod converted to 5sp. It has a new 5.0 from Blueprint. GT40 intake, no smog and still has egr. I have replaced every sensor and solenoid on the car including oxygen sensors. It also has an off road h pipe.
I just ran the koeo and get 11, 10, 91. So it appears everything is good but the driver side 02 sensor.
I don't know if it matters or not, but it does still have the aod computer

Thanks
 
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jrichker

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You should have gotten engine not running and engine running codes, so you are missing some useful data..

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 24 Aug 2018
1.) To correct the RH & LH mismatch on 91-93 5.0 Mustangs
2.) To add Tmoss’ wiring diagrams for 88-95 Mustangs


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 sensor circuit. 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:
a9x-series-computer-connector-wire-side-view-gif.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
94-95_5.0_EEC_Wiring_Diagram.gif


Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
91-93_5.0_EEC_Wiring_Diagram.gif


Complete computer, actuator & sensor wiring diagram for 88-90 Mass Air Mustangs
88-91_5.0_EEC_Wiring_Diagram.gif
 

Barron91lx

Member
Dec 21, 2018
26
3
13
48
Newton, KS
Thanks for all of the info. I think I'll look for a vacuum leak this weekend. I made a leak detector but haven't used it yet.
I started it tonight and just let it idle for several minutes. It idled around 750 rpms then after about 5 minutes suddenly idled up to 900 which is where I set it.
I did replace both O2 sensors a few months ago. Do you think one could be bad?
 

Wayne Waldrep

Before I post a pic, do you have one of yours?
15 Year Member
Apr 14, 2003
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Cuba, AL
My car does exactly the same thing. There's a curve in the road that I know is the spot where the missing feeling shuts off every single day. It's annoying. I have no idea and have given up looking for it. Just know yours isn't the only one.
 
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Barron91lx

Member
Dec 21, 2018
26
3
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48
Newton, KS
My car does exactly the same thing. There's a curve in the road that I know is the spot where the missing feeling shuts off every single day. It's annoying. I have no idea and have given up looking for it. Just know yours isn't the only one.
I'm leaning towards a vacuum leak. I think I'll check it tonight. My 15 yr old dives it to school daily and I just want to get it fixed. Does yours smell very rich? That is another annoying thing about it and I think it's connected.
 

Barron91lx

Member
Dec 21, 2018
26
3
13
48
Newton, KS
I'm just wondering why the 91 code is the only one coming up if it's detecting a lean condition. Why don't I get codes from both O2 sensors?

Thanks
 

Wayne Waldrep

Before I post a pic, do you have one of yours?
15 Year Member
Apr 14, 2003
998
522
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Cuba, AL
Yes, it smells very rich. I remind myself of the old mechanics from when I was a kid. Walk close to them and they smell like gas....lol. I don't have a vacuum leak. That's one of the few items I'm actually sure of.
 

Blucifer99

I only understand every 3rd word
Jul 15, 2018
462
34
38
36
Port Mcnicoll Ontario
I'm just wondering why the 91 code is the only one coming up if it's detecting a lean condition. Why don't I get codes from both O2 sensors?

Thanks

Exhaust leak at the flange... Thats my guess for the code 91... I had that problem. Code 91. Checked everythin. Then while switchen o2s sides i noticed my flange bolts were loose. Torqued them down. Goodbye 91
Also heres something to check for before u go chasing vacuum leaks. Check your throttle body blade for a hole.. Previous owner may have did the quick fix thing for a crapped out iac valve.. If u cannot get ur car to idle lower then 800rpms and have searched everywhere for a vac leak and couldnt find one. Check that throttle blade
 
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Barron91lx

Member
Dec 21, 2018
26
3
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48
Newton, KS
I wanted to update for anyone who cares. I checked for vacuum leaks and found a sizeable one in the lower intake front gasket. I had noticed oil around the distributor also. When I removed the lower I could not believe what I saw.
I had put this lower intake on my new Blueprint engine last summer. I took great care in making sure it was done right. I took it to a dealership last fall because of excessive noise from the lifters. One had failed so it was warrantied. When the dealership shop put it back together, their method was different. Check the pictures
D3F9F918-6496-4DB1-837B-44B8DD80EFF2.jpeg
95D34BBC-AA92-429B-9804-3481C3F1158C.jpeg
 

Barron91lx

Member
Dec 21, 2018
26
3
13
48
Newton, KS
Exhaust leak at the flange... Thats my guess for the code 91... I had that problem. Code 91. Checked everythin. Then while switchen o2s sides i noticed my flange bolts were loose. Torqued them down. Goodbye 91
Also heres something to check for before u go chasing vacuum leaks. Check your throttle body blade for a hole.. Previous owner may have did the quick fix thing for a crapped out iac valve.. If u cannot get ur car to idle lower then 800rpms and have searched everywhere for a vac leak and couldnt find one. Check that throttle blade
Canyou tell me what flange you are referring to?
 

