NEED HELP: Code 91

[FoMoCo17]

1990 Mustang GT - Supercharged - Kenne Bell

Okay been having this problem for a while ... cannot drive car. Start it up and it cuts off like IAC problem but my main concern is the Code 91 I am continually getting. Was getting Code 91 (every test), Code 12 (every test), Code 13 (before ... raised idle and it went away), Code 18 (every now and then). I think the main problem is the Code 91 because when it goes thru the KOER Test, you can hear that it is running bad and cannot adjust the engine speed properly due to the lean condition. Then, the smell of raw gas is just overwhelming.

I blocked off EGR to take that out of the equation. Checked Thermactor system everything works. Still get codes with Thermactor system disconnected which would serve to lean it even more. Replaced upper intake gasket to no avail. Replaced oxygen sensors and even switched them from side to side to no avail. Tested drivers side oxygen sensor and found heater receiving 12 volts and that the resistence was within specs. Haven't checked for shorts to ground or opens yet though. Performed a Balance test but always get a Code 9. Looked in my EEC IV book written by Charles Probst on Page 227 .... he states that a Balance test will not identify a lean nor non-firing injector ... how can this be??? Doesn't the decrease in RPM still point to that cylinder since it is not contributing to the overall RPM? I don't understand anyone that does please explain why a lean injector or non working injector wouldn't show up???

All of my plugs look kinda of lean. Number 6 cylinder had a yellowish-orange center porcolean color so I put in one from another set but do not understand why it was discolored. Had some rust on outside between metal and porcolean that could have run between tot he porcolean and metal to the inner porcolean ... dont' know. I also ran the engine when it was stumbling terribly and removed the fuel regulator hose and it ran a bit better but not by much so it does seem like a lean condition. It is most evident at idle and off-idle just completely terrible running. One time, I ran it at 2000 RPM for the Cylinder Balance test and the headers were glowing orange so I am concerned about running this car too much with this condition. The computer is definitely over-compensating ... wouldn't that tell me that the oxygen sensors ARE IN FACT WORKING?

Spoke to a guy at Kenne Bell who suggested replacing upper intake manifold gasket. Did it but no change. He stated then changing the lower but stated that unless the heads have been planed or with a different intake they should not pose a problem. Mine are stock and it was changed years ago and sealed to the best of my ability. Have not done a compression test yet especially on drivers side. Did one a year ago ... everything looked good but cylinder #6 was a bit low but still within 10% of the others. Guess what I'm saying, this wild-goose chase is beating me up!!!

Checked injectors with stethoscope and they all appear to be working. Can't do anything more with them since they are all under the supercharger. They are however new ACCel units replaced last year. I am so tired of trying to find tests to uncover what this problem is but have to get it fixed!!! And, every test takes so long or I can't get to something or it takes all day and is inconclusive. Thus, anyone have some diagnostic ideas I haven't already tried in defining what this lean condition is and how to solve it??? I do not want to keep going into the engine and not finding things and wasting a lot of time, money and stressing me out! Between this crappy OBD I system and faulty autoparts store parts, I do not relish working on cars anymore (hobby for 25 years or more) because someone else fools you with something crappy to steal the dollars out of your pocket. I have been dealing with a major parts reliability problem for the last three years because everyone is buying overseas and charging regular prices to line their pockets ... no part replaced can be considered or eliminated ... just driving me crazy!!!

Any help would be appreciated ... I shall try anything as time allows to get this solved but would like to do it methodically and w/o tearing everything apart! Thank You!

 

[5.0Droptop]

What fuel pressure are you running

 

[5.0Droptop]

Something else came to mind. Maybe your distributor is a tooth off causing timing issue

 

[FoMoCo17]

I have checked the fuel pressure several times and it is always good. It is 41-42 PSI unplugged at the regulator and round 32-34 PSI plugged in and is always steady. FPR also holds pressure for a long time when engine is shut off.

