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2G Misfire when warm at low RPM - stock, no CEL

ErikTande

Supporting Member
1,470
502
Dec 17, 2012
Centennial, Colorado
Got a 1997 Talon TSi I'm having a misfire / stumble / miss problem with. It's fine when it's cold, but when it's warm it starts to misfire at low RPM and at idle. Higher RPMS seem ok. Car is bone stock.

Plugs and wires have been changed, and I assumed it was the ICM so I swapped that out. No change. It was a junkyard part ICM though so it's possible they were both bad...

No history on the car, I just got it.

What do I check next? Thanks!
 
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brian9397gsx

Proven Member
1,559
93
Apr 28, 2008
sioux city, Iowa
Idle misfires are usually a fuel delivery issue or low compression. Start with a compression check which is pretty simple then pull the injectors out and clean them. Its pretty simple I made my own rig with some pipe, hose and hooked up shop air then fired the injector with a battery. Those are the two areas I'd start with.
 

ErikTande

Supporting Member
1,470
502
Dec 17, 2012
Centennial, Colorado
Idle misfires are usually a fuel delivery issue or low compression. Start with a compression check which is pretty simple then pull the injectors out and clean them. Its pretty simple I made my own rig with some pipe, hose and hooked up shop air then fired the injector with a battery. Those are the two areas I'd start with.

Even though it's fine when it's cold? The engine has to be warm for it to start happening. Once it gets up to operating temp it takes another 5-10 mins before it starts missing. And the miss is intermittent.

I know the power transistor / ignition control module is a common culprit for misfires when warm. I might try a brand new one then move on to replacing the coils next.
 
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1990TSIAWDTALON

Moderator
9,279
5,073
Nov 14, 2013
Independence, Kansas
I second a compression test just to make sure of the health of the motor.
 

ErikTande

Supporting Member
1,470
502
Dec 17, 2012
Centennial, Colorado
I second a compression test just to make sure of the health of the motor.

I plan on it, but poor compression on a cylinder wouldn't cause a misfire only after the car is thoroughly warmed up. That doesn't seem like a logical step to fixing the issue at hand. Unless I'm misunderstanding something.
 

1990TSIAWDTALON

Moderator
9,279
5,073
Nov 14, 2013
Independence, Kansas
Well Erik, I have had a DD Honda that didn't misfire until it warmed up and we had a HG on its way out is the only reason I thought that would be a good thing just to "cross off of the list". :)
 

ErikTande

Supporting Member
1,470
502
Dec 17, 2012
Centennial, Colorado
Well Erik, I have had a DD Honda that didn't misfire until it warmed up and we had a HG on its way out is the only reason I thought that would be a good thing just to "cross off of the list". :)

Word. I'm gonna try the new ICM and then coil packs first. If that doesn't do it I'll get around to the compression test.
 

brian9397gsx

Proven Member
1,559
93
Apr 28, 2008
sioux city, Iowa
Well Erik, I have had a DD Honda that didn't misfire until it warmed up and we had a HG on its way out is the only reason I thought that would be a good thing just to "cross off of the list". :)

Precisely because what are you out doing a compression test? I've had a few like that with old 4.0s and others.
 

Tyeler18

Proven Member
2,475
204
Dec 16, 2008
Casa Grande, Arizona
Before throwing parts at it, find out which cylinder is causing it. Injectors are common source of warm misfires at idle. They build up varnish over the years and when warm the varnish turns to goo on the pintle making them sticky. Valves do the same thing, a warm compression test would help determine if a valve is getting sticky causing a low compression issue. But you need to determine which cylinder it is first.

I have a hard time believing two used ICM's are showing the same issues. Generally a failing ICM or coil pack will cause both cylinders on that circuit to go down. I'd bet money on an injector, but I wouldn't spend a dime until you figure out which cylinder it is. Once you do, swap injectors around, do a compression test, and work from there.
 

ErikTande

Supporting Member
1,470
502
Dec 17, 2012
Centennial, Colorado
Before throwing parts at it, find out which cylinder is causing it. Injectors are common source of warm misfires at idle. They build up varnish over the years and when warm the varnish turns to goo on the pintle making them sticky. Valves do the same thing, a warm compression test would help determine if a valve is getting sticky causing a low compression issue. But you need to determine which cylinder it is first.

I have a hard time believing two used ICM's are showing the same issues. Generally a failing ICM or coil pack will cause both cylinders on that circuit to go down. I'd bet money on an injector, but I wouldn't spend a dime until you figure out which cylinder it is. Once you do, swap injectors around, do a compression test, and work from there.



Nice, I didn't know that about injectors. Thanks! What's the best way to figure out which cylinder is missing? I mean, if it's valves then the compression test will show it, I get that. But is there a quick way to tell if it's a fuel or spark issue on a particular cylinder?

I have a spare set of injectors sitting around from a 1g, I'll swap those in and do the compression test if that doesn't fix it.

On a side note, this car sat for at least 2 years before I got it. I've put like 700 milles on it with a few fresh tanks of gas though so it's probably not old gas doing it.
 
