IDLE Misfire

[pormgb]

Got an LS6 that misfires on all cylinders at idle, as soon as you touch the gas the misfires stop.

I am monitoring the misfire with the scan tool, the idle misfires are enough to light the CEL (P0300). This motor has new coils, and wires. I have just added headers and a new exhaust system, the motor does need to be tuned but I am not sure why I only see misfires at idle.

Could bad idle tuning cause these misfires? Idling around 800 rpm.



- 2002 LS6, about 2000 miles on fresh motor.
- 226, 229, 585, 575 116 LSA Cam
- 85mm MAF

[statesman]

Are you in running open or closed loop?

Are you running MAF only, SD or MAF/SD hybrid?

Post your current tune and log.

[pormgb]

Are you in running open or closed loop?

Are you running MAF only, SD or MAF/SD hybrid?

Post your current tune and log.

Pretty stock closed loop tune.

Forgot to mention 30lb injectors.

I also noticed that when the car warms up, it idles around the 12s AFR, as soon as I hit the gas AFRs appear normal.

[joecar]

Did this just start happening out-of-the-blue, or did it start happening when parts were replaced, or was it already happening (since it was tuned)...?

[joecar]

BTW (unrelated to thread): your PE table at EQR 1.75 will cause engine to bog down when PE enables.

[pormgb]

Did this just start happening out-of-the-blue, or did it start happening when parts were replaced, or was it already happening (since it was tuned)...?

The misfires have always been there, always had P0300, changed all ignition parts but still had P0300.

I just noticed the rich idle but it doesn't always happen, it seems to happen when the car has fully warmed up.

I initially thought it was something to do with plugs and wires but there is no way I could have the problem on both banks of cylinders, I have dual WBOSs and they both read 11s to 12s.

I get the 11s and 12s when idling hot, when coming to a stop, sometimes the car almost stalls. When I first drive the car its fine, as it warms up the problem occurs.

[pormgb]

BTW (unrelated to thread): your PE table at EQR 1.75 will cause engine to bog down when PE enables.

Ok, that can be changed.

[statesman]

What spark plugs are you using and what have you got them gapped at?

[pormgb]

I am using NGK V-Power TR-55 Spark Plugs gaped at .40.

[statesman]

Can you do a log of a drive and include,

RPM
MAF
MAP
LTFT's
STFT's
ECT
IAT
VSS
TP
VOLTS

[pormgb]

Can you do a log of a drive and include,

RPM
MAF
MAP
LTFT's
STFT's
ECT
IAT
VSS
TP
VOLTS


Ran the car with these PIDs, the car ran terrible, almost stalling when slowing down. I am seeing AFRs in the 11s when slowing down and the motor is struggling to accelerate.

I have attached the log file.

[statesman]

You didn't log your LTFT's so it's a bit hard to see exactly what's going on, but your STFT's don't look right... bank 1 goes crazy positive in places and then both banks go to zero and stay there.

Can you do another log, this time include the LTFT's and can you also include the front O2 sensors (GM.HO2S11 and GM.HO2S21). You can remove the STFT and LTFT averages... I don't need them.

[pormgb]

You didn't log your LTFT's so it's a bit hard to see exactly what's going on, but your STFT's don't look right... bank 1 goes crazy positive in places and then both banks go to zero and stay there.

Can you do another log, this time include the LTFT's and can you also include the front O2 sensors (GM.HO2S11 and GM.HO2S21). You can remove the STFT and LTFT averages... I don't need them.


Here you go.

This is weird, I almost have no AFR control, I am seeing in the 20s when driving, then drops down to the 11s.

I also added the external Wide Band PID.

[statesman]

Your O2 sensors have expired... you need to replace them.

[pormgb]

Your O2 sensors have expired... you need to replace them.

That's weird, they are only about 5 years old with no more than 10,000 miles on them.

Thanks!!

[statesman]

Have you used leaded fuel at all?... that will kill them real quick.

[pormgb]

Have you used leaded fuel at all?... that will kill them real quick.

No, I am in California and use regular pump gas.

I did build a new exhaust system and the headers run much hotter than the old manifolds, I never thought too much heat can kill O2 sensors.

[joecar]

Headers are thinner material than the cast iron manifolds, so headers shed heat... (they don't really run hotter than manifolds).

[joecar]

Does your exhaust system have any air leaks...?

[pormgb]

Does your exhaust system have any air leaks...?

I do have some leaks but I welded the O2 bungs before the flange on the header side, the only air leaks that could affect the O2 would be where they bolt to the heads.