Timing issue. It is set at 8 degrees BTC (as per Kenne Bell) and steady w/ SPOUT disconnected). When reconnected, it jumps up about 15-20 degrees and moves around a bit but this appears to be normal on 5.0's

When distributor was put in, it was installed at 8degrees before TDC and set directly pointing to number 1 cap tower so it should be correct. However, I did read on one website that even though the timing is right on, the fuel injection could be off but didn't fully understand this and why Ford would make it so it could literally be put in wrong with regard to the fuel injection system. It was running okay before this particular problem occured though a bit rough at idle. Now, it is real bad and only runs a bit better when warm. Is anyone aware of stabbing the distributor wrong regarding the fuel injection system or is the information I ran across a while back just misinformation. If the distributor is installed correctly ignition-wise, can it be installed fuel injected-wise wrong??? I do not see how since it is all centered and locked on one shaft ... but I could be wrong ....

 

[jrichker]

Code 91 is an O2 sensor problem. It has nothing to do with ignition timing.

Note that there is no such thing as one tooth off on a 5.0 Mustang. If it doesn't align perfectly with #1 position, you can turn the distributor until it does. The only problem is that if you are too far one way or the other, you can't turn the distributor enough to get the 10-14 degree optimum timing range.

Code 41 or 91 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 22-Jun-2009 to include 3 digit code and wiring for 94-95 5.0 Mustangs

Code 41 is a RH side sensor,
Code 91 is the LH side sensor.

Code 172 is the RH side sensor
Code 176 is the LH side sensor

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


Testing the O2 sensors 94-95 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. 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.

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

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.

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

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.
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up the driver’s side, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41. The cure is to remove the crossover tube and thoroughly clean the insides so that there is no carbon blocking the free flow of air to both heads.

 

[FoMoCo17]

JRichker I agree w/regard to the timing issue. This is where i got my info from a while back that has mad me scratch my head (Reinhold Racing) It appears to be misinformation!

Reinhold Racing - Mustang EFI Information

Read under distributor phasing and correctly installing a distributor. If these guys are wrong, which it appears to me, they should take this garbage down.

Back on topic:

I replaced the oxygen sensors and this did nothing. During the KOEP test, if the mixture is lean due to a lean condition as opposed to an inoperative oxygen sensor, then is it really an oxygen sensor condition per se. From what I can infer from all of the info is that I have a lean mixture ... could be in oxygen sensor circuit or a lean mixture caused by so many other things .... is this correct? Or, is is a strictly oxygen sensor problem???

 

[ID89GT]

Something else to think about is an exhaust manifold gasket.. or maybe a cracked header/manifold that is causing air to come in.

 

[FoMoCo17]

that's a problem since it could be so many things from injectors, head gasket, vacuum leak, intake gasket (lower), cylinder, exhaust leaks etc.. Just don't want to start ripping things apart and then not find anything. Just replaced upper intake gasket and re-installed supercherger etc.. Still waiting on JRichker or someone else to tell me for sure if the Code 91 explicitly means oxygen sensor circuit failure or that the oxygen sensor is indicating a lean condition due to another condition causing the lean condition? Then, if it is indicating other cuases of a lean condition where to begin to isolate this problem the easiest way ... it could be anything!

 

[FoMoCo17]

Just found it again about installing distributor to fire correctly for ignition system but also explaining it could be wrong for fuel injection system at:

http://*******.com/index.php?topic=119.0;wap2

Who's right on this one???

 

[jrichker]

The “one tooth off fable”

Before installing the distributor, you need to insure that #1 cylinder is on compression stroke when you put the distributor back in place.

When the spark is right the injector timing is right. On a 5.0 EFI Mustang you can't get the distributor installed far enough to cause problems if the rotor is pointed at the stock #1 position on the distributor cap. The TFI module will either run into the radiator hose outlet on the intake manifold or the A/C compressor when you set the timing with a timing light.

