ECM 41 and 91 KOER codes

[shotsy242]

Hey Guys,

I've got the dreaded ecm 41 and 91 codes on my 92 Fox with KOER and i've thrown the book at it.

Fuel pressure is 39/40 with the vacuum disconnected from the regulator.

--Changed the regulator and fuel pump anyway

-- New Ignition/complete tuneup, plugs, wires, cap, rotor, coil etc.

-- New PCV valve

-- base timing set at 14 deg

-- New 02 sensors on both sides (good power and ground all the way to the sensors)

-- Egr Delete with the resistor triangle installed

-- smog pump delete (Thermactor tube just bent over? done by prev owner)

-- Long tube headers

-- cat delete with H-pipe installed

-- All vacuum lines capped off except for brake booster and fuel pressure regulator

The only thing I can think is that I'm sucking air through the thermactor tube thats bent in half on the passenger side, I've got the plugs to go into the heads but it's going to be a PITA getting them in back there.

Anyone have any other ideas, tips or advice?

 

[jrichker]

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.

Code 41 is a RH side sensor,
code 91 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
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.

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

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 to that there is no carbon blocking the free flow of air to both heads.

 

[shotsy242]

Thats the same canned answer that I've seen ten times in this forum. Sir, did you even read what I had done to the car so far? About all I can see form the directions above that I haven't already done is that I need to check the 02 sensor output at the ecm on pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). for .2 to .9 volts at idle. Both 02 sensors are brand new and getting good power and ground as stated above. I'll check sensor output at the ecm in the morning...

Point blank range, if the thermactor tube is letting fresh air get into the engine would it cause a lean condition? Also if it's tied right into the exhaust ports on the heads why dont I get exhaust gas out of the end of the thermactor tube?

Sorry if it sounds like I'm being an arse, not how I intend to come across just frustrated and looking for some help.

Thanks

Ken

 

[jrichker]

Canned answer - if you haven't exhausted all of the directions in the post, it isn't stale stuff. Go back and do all the tests and then voice your opinions. There is no cheap way you will find O2 harness problems without doing the voltage measurement tests at the computer.

 

[TenBlade]

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.

Code 41 is a RH side sensor,
code 91 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
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.

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

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 to that there is no carbon blocking the free flow of air to both heads.

Sorry to bring up an old post but seemed like the easiest. I Have a 347 with long tubes. New ACT, coolant sensor, MAF, and bosch 02 sensors, IAC. Sometimes it idles great and sometimes it is loopy and dies when I come up to a stop sign.

I keep getting these codes 41 and 91. I have the orange ground to the intake. When I test the wires to my 02 sensor (after unplugging I get 12v on the hot wire and continuity to the ground on both harness.) I have probed the ECU. On 29 I originally had 0.5v (positive) but then it slowly fell past 0 to -0.4 (yes, negative). I measured 43 and got a negative number as well (it fluctuates but remains negative). (red to the ECU and black to ground and I checked at the battery to be sure the polarity was right and the voltmeter was working). I'm not sure what this means. Any ideas?

-Mike

 

[Mustang5L5]

The only thing I can think is that I'm sucking air through the thermactor tube thats bent in half on the passenger side, I've got the plugs to go into the heads but it's going to be a PITA getting them in back there.

Anyone have any other ideas, tips or advice?

If it's open to outside air...that would be the cause. You need to seal the system.

 

[TenBlade]

If it's open to outside air...that would be the cause. You need to seal the system.

Actually that part of the post is 6 months old. Sorry. I probably should have started a new thread.

-Mike