Anyone ever get code 91 and fix it.... Running lean.

[ram360]

After I drive my car for like 3 or 4 miles my check engine light comes on. It will not turn on until I drive that far. I can then shut it off and the check engine light will remain off again for another few miles and then turn on.

I pulled the codes and fixed all the other codes but this one has me stumped. I have a 255lhp in tank pump, 24lb injectors, adj fp reg set at 39psi. I have a visible fp gauge in the car and my fp is constant all the time. Plus everything is new.
I keep getting code 91 which is (System running lean - Fuel control) My oxygen sensors are new bosch (I put less than 200 miles on them) I pulled the plugs and they everything is running right. The car runs fine no signs of a fuel problem when driving it but this damn code keeps poping up. I'm totally lost and about ready to take it to a garage already b/c I'm out of things to try. Anybody have a similar problem before. I'm thinking about upping the fp a little but not sure if that's going to help anything.

 

[vristang]

I would up the pressure and see what happens. I have heard of some people having to run 40-40psi with what you would think was a somewhat mild combo.Also you could have some bad wiring, or a dirty connector which is messing up the signal to the computer just enough to trip the code. I am taking a guess on that, but it makes sense to my pea brain.

 

[RsStanG1987]

Yeah what he said. Thats what you get when your wiring is from ebay... E-bay= Shady wiring... J/K hope you fix it.

 

[ram360]

I would up the pressure and see what happens. I have heard of some people having to run 40-40psi with what you would think was a somewhat mild combo.Also you could have some bad wiring, or a dirty connector which is messing up the signal to the computer just enough to trip the code. I am taking a guess on that, but it makes sense to my pea brain.

Seems logical to me. I'm going to mess w/ the fp tomorrow and see what happens. Maybe I'll clean the wiring for the 02's as well since I forgot to do that today.

 

[ram360]

J/K hope you fix it.

If all else fails I'll sell the stang and get the IROC i've been wanting for awhile now.

 

[RsStanG1987]

Yeah i seen one on main street in pittston behind some shady guys house. Looks like a sleeper that was punched in the face. I think its like 400 you should buy it and you could be the cool guy with the iroc at work....

 

[HISSIN50]

A vac leak, clogged fuel filter, bad FPR, bad pump and several other things can cause that code.

Good luck.

 

[RsStanG1987]

A vac leak, clogged fuel filter, bad FPR, bad pump and several other things can cause that code.

Good luck.

Everything he put on the car is brand new. FPR, filter, pump, and pretty much anything else you can name is new. He also got the code for the vehicle speed sensor is bad. He changed that and gets the same code.. Any ideas there?

 

[jrichker]

Everything he put on the car is brand new. FPR, filter, pump, and pretty much anything else you can name is new. He also got the code for the vehicle speed sensor is bad. He changed that and gets the same code.. Any ideas there?

Start looking very carefully at the wiring and connectors under the car. Two problems in the same physical area with new sensors are a good indication of wiring problems. You may have some corroded connections or damaged wiring. Use an ohmmeter to check the resistance of the wiring. More than 1 ohm on a wire is a sign of problems.


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.

AutoZone wiring diagrams

http://www.autozone.com/servlet/UiB..._us/0900823d/80/16/71/3c/0900823d8016713c.jsp for 79-88 model Mustangs

http://www.autozone.com/servlet/UiB..._us/0900823d/80/19/59/5a/0900823d8019595a.jsp for 89-93 model Mustangs

http://www.autozone.com/servlet/UiB..._us/0900823d/80/1d/db/3c/0900823d801ddb3c.jsp for 94-98 model Mustangs

Check to see if the O2 sensor ground that is in the fuel injector wiring harness is grounded properly. It is an orange wire with a ring terminal on it.

 

[Matts89LX5.0]

up fuel pressure to 40-42 psi

 

[ram360]

I upped the fp to 42 psi w/o the vacuum. Cleared the codes. Drove it 20 miles and pulled them again. Only codes left are 34 and 91 now. Dunno if the others show up again after I drive it more. It's kinda weired b/c the check engine light does not come on immediately. I takes at least 6-8 miles before I comes on. I just don't understand it. I cleaned the connections for the 02's and noticed 2wires were pulled out of the connector in the 02 harness (passenger side). I pushed them back in and they appear to be secure now. I wish I had a spare harness to replace it with to see if that's the problem. Not sure If I can check resistance w/ a voltometer? That's all I have, plus I'm not familiar w/ the process. Care to elaborate on this Jrichker? thanks.

 

[jrichker]

Check out the part of this test path on how to read O2 sensor voltages at the computer.

MOst of the common multimeters have a resistance scale. Be sure thth the O2 sensors are disconnected and measure from the O2 sensor body harness to the pins on the computer.

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.

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more 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

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.

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.

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

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

 

[DMAN302]

Did anyone mention checking the hego ground (orange ground wire found on the rear of the block by the firewall...could be damaged, separate. this ground is realated to the funtioning of the oxygen sensors..maybe another option to look at. Also check the vac lies in or near the passenger firewall..there is a small vaccume resivoir just inside the fender near the firewall on the passengerside. I did find a vac leak JUST inside the fender..it was causing some significant issues on my car as well. I believe SD converted cars to be more vac sesitive for some reason, though I'm sure that is not accurate.

 

[rowdie- stang]

I had that same problem...The wire harness was burnt. It was to close to the header