The exception to this statement is that if you install the distributor so far off that you can no longer get the rotor to point at the #1 position. If you move the spark plug wires so that the rotor points to the revised #1 spark plug position, this will throw the injector timing off.

The ignition timing signal and the injector timing are triggered by the same mechanism: the distributor PIP pulse. There is a fixed relationship between the two; you can't change one without changing the other. The PIP shutter wheel does have the #1 slot wider than the other slots and this tells the computer where the firing order starts. That allows the computer to know when to start the sequence to pulse the injectors so that they spray only when their intake valve is open.

A 4 cycle, 5 event engine takes two turns of the crank to complete a power cycle that has a duration of 720* (*=degrees). In our theoretical engine, each of the 4 cycles is separated by 180*.

At 0* TDC (Top Dead Center), both intake and exhaust valves are closed. The spark plug fires, and the expanding gases force the piston downward.

At 180* later, BDC (Bottom Dead Center), the exhaust valve starts to open and the piston starts upward towards TDC, forcing the spent gases out the exhaust valve.

At 360*, TDC, the exhaust valve closes and the intake valve opens and the piston starts downward and the resulting vacuum pulls the air into the cylinder. Since this is Sequential EFI, the injector starts to spray fuel into the port. The start and stop of the injector spray (injector duty cycle) is calculated by the computer to deliver the right amount of fuel according the sensor inputs.

At 540* BDC, the intake valve closes, if the fuel injector was spraying, it stops spraying. The piston starts upward on the compression stroke, compressing the air/fuel charge.

At 720* TDC, the cycle starts all over again with the firing of the spark plug.

Note that in real life, the events of ignition, exhaust, intake, fuel injection, and compression will not start on 180* boundaries, but will vary slightly to change the performance characteristics of the engine.

 

[FoMoCo17]

I agree wholeheartedly ... its just amazing how much misinformation is out there that has to be sorted through because one has to look at everything. And as usual, this subject has wasted a lot of time and energy in an off-topic that does not help me any further.

Correct me if I'm wrong. When the Code 91 is issued, it means that the drivers side engine bank is lean ... and it could be caused by the oxygen sensor but could be caused by many other things also??? That's what I'm seeing after I have interpreted everything. And if this is the case, what is the best way of attacking this lean condition. I posted all that i have done to date but nothing is conclusive ... just do not want to tear things apart and not find anything without proper diagnosis first. Any other things I should check that I haven't mentioned???

 

[jrichker]

This isn't going to be quick and easy, but I have supplied you with what I know that can cause a lean condition.

The code 91 is a lean condition:

1.) Defective sensor - swap it with the sensor on the other bank. If it follows the sensor, the sensor is bad.
[Testing the O2 sensors 87-95 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.

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 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink 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 (LH O2 with a Gray/Lt blue wire) and 43 (RH 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.


Testing the O2 sensors 94-95 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. 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.

2.) Bad O2 sensor wiring harness.
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 Os sensor harness and the Dark Green/Pink wire on the computer pin 43

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 Dark Green/Pink Gray/Lt blue wire on the RH Os 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 Os 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

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.

3.) Stuck closed fuel injector - use the cylinder balance test to spot injectors that are stuchk closed. The cylinder they supplywill not exhibit a drop in RPM when it is tested.

Cylinder balance test
Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. Remember to keep the clutch pedal (5 speed) depressed to the floor during the test.The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure

4.) Vacuum leak:
Finding vacuum leaks

Revised 04-Aug-2011 to add pintle cap, PCV grommet & power brake check valve grommet to checklist.

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 650-7250 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 propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

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.

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 ( BWD - Home ) as being resellers of Borg-Warner parts:
Welcome to PartsPlus.com or Auto Value / Bumper to Bumper Quality Parts & Service - Home of the Aftermarket Auto Parts Alliance Group or Auto Parts Stores, Brakes, Tires & Automotive Parts | Pep Boys or Federated Auto Parts - Automotive Aftermarket

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

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.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.

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http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

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