Severe Miss @ WOT only above 4.5k RPM

[jmgtp]

LX9 swap/5 speed/OBD I

I was datalogging last night and expereinced a violent miss in the high RPM range (above 4500) at full throttle. It was as if it completely lost spark for a fraction of a second and then came back. This would happen a few times a second, the tachometer would also go kind of crazy when it occured. Other times it would rev out just fine.

Since the tach went nuts I'm fairly confident this is an ignition issue. Leaving me with not too many parts to sort out as bad:

CPS - pulled the LQ1 age unknown but I do remember replacing it once years ago
CPS Wiring - new with motor swap, but easily inspected
External Crank Trigger Wheel - 1 piece unit, not much to go wrong
CPS mount for Crank Trigger - adjustable, I can check to make sure it is not loose
ICM - I have a bunch so I don't remember what its from or age
Coils - same as ICM
Ignition Wires - new with motor swap, less than 2k miles

The motor runs just fine under normal driving, and I absolutely cannot replicate this problem in my day to day commute. I think that safely eliminates ignition wires, the CPS/wiring and the external trigger wheel hardware. I don't know if the miss is RPM related or LOAD related because I only encountered it under load (WOT) and at high rpm.

My thought is that the ICM is failing under the demand of high rpm. I have another set of Coils/ICM (unknown condition) that I can swap in easily enough but just wanted to see what others thought.

How susceptible are the ICM/Coils to heat on the pushrod motors? They are mounted right above the exhaust manifold so I'm sure they get their fair share of radiant heat. Then again, there are countless 3x00s running like this everyday so I think a proper functioning ICM is up to the task.

[TGP37]

I know the tach can go nuts when trying to start a car with low voltage. What was your voltage like?

I heard cold water on the coils can remedy the problem for a moment, help diagnose over heating.

Is your crank sensor signal wires running anywhere near the alternator? Those wires are delicate to interference hence why they are twisted, but I'm sure you knew that. However, a strong source of interference can still cause issues. Running the CPS wire right up against the alternator (in theory) could cause issues. I never encounter this personally but I heard about.

[jmgtp]

Nah, voltage was good. It's not just the tach but a coninciding engine miss as if it lost the crank signal or te ICM failed.

edit: and the crank wiring does go fairly close to the alternator but I can't see that becoming an issue out of nowhere. It's been running fine all through the rev range for 6 months routed like that. I'm not going to attempt pouring water on the coils/icm seems risky.

Plan of action is to swap in another set of ICM and coils that I have in the parts bin - problem with that being I do not know their condition.

[TGP37]

Distilled water is non-conductive. Plenty of people had poured water over the coils to cool them down as a test. I never done it and I agree with your hesitation to do so.

Can you log which cylinder is missing? Ruling out an engine wide issue or a specific cylinder issue.

[pocket-rocket]

Can you log which cylinder is missing?

OBD1, no can do :/

[jmgtp]

OBD I doesn't have the capacity to narrow down a misfire to a specific cylinder.

BUT... the results are in, sort of. I replaced the ICM and Coils with a spare set. Fired the motor up and it ran fine. I noticed it is idling smoother. It wasn't rough before but the new ICM/Coils seem to have smoothed it out some. As to whether or not it fixed my high rpm miss, that I don't know because after I started it up I heard this chirping coming from the accessory belt. I pulled the belt and checked each pulley. Water pump had some slop in it. DAMN. At least I had a spare pump from the dohc motor so I didn't have to spend any money. Removed the LX9 pump (more on that in a minute) and installed the dohc pump. Didn't have any fresh coolant so I couldn't refill and drive. I did fire it up for about 5 seconds to verify the noise was gone - it was.

Now, this LX9 pump, check this pic out. What is this funky impeller. Its a weird shape and is stamped steel. The shaft has a bit of wobble which was my chirp sound. At 52k miles I'd think this was the original waterpump but it sure looks like an aftermarket one. Is this what GM is using now?

Anyway, I'm going to pick up some new coolant tomorrow after work, fill, bleed and then hopefully rev to 6k with a solid ignition.

[pocket-rocket]

That's the junk piece of crap stamped steel impeller used on reman water pumps. The original would have been cast and had many more fins. A DOHC will not run cool with one of those, and neither will a performance oriented 3x00. That is for sure a replacement, and the one to avoid. 52k sounds fishy...

[TGP37]

I had a water pump like that. I ditched it for the revised one, new.

[Superdave]

I think that's the standard one that comes in the 3500.. i remember taking mine out several years ago and it being similar to that.


I never had cooling problems with it.


If the missfire is still there, check the CPS and the CPS harness.

[jmgtp]

ICM and Coils did the trick. Revs clear to 6300 rpm.

[3400-95-Modified]

IIRC thats the same style pump that was on my 05 3400 and it's still there to this day... never had issues with it.