Trouble codes, misfir, O2 sensor

[sprocket]

Hello, I have a '96 Mark VIII, 204,000 miles.

I'm getting occasional misfires. The CEL flashes. It usually lasts about two minutes at most, occasionally will come back several minutes later, and often clears up for several days at a time. It appears to be worse on hot days with the A/C on. Here's the codes I am getting:

P0141
P0156
P0161
P0301
P0307
P0308

The misfires are from both coil packs and I think from three different coils. I can't see it being a coil pack issue. I'm a little confused. It looks like I have a bad bank 2 sensor 2 O2 sensor. But there's is also a code for the bank 1 sensor 2 O2 sensor.

But both those sensors are past the catalytic converter right? They don't control engine management?

Are the misfires resulting in fuel in the exhaust and that is why those O2 sensor codes are set?

Bad PCM?

Thanks for any help

 

[sprocket]

The plugs are copper and they probably have 30,000 on them.

 

[chris2523]

P0141 - O2 Sensor Heater Circuit Malfunction (Bank1, Sensor2)

P0156 - 02 Sensor Circuit Malfunction (Bank 2 Sensor 2)

P0161 - 02 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2)

P0301 - Cylinder 1 Misfire Detected

P0307 - Cylinder 7 Misfire Detected

P0308 - Cylinder 8 Misfire Detected


the misfires are not causing those O2 sensor failure codes.
either both downstream O2s have failed, or there is a wiring issue, or there is a PCM issue.
does the car have cats still?

copper plugs are cheap enough and 30k is a good run for copper plugs.
i'd change plugs.

 

[unity]

Plug change for sure, check the gap when you do. Normally enough misfires can cause an O2 sensor fault for rich sine fuel is not being burnt, but as listed above none indicate a mixture issue.

Also your cylinders fire in pairs. Combustion spark and a wasted spark. So the computer is really only driving four sparks off of four coils. I dont know the pairings, but if two fail that are paired its typically the coil pack. Also you can swap coil packs to see if the issue moves.

 

[sprocket]

Plug change for sure, check the gap when you do. Normally enough misfires can cause an O2 sensor fault for rich sine fuel is not being burnt, but as listed above none indicate a mixture issue.

Also your cylinders fire in pairs. Combustion spark and a wasted spark. So the computer is really only driving four sparks off of four coils. I dont know the pairings, but if two fail that are paired its typically the coil pack. Also you can swap coil packs to see if the issue moves.

OK thanks for the help guys. I will change the plugs and see what that gets me. None of the misfires are from paired cylinders so I don't think it's a coil pack. I probably do have one bad O2 sensor at least. In fact, I seem to remember that I do and I never changed it because it is downstream of the cat and the CEL is on anyway because I eliminated the EGR valve and never had it tuned out.

 

[sprocket]

So far so good with new plugs. I'll know for sure when I drive it to work and back tomorrow.

 

[unity]

The downstream O2 (after cats) are not often in need of change. They have zero impact on AF ratios. Their only job is to make sure the cats are working right. They can go bad, but being after the cats they generally last at least twice as long as the pre-cats. You will also notice these do not have heating circuits.

 

[sprocket]

Misfire returned on cylinder #1. It happened on the way to work yesterday and then again at lunch. I pulled codes on lunch break. It hasn't happened since. Could it be that I should have used dielectric grease? Hopefully it was a fluke and now #1 has a good connection. If it happens again, I will swap two plug wires and see if the problem moves.

Also I am using plug wiring for like a '93 which, for whatever reason, is different than what the '96 manual shows. It shouldn't matter because the differences are at the same coils which fire at the same time. It just makes for neater wiring. That being said, could the PCM think the wrong cylinder is misfiring because I have it wired different? How does it know which cylinder it is?

 

[driller]

I forget the exact detail, but from what I recall there is a polarity difference between the cylinders on the same coil.

 

[chris2523]

misfire detection is based on crankshaft angular rotation.
when the crank is accelerating, or holding an rpm, then suddenly drops when cylinder 1 is supposed to fire, the computer interpolates that as cylinder 1 misfire.