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CrackedDSM

Proven Member
1,921
593
Dec 17, 2009
Pensacola, Florida
Easy test is just unplugging the injector of each cylinder and seeing how it affects the motor. Usually if one cylinder is weaker, unplugging it will make very little difference. The "backbone" cylinders though will damn near kill the motor when you unplug them.


It only misfiring when it's warm could be a vacuum/boost leak too. When it's cold everything's barely sealed, when it's warm it warps/moves/whatevers enough to open up and cause some issues. I've had that issue before too on an intake manifold.
 

Tyeler18

Proven Member
2,475
204
Dec 16, 2008
Casa Grande, Arizona
Nice, I didn't know that about injectors. Thanks! What's the best way to figure out which cylinder is missing? I mean, if it's valves then the compression test will show it, I get that. But is there a quick way to tell if it's a fuel or spark issue on a particular cylinder?

I have a spare set of injectors sitting around from a 1g, I'll swap those in and do the compression test if that doesn't fix it.

On a side note, this car sat for at least 2 years before I got it. I've put like 700 milles on it with a few fresh tanks of gas though so it's probably not old gas doing it.

I wouldn’t rule out old gas IN the injectors. A layer of varnish is super hard to clean out without a strong chemical cleaner. Sea foam etc works OK over a long period of time but actually clearing it out the best way is to have them properly cleaned. Over time fresh gas will slowly clean them out too but once it’s varnished it’ll be a long process to remove.

As far as checking them, pull a plug wire one at a time and see which don’t effect the idle. That’s your dead cylinder. Listen for spark as you pull the wire and that’ll give you a reasonable indication on whether or not it’s spark. You should be able to compare the arcing sound from a good cylinder to the bad to ensure it has enough spark to consider OK. Before throwing more variables in (different injectors), either swap the injectors around and see if it follows the injector or grab a stethoscope and listen to each one firing. They should all have the same audible clicking. If ones gummy or flowing poorly generally it’ll click softly compared to the others. Once you find the injector (assuming that’s what it is) move it to another cylinder to make sure the miss follows that injectors. A weak injector driver could potentially cause it as well as an injector resistor in the resistor box shorting our hot.
 

ErikTande

Supporting Member
1,470
502
Dec 17, 2012
Centennial, Colorado
I'm home troubleshooting, unfortunately it's nearly impossible to narrow the problem down as it's intermittent. I'm going to let it cool down for and hour, do a compression test, and if that looks fine I'm going to swap the injectors. I'll keep you guys updated.

Here's a video, it doesn't seem horrible here, but when it does it when driving it really bucks and sputters / stumbles. You can hear it really clearly at 14 seconds

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ErikTande

Supporting Member
1,470
502
Dec 17, 2012
Centennial, Colorado
Compression test was on a fairly cool engine, (it had been sitting for 3 hours after running)
1. 130
2. 135
3. 140
4. 123

Not bad.13% difference between the highest and lowest but meh, it's fine IMO. Also, I'm at 6000 feet elevation so gotta correct in .86% or so, so should be fine.
 

ErikTande

Supporting Member
1,470
502
Dec 17, 2012
Centennial, Colorado
Swapped the ICM (power transitor), no change. Put in new coil packs, no change. Already has new plugs and wires.

Gonna swap the injectors next and see if that does it. Like I mentioned before, it's intermittent so it's nearly impossible to narrow the problem down.
 

brian9397gsx

Proven Member
1,559
93
Apr 28, 2008
sioux city, Iowa
I watched your YouTube video I'm not sure about injector anymore but I'd still start there. I'm really surprised that it's not throwing any codes for a cylinder mis from the way it was on the video. Anyway great score on the car.
 

boostdawd

Supporting Member
1,437
324
Apr 6, 2010
phoenix, Arizona
Also might want to open up the valvecover and check the rockers. I had a problem with a misfire and it was one of the rockers was misaligned causing problems with the valve.

If u have replaced icm, coilpack, plugs and wires and injectors then taking a look at the ecu might be something to consider.
 

CrackedDSM

Proven Member
1,921
593
Dec 17, 2009
Pensacola, Florida
How are the coolant temp sensors and MAF sensors btw? Can you quickly swap some out? Also, what CAS does it have on it? Check the CAS wiring too.


It is also worth swapping in another ECU because it being random is weird. Like a a sensor is dying or your ecu is dying.
 

ErikTande

Supporting Member
1,470
502
Dec 17, 2012
Centennial, Colorado
I haven't done any more work on it yet (no time), but I did take the car to get emissions done and get it registered. It had the random miss while on the rollers doing the test. I BARELY passed. You can see my NOx was 1.99 with the limit of 2.00.

This could be indicating a a lean condition, which could be a bad injector we've been speculating about. So I'm going to swap those next. Then check ECU if that doesn't work.

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BLACK'98DSM

Proven Member
3,557
1,266
Feb 9, 2019
Alabama
Just wanted to update this thread, I got it fixed, it was a bad cam angle sensor causing the random misfire.

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That's a crank angle sensor, not a cam angle sensor. :D
 
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