Random misfires at idle, all cylinders
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From: Northwest AR
Random misfires at idle, all cylinders 02 T/A, m6
TSP 1 7/8 headers
SLP LM1
SLP lid
Stock engine rebuilt with 98 cam
Tune has copied tables from 98 tune to work with the cam
Car runs great in the mid-upper rpm and WOT
At idle the car has misfires randomly on all cylinders. Not sure how long it keeps track but a Snap On MT2500 shows a misfire history for 100k+ (some much much higher) for each cylinder.
As soon as you bring up the rpms, the misfires stop popping up. Car has idled rough like it has a huge cam since the rebuild. Timing set was triple checked by the owner to make sure it was dot to dot.
Going to try replacing the crank sensor with 1 I have that is known to be good, but what else could cause this?
TSP 1 7/8 headers
SLP LM1
SLP lid
Stock engine rebuilt with 98 cam
Tune has copied tables from 98 tune to work with the cam
Car runs great in the mid-upper rpm and WOT
At idle the car has misfires randomly on all cylinders. Not sure how long it keeps track but a Snap On MT2500 shows a misfire history for 100k+ (some much much higher) for each cylinder.
As soon as you bring up the rpms, the misfires stop popping up. Car has idled rough like it has a huge cam since the rebuild. Timing set was triple checked by the owner to make sure it was dot to dot.
Going to try replacing the crank sensor with 1 I have that is known to be good, but what else could cause this?
Thread Starter
11 Second Club
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From: Northwest AR
Maf is clean and screened.
Intermittently throws codes for 02 sensors but it was acting bad first, really want to try to figure out the cause before ruining new sensors.
I've been doing some research and it seems o2 extension harnesses that have gotten wet can be a problem but I thought the connectors on ls1 cars were weather tight?
Intermittently throws codes for 02 sensors but it was acting bad first, really want to try to figure out the cause before ruining new sensors.
I've been doing some research and it seems o2 extension harnesses that have gotten wet can be a problem but I thought the connectors on ls1 cars were weather tight?
On The Tree
Joined: Apr 2011
Posts: 181
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Im assuming issue occurs once car is warmed up and the vehicle is in closed loop (o2's warm and working)? If issue in closed loop can't you view the o2 sensor voltage at idle with your Snap On tool to see if they are the cause?
Misfires are a natural consequence of the bad-*** lopey idle you wanted.
You just have to bump up the misfire table values until it quits bugging.
Or just kill the detection altogether, but then you won't know about any
real ones.
A bad sensor will screw a whole bank and a bad cylinder (spark, fuel)
will stand out in the numbers Misfire Current Cylinder 1...8. This is just
one of those things that you have to deal with when you cam it up.
Or lighten flywheel mass, like performance converters and lightweight
clutch / pressure plate. Low end mixture error and spark jumping
around like crazy trying to stabilize idle help none either.
You just have to bump up the misfire table values until it quits bugging.
Or just kill the detection altogether, but then you won't know about any
real ones.
A bad sensor will screw a whole bank and a bad cylinder (spark, fuel)
will stand out in the numbers Misfire Current Cylinder 1...8. This is just
one of those things that you have to deal with when you cam it up.
Or lighten flywheel mass, like performance converters and lightweight
clutch / pressure plate. Low end mixture error and spark jumping
around like crazy trying to stabilize idle help none either.
Thread Starter
11 Second Club
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From: Northwest AR
The car has a stock 98 camaro cam. But it sounds like it has a bigger cam than my 230/224 on 111.
And basically a stock tune, only copied and pasted some tables to correspond to the cam being larger than the 01 cam. The idle was not altered. It does have a monster stage 3 clutch and lightweight flywheel, but it had this same issue before they were installed.
We found a small vacuum leak on a pcv hose but fixing it did nothing to correct the idle.
Also of interest the LT fuel trims are extremely high
And basically a stock tune, only copied and pasted some tables to correspond to the cam being larger than the 01 cam. The idle was not altered. It does have a monster stage 3 clutch and lightweight flywheel, but it had this same issue before they were installed.
We found a small vacuum leak on a pcv hose but fixing it did nothing to correct the idle.
Also of interest the LT fuel trims are extremely high
check the fuel pressure regulator, We were fighting similar in my little brothers 79 k10 that we swapped a 2000 5.3l into and found that the fuel pressure regulator was bad. It was pumping fuel straight through the FPR and up the vacuum hose to the intake manifold and out into the intake manifold.