The car was running fine until about two weeks ago, the first sign of trouble was the rich idle. I realize now that when the car is cold its in open loop, that's why I could drive away fine but when the car warmed up and went into closed loop, the idle AFRs rich.

[joecar]

I do have some leaks but I welded the O2 bungs before the flange on the header side, the only air leaks that could affect the O2 would be where they bolt to the heads.

The car was running fine until about two weeks ago, the first sign of trouble was the rich idle. I realize now that when the car is cold its in open loop, that's why I could drive away fine but when the car warmed up and went into closed loop, the idle AFRs rich.ok, this means that something suddenly changes (sensor failed, wire broke, etc...)...

[pormgb]

ok, this means that something suddenly changes (sensor failed, wire broke, etc...)...

Yep, from the last log, it looks like a sensor has failed.

[pormgb]

Your O2 sensors have expired... you need to replace them.



I checked the log again and I do see the Oxygen sensors fluctuation, the do have large swings.

Why do you thing they are bad? could there be something wrong making them see a widely fluctuating AFR?

[pormgb]

Picked two new O2 sensors, the old ones tested good.

Installed the new O2 sensors and still have the same problem, as the car idles, AFRs increase then decrease.

I even done a propane test for air leaks, found none, I have attached the latest short log idling with new O2 sensors.

I also noticed no STFT activity, this is at idle at normal operating temperature.

[joecar]

HO2S11 stays at about 600mV... something is wrong here... do you have a spare PCM to try...?

[statesman]

Picked two new O2 sensors, the old ones tested good.

Installed the new O2 sensors and still have the same problem, as the car idles, AFRs increase then decrease.

As joecar has already pointed out, your O2's are not switching properly... particularly bank 1. This is usually the sign of a dead or dying O2 sensor, but in your case it's some other problem. Check your wiring for damage and, as joecar has already suggested, you might need to get a hold of another PCM to try out.

[pormgb]

As joecar has already pointed out, your O2's are not switching properly... particularly bank 1. This is usually the sign of a dead or dying O2 sensor, but in your case it's some other problem. Check your wiring for damage and, as joecar has already suggested, you might need to get a hold of another PCM to try out.

Joecar, Statesman,

Yea, something really weird is going on here, there are no codes and that's whats makes this so strange.

I have just flashed the PCM to Open Loop, it ran that way for a few years before coming to CA, I will also pickup a new PCM and check the current PCM's pins and wiring.

[pormgb]

Got a new PCM, same problem, this time I saw idle AFRs in the 19s.

I have attached a short log at idle and with the OL tune, I don't think I even got out of open loop.

When I rev the car, AFRs go up then down to the 12s then up to around 19, when the AFRs go up the car starts to sound like a train, I also noticed the idle being very rough.

I also tested the car with the OL tune, same problem, I am beginning to think this could be some sort of vacuum leak, it feels like air is being added leaning out the mixture. I also see the same AFRs on both banks.

[statesman]

Your narrowband O2 sensors are flat around 450 through both logs... there's definitely something wrong there. You don't have your widebands logged so I can't see what they're doing.

Do you have narrowband and wideband sensors fitted or do you only have widebands and emulate the narrowband signal for the PCM?

[pormgb]

Your narrowband O2 sensors are flat around 450 through both logs... there's definitely something wrong there. You don't have your widebands logged so I can't see what they're doing.

Do you have narrowband and wideband sensors fitted or do you only have widebands and emulate the narrowband signal for the PCM?

I have stock narrowband's with dual widebands, the left WB is an LC1 with a gauge, the right WB is an LM1 that provides the logs for EFILive, both show the sam readings.

Here is an account of the wideband readings.

Car starts right up with AFRs in the high 12s then settles into the mid 14s. As so as the car starts to warm up the AFRs start going up to the 17th and fluctuate, when I rev the car AFRs go all over the place between 17s and 13s. At idle when warm, AFRs will fluctuate between the 13s and the 17s, you can hear the motor note change as the AFRs go up.

[statesman]

Okay, you've got a new PCM and new narrowband sensors. The only other thing I can think of is the wiring between the O2 sensors and the PCM.

Is the PCM new and was the OS flashed in by a dealer with a tech2?

[pormgb]

Okay, you've got a new PCM and new narrowband sensors. The only other thing I can think of is the wiring between the O2 sensors and the PCM.

Is the PCM new and was the OS flashed in by a dealer with a tech2?


OK, I will check all O2 PCM connections.

I picked up a used PCM and done a complete re-flash both OS and tune.

One thing that has me a bit confused. When in open loop, O2s are not used, at warmup and with my OL tune the problem exists.

