Correction: Negative trim means that the computer detects running rich. By the sound of it, there may be some exhaust leaks, though nothing major before the cats.
00 Bosch 4.0 misfire cylinders 3, 5 gets better when warm
The truck sat for a over a year before I bought it. I replaced the BECM, ECU and instrument cluster and it started to run. I replaced the leaking heater core o-rings (the carpet on the right hand side was soaked).
I replaced the spark plugs and did a compression check. a couple of cylinders showing 200-210 psi, a few more 180. #3 was 170, #5 was 70 (yes, seventy). I inspected the pistons using a 6mm endoscope, all pistons seem covered with black deposits. #5 seems particularly filthy. The truck can idle forever without showing any signs of overheating. The plugs from #5 and #7 were filthy black, no sign of coolant getting into the cylinder. I put some oil into the cylinders and did a compression test. The compression was higher than the tester will show.
I soaked the #3 and #5 cylinders with Marvel Mystery Oil. The engine runs better now, but still has a miss while cold which gets better when the engine heats up to normal temperature. I cleared the codes, but misfire in #3 and #5 came back. I also have a P1300 which is "multiple cylinder misfire".
When the engine is warm I get the following fuel trims:
STFT (Short term) 0% both banks
LTFT(Long term):
- PID 0x08 - Bank 1: -1.56
- PID 0x09 - Bank 2: -0.78
I suppose that the left side (Bank 1) is misfire is what is causing more negative trim, but it doesn't seem like a big deal (less than 5%), or is it? I suspect that the ECU thinks that the engine is running slightly lean, probably due to misfires?
Also both post-cat sensors (sensor 2) show 99.22% fuel trims (PIDs 0x15 and 0x19). Not sure if it even has post-cat O2 sensors, I haven't looked. If it has them, does it mean that cats are shot? I get no 02 sensor codes.
My current working theory is that the truck may have misfired for a while and the piston rings in cylinders 3 and 5 are stuck. I am not suspecting a valvetrain-related problem or a cracked block because:
- the compression improved with oil added to the cylinder
- no signs of overheating or pressurizing the cooling system with exhaust gases that I can see.
- Truck takes a while to warm up. The miss gets better when engine is warm
- the truck started running better after soaking cylinders with MMO (and changing the plugs).
I am planning to replace wires and coils next, dump some 44K in the tank and run the truck for a while. Re-do the compression check after that. I am figuring with a known good ECU, new wires, coils and plugs there should be no ignition-related reasons for misfiring. And since the fuel trims are not ugly I am not suspecting major issues with fuel supply. If it still misfires with new coils and wires, it can only be the compression.
I am also planning to run it with Rislone compression restoration additive in the oil. I did free up stuck rings with it on more than one occasion.
Is there anything else that should I look at? Any other ideas?
P.S. It's getting cold here and I am not looking forward to pulling the engine and re-ringing it. Worst case, I can probably pull the head and oil pan off and do an "in-frame Tijuana ring job", but I am not a fan of those kinds of shortcuts. Besides, I am not sure if it can be done with aluminum pans on Bosch engines.
I replaced the spark plugs and did a compression check. a couple of cylinders showing 200-210 psi, a few more 180. #3 was 170, #5 was 70 (yes, seventy). I inspected the pistons using a 6mm endoscope, all pistons seem covered with black deposits. #5 seems particularly filthy. The truck can idle forever without showing any signs of overheating. The plugs from #5 and #7 were filthy black, no sign of coolant getting into the cylinder. I put some oil into the cylinders and did a compression test. The compression was higher than the tester will show.
I soaked the #3 and #5 cylinders with Marvel Mystery Oil. The engine runs better now, but still has a miss while cold which gets better when the engine heats up to normal temperature. I cleared the codes, but misfire in #3 and #5 came back. I also have a P1300 which is "multiple cylinder misfire".
When the engine is warm I get the following fuel trims:
STFT (Short term) 0% both banks
LTFT(Long term):
- PID 0x08 - Bank 1: -1.56
- PID 0x09 - Bank 2: -0.78
I suppose that the left side (Bank 1) is misfire is what is causing more negative trim, but it doesn't seem like a big deal (less than 5%), or is it? I suspect that the ECU thinks that the engine is running slightly lean, probably due to misfires?
Also both post-cat sensors (sensor 2) show 99.22% fuel trims (PIDs 0x15 and 0x19). Not sure if it even has post-cat O2 sensors, I haven't looked. If it has them, does it mean that cats are shot? I get no 02 sensor codes.
My current working theory is that the truck may have misfired for a while and the piston rings in cylinders 3 and 5 are stuck. I am not suspecting a valvetrain-related problem or a cracked block because:
- the compression improved with oil added to the cylinder
- no signs of overheating or pressurizing the cooling system with exhaust gases that I can see.
- Truck takes a while to warm up. The miss gets better when engine is warm
- the truck started running better after soaking cylinders with MMO (and changing the plugs).
I am planning to replace wires and coils next, dump some 44K in the tank and run the truck for a while. Re-do the compression check after that. I am figuring with a known good ECU, new wires, coils and plugs there should be no ignition-related reasons for misfiring. And since the fuel trims are not ugly I am not suspecting major issues with fuel supply. If it still misfires with new coils and wires, it can only be the compression.
I am also planning to run it with Rislone compression restoration additive in the oil. I did free up stuck rings with it on more than one occasion.
Is there anything else that should I look at? Any other ideas?
P.S. It's getting cold here and I am not looking forward to pulling the engine and re-ringing it. Worst case, I can probably pull the head and oil pan off and do an "in-frame Tijuana ring job", but I am not a fan of those kinds of shortcuts. Besides, I am not sure if it can be done with aluminum pans on Bosch engines.
1 - 3 of 3 Posts
A re-ring with the engine in place is easy, heads off, sump off and everything is there looking at you. No need to take the engine out to do it at all. It does sound like stuck rings. If you aren't using the car regularly, I'd be inclined to fill those two cylinders with diesel and leave it to soak for a few days. After it has stood, spin it over on the starter with no plugs in to get what remaining diesel is above the pistons out and do an oil change. If the rings are simply gummed into their grooves, that should free them off. All NAS cars, and only NAS cars, had post cat lambda sensors.
1 - 3 of 3 Posts
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