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Try doing a Crank Trigger Relearn. Book says you're supposed to do this after an engine build, but most folks don't bother and have no misfires.
How many steps is the IAC keeping at idle? Should be around 40-60 steps. If it's much higher then the engine may not be getting enough air. Also, what is the spark doing at idle? It should bounce around a good bit but stay between 18-28 and average about 22-24. Best of luck.
How many steps is the IAC keeping at idle? Should be around 40-60 steps. If it's much higher then the engine may not be getting enough air. Also, what is the spark doing at idle? It should bounce around a good bit but stay between 18-28 and average about 22-24. Best of luck.
Thread Starter
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From: Northwest AR
Thought about crank trigger relearn but the car has behaved this way for a long time (over a year) without throwing a code. Might go ahead and do that anyway.
IAC was showing 0 at idle, likely due to adjustment on the TB blade...any less and it tries to die a lot.
I'll have a look at spark next chance I get. I can also try to upload a EFI live log if that would help.
IAC was showing 0 at idle, likely due to adjustment on the TB blade...any less and it tries to die a lot.
I'll have a look at spark next chance I get. I can also try to upload a EFI live log if that would help.
Did you mean the TPS (throttle position sensor) was 0%? If your Idle Air Count was 0 then you might need to replace it. IAC logs should dance around quite a bit at idle.
Thread Starter
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From: Northwest AR
No the IAC. It moves once you rev and let off, but at idle it sits at 0. It used to move around but he adjusted the set screw enough the car would start and idle without feathering the throttle.
I can't imagine what is wreaking this much havoc on a damn near stock car. It literally sounds like he's running a ms3 cam at idle. For a while I thought someone might have sold him a aftermarket cam instead of stock by accident, but it traps normal for his bolt ons.
I can't imagine what is wreaking this much havoc on a damn near stock car. It literally sounds like he's running a ms3 cam at idle. For a while I thought someone might have sold him a aftermarket cam instead of stock by accident, but it traps normal for his bolt ons.
Dude, that's messed up. IAC is not involved in the idle like it's supposed to be. The purpose of the IAC is to smooth out idle transitions, provide extra idle air when A/C kicks on, and most importantly help control idle speed. Opening the throttle blade that much takes it back to the carb era idle control.
I suggest closing the throttle blade until the IAC steps are > 30. You may have to increase the Desired Airflow table a little to get more air into the engine. Also, I'm assuming stock range idle speeds?
Idle spark is even more sensitive than IAC. Make sure the Idle Flare control tables are stock. Even small changes to the spark underspeed/overspeed tables can produce unexpected results.
Another question, what's the air fuel ratio during idle? If it's running even a little lean like above 15:1 then it will idle rough and burn the eyes.
I suggest closing the throttle blade until the IAC steps are > 30. You may have to increase the Desired Airflow table a little to get more air into the engine. Also, I'm assuming stock range idle speeds?
Idle spark is even more sensitive than IAC. Make sure the Idle Flare control tables are stock. Even small changes to the spark underspeed/overspeed tables can produce unexpected results.
Another question, what's the air fuel ratio during idle? If it's running even a little lean like above 15:1 then it will idle rough and burn the eyes.
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From: So.Cal.
Thought about crank trigger relearn but the car has behaved this way for a long time (over a year) without throwing a code. Might go ahead and do that anyway.
IAC was showing 0 at idle, likely due to adjustment on the TB blade...any less and it tries to die a lot.
I'll have a look at spark next chance I get. I can also try to upload a EFI live log if that would help.
IAC was showing 0 at idle, likely due to adjustment on the TB blade...any less and it tries to die a lot.
I'll have a look at spark next chance I get. I can also try to upload a EFI live log if that would help.
No the IAC. It moves once you rev and let off, but at idle it sits at 0. It used to move around but he adjusted the set screw enough the car would start and idle without feathering the throttle.
I can't imagine what is wreaking this much havoc on a damn near stock car. It literally sounds like he's running a ms3 cam at idle. For a while I thought someone might have sold him a aftermarket cam instead of stock by accident, but it traps normal for his bolt ons.
I can't imagine what is wreaking this much havoc on a damn near stock car. It literally sounds like he's running a ms3 cam at idle. For a while I thought someone might have sold him a aftermarket cam instead of stock by accident, but it traps normal for his bolt ons.
With TB blade in original/unadjusted position, there is too little idle air...
either way, sound like the idle air table is wrong...
and maybe spark is at too far at end of the range which is preventing the PCM making speed corrections.
Last edited by joecar; Sep 15, 2011 at 04:21 PM.