When the car initially starts, things look OK but after a few minutes you can see the AFRs going off, at this point the car is still in OL.

[RADustin]

you may be getting a false lean reading from the WB02 as it can't read the unburnt fuel. could this be true? does it seem rich or does it seem legit lean?

are you in PE all the time or something weird?

the LTFT increment without the STFT ever registering.

hm.

[pormgb]

you may be getting a false lean reading from the WB02 as it can't read the unburnt fuel. could this be true? does it seem rich or does it seem legit lean?

are you in PE all the time or something weird?

the LTFT increment without the STFT ever registering.

hm.

No, there is no way both WB02s are off, I can feel the motor start to shudder as AFRs increase above 15 AFR.

I have also tried many tunes, older CL tunes and an OL tune, the problem is consistent across all tunes.

I have also not seen any codes through this whole process.

[RADustin]

have you unpluged MAF while running to ensure a code is thrown?

what about making an OLSD tune?

your PE is rich as hell. 1.75 eq ratio, ~8.4 lamda. it also engages at 15kpa- so is it always active?

open loop idle is around eq of 1- so makes since until you warm up it runs fine.

desired airflow B4307 looks a little crazy, high? maybe the ecm is learning down the airflow and it starts leaning it out. have you dont RAFIG?

all your spark tables should have the same timing for idle cells. you may make this region large on hi/low octane maps.

just a few comments.


what else happened 2 weeks when it started runing bad? Put a tune on it? Did any sort of mechanic work? Anything?

[pormgb]

have you unpluged MAF while running to ensure a code is thrown?

what about making an OLSD tune?

your PE is rich as hell. 1.75 eq ratio, ~8.4 lamda. it also engages at 15kpa- so is it always active?

open loop idle is around eq of 1- so makes since until you warm up it runs fine.

desired airflow B4307 looks a little crazy, high? maybe the ecm is learning down the airflow and it starts leaning it out. have you dont RAFIG?

all your spark tables should have the same timing for idle cells. you may make this region large on hi/low octane maps.

just a few comments.


what else happened 2 weeks when it started runing bad? Put a tune on it? Did any sort of mechanic work? Anything?

Wow, thanks for this feedback!!

Found the problem, O2 heater fuse was blown, I will test the car tomorrow.

[joecar]

...

Found the problem, O2 heater fuse was blown, I will test the car tomorrow.ok, that is a corner case (why it seemed weird)... good job :cheers:

[statesman]

Found the problem, O2 heater fuse was blown, I will test the car tomorrow.

It had to be the one thing that I didn't think of. :doh2:

[pormgb]

Yea, but why no DTCs.

I checked the tune and all the front O2 MILs and DTCs are enabled, shouldn't the PCM have known there is a heater issue?

[statesman]

If they are set to "Not Reported" then the PCM will ignore them and no DTC is set.

[pormgb]

If they are set to "Not Reported" then the PCM will ignore them and no DTC is set.

Yes, I will double check.

Thanks!!

[pormgb]

Got the car out today, drove quite well but I still think something is wrong.

While idling I still see the AFRs rise and drop, it just doesn't look normal.

I have attached a log at idle, take a look and let knoe if you see anything weird, I have also attached the latest tune.

[statesman]

Your trims are very high.

What mods have been done to this engine?

[pormgb]

I have an 85mm TB, short tube headers and a 226,229 585,575 cam.

I don't think the problem is in the tune or PCM, If I use my Open Loop tune, the car will idle steady around 17 AFR. Also, both WB show the same AFRs so I think something mechanical.

I did notice last night that the cap that covers the brake booster vacuum feed on the back of the intake manifold was very loose, I put a couple of zip ties around it.

The way how the AFRs fluctuate, make me think I have a fuel or vacuum issue, I will do some more digging today.

[pormgb]

Ran the car today, no luck.

Car starts and runs but Idle AFR at initial start up are in the 15s, I am now completely lost with this issue.

[pormgb]

Finally made some progress, fixed a vacuum leak and AFRs seem to have stabilized.

here is the latest short log, I do still see fuel trim activity so some work is still needed at idle.

Does anything stick out on this log?

[statesman]

Your trims look a lot better now but, like you said, they still need some work.

The only thing which sticks out to me now is the big dip in your rpms after one of your throttle blips. You need to tune that out.

[pormgb]

Your trims look a lot better now but, like you said, they still need some work.

The only thing which sticks out to me now is the big dip in your rpms after one of your throttle blips. You need to tune that out.

Yep, I trying to figure that out, I think its something to do with the VE table between 400rpm and 800rpm.