Barron91lx

Member
Dec 21, 2018
26
3
13
48
Newton, KS
Thank you. I'm still trying to figure out the code 91. The o2 sensors were replaced a few months ago and, now that I fixed the ridiculous intake gasket hack job, my vacuum leak test looks good. The car is still stuttering until it gets to temp
 

Blucifer99

I only understand every 3rd word
Jul 15, 2018
462
34
38
36
Port Mcnicoll Ontario
Thank you. I'm still trying to figure out the code 91. The o2 sensors were replaced a few months ago and, now that I fixed the ridiculous intake gasket hack job, my vacuum leak test looks good. The car is still stuttering until it gets to temp

Well code 91 does mean lean. So chances are theres either a. Exhaust leak or a vacuum leak. Which u said u have fixed. Or it could be a fuel delivery problem. Have u checked fuel pressure or replaced ur fuel filter? Could be a weak fuel pump causing a lean condition on one side of the cylinder banks.. Possibly a pressure drop across the fuel rail. If fuel pressure checks out. I recommend a smoke test to see if ur missing something leaking ..[/QUOTE]
 

Barron91lx

Member
Dec 21, 2018
26
3
13
48
Newton, KS
I have replaced everything in this car, including the engine last year. I replaced the o2 sensors and upgraded the fuel pump a few months ago. I put in refurbished 30 lb injectors and fuel pressure guage last summer and every sensor is new. I bought an a9-l computer and put it in and it shows the same 91 code. The smoke test looks perfect except a little smoke coming from the egr, which is new, and the tps, also new. I'm going to try swapping the o2 sensors this weekend.
You think an exhaust leak could be causing these problems?
 
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Blucifer99

I only understand every 3rd word
Jul 15, 2018
462
34
38
36
Port Mcnicoll Ontario
I have replaced everything in this car, including the engine last year. I replaced the o2 sensors and upgraded the fuel pump a few months ago. I put in refurbished 30 lb injectors and fuel pressure guage last summer and every sensor is new. I bought an a9-l computer and put it in and it shows the same 91 code. The smoke test looks perfect except a little smoke coming from the egr, which is new, and the tps, also new. I'm going to try swapping the o2 sensors this weekend.
You think an exhaust leak could be causing these problems?

well yea a exhaust leak will allow air to pass the o2 sensor causing a false lean condition.. id switch o2s sides and see if the code 41 pops up... If i remember correctly code 41 is lean passenger side.. dont quote me on that.. but i had a code 91 and it was a exhaust leak. i found it while i was switching o2s to see if the problem followed the o2 sensors
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
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30lb injectors sound kinda big for a mild build.
have you gone through the surging idle checklist?
can you tell at what engine temp the car stops running ruff?
 

Blucifer99

I only understand every 3rd word
Jul 15, 2018
462
34
38
36
Port Mcnicoll Ontario
I have replaced everything in this car, including the engine last year. I replaced the o2 sensors and upgraded the fuel pump a few months ago. I put in refurbished 30 lb injectors and fuel pressure guage last summer and every sensor is new. I bought an a9-l computer and put it in and it shows the same 91 code. The smoke test looks perfect except a little smoke coming from the egr, which is new, and the tps, also new. I'm going to try swapping the o2 sensors this weekend.
You think an exhaust leak could be causing these problems?

Yea i didnt notice that you said your running 30lb injectors... what kind of build do you have for running 30s?.. like im running 30lbs at 43.5psi but ive got trickflow170cc heads Tf stage one cam,70mm throttle body cobra upper intake with a heavily ported cobra gt40 lower.. no Mass air flow as im tuning it with megasquirt2pnp,,
depending on what your combo is.. you might be pumping mass loads of fuel into it and while its cold it could be causing maybe a rich misfire tricking the 02 into thinking its lean.. but thats just a theory of mine tho..