Giving up on random misfire at idle

**gingerdude** · 07-27-2017

So I have a 47,000 mile 01 SE. I was garage kept by my grandparents and now it's mine. Great condition.

The only problem I have currently and have had since I got it is that it has a random misfire at idle (noticeable in the car and confirmed with Torque app). FYI Torque app shows nothing out of the ordinary for anything besides these misfires.

So I have replaced everything as a abundance of caution given the vehicle sat for years and years.

-Wires and plugs
-Fluids and filters
-PCV and FPR
-Head and intake gaskets (confirmed no leaks with the proper physical tests and an oil analysis)
-Cleaned out everything while I had the engine torn down
-Basically anything that would be routine maintenance, plus all the trouble points on this car have been touched.

I even swapped a known good ignition module and coils over from another GP and it STILL misfired.

So today I was at the junk yard looking for something else, so I decided to pull an ECU (same model and year) thinking this has to be the problem. Well I put the ECU in. She drives great, but STILL misfires.

?!

But yeah so at this point I'm just going to say that's just what this car does. Lol.

**FordMan77** · 07-28-2017

What brand and type of plugs did you install? Are you sure the wires are in the correct order? It happens more than you think and the car will still run fine just with a slight miss.

**gingerdude** · 07-28-2017

They are in the right place. I used the recommended AC Delco plugs. I have put like 10,000 miles on it since I did the work. Plus, I noticed this issue before I did any work.

At this point the only things I haven't swapped or replaced that could be related are the Cam and Crank sensors.

The weird thing is that it doesn't happen all the time. Sometimes I can sit with my scanner for 15 minutes and not see a single misfire. Then I restart the car and it immediately starts the issue. Sometimes I'll get off the highway after a long run and it runs perfectly smooth. Other times you can easily feel the random misfire after a long drive. Makes no sense! It's even got my mechanic buddies stumped. I've just been rolling with it.

**fst** · 07-28-2017

with your torque app, do you know which cylinder(s) it is misfiring at idle? *subscrived btw cause a local also has the same issue wit his for 2 years and he replaced virtually everything and still gets a misfire on only cylinder 4. he replaced the following:
LIM gaskets, injectors, BBV, fuel pump, fuel filter, plugs, wires, FPR (I think?), valve cover gaskets

the only things left untouched I believe is the ignition control module

**rickisrad** · 07-28-2017

I am on my forth grand prix and all of them had random misfires. Not enough to throw a code but I knew they were there which was annoying.

in your case it may be bad injector(s) and also check the injector wiring. critters love to chew on stored cars. I've repaired several Harnesses from this.

**rickisrad** · 07-28-2017

also may need a case learn??

**gingerdude** · 07-28-2017

Fst yes I do. So the misfire will happen on every cylinder randomly, but strangely enough it seems to happen the most on 4 on my vehicle as well. I've done all the tests too. Compression was 170 - 180 across the board for example.

Rickisrad yes this actually isn't the first time I have heard this. I'm thinking it has to be normal for these vehicles. When I did the heads I cleaned and flow tested the injectors. They were good. Wiring is good (from what I have had out and inspected). Plus my grandparents garage was attached so I doubt anything got into the car. My grandpa worked for GM so he was meticulous with the upkeep on this vehicle even though they hardly drove it. CASE learn? Possibly? I put the original ECU back in, so highly doubt it.

I've thought about taking it up to the dealer for a one hour diagnostic on the Tech 2.

**Moozery** · 08-04-2017

This is exactly what my 2004 GTP has been doing since I got it a few months back, and my 04 GT never did this. Mine's mostly cylinder 4 too.
When I first got my car it was bad enough to throw codes if you let it idle long enough. As I've been trying different things here and there it's never been fixed completely but it's been getting noticeably better with everything I do.

I've been closely observing it for a while and it almost seems to be a heat issue to me, I was starting to wonder recently if maybe leftover oil gunk from my bad LIM gasket (before they got replaced) is not allowing the coolant to pull the heat effectively enough.
I was going to do some thorough flushing next week and see if that does anything.

The past few weeks I've been running my AC more often to force the fans on and I'm noticing a bit of an improvement. Maybe see if you notice the same?

**green97** · 08-04-2017

Have you done a crank relearn procedure. Swapping ecu or crank sensor or rotatinig the crank can require this. I had same code after swapping motors and crank relearn fixed it.

**gingerdude** · 08-18-2017

I also wonder if it's heat related. Seems to be most noticeable after long drives when I finally come off the highway or interstate and idle.

I haven't done a crank relearn. I wish I had access to a scan tool that could do it.

**stealthee** · 08-18-2017

I'm with Rick on this. I noticed the misfire at idle on mine several years back and thought something was wrong, so I went through and did plugs, wires, etc. It was still there til the day I sold the car.

**fst** · 08-18-2017

verified if there's any vac leaks?

**Moozery** · 08-18-2017

Well i finished doing 5 good flushes with prestone rad flush last week and i perfectly filled a 5 gallon pale with a bunch of nasty brown coolant. Combined with this and forcing the fans on, ive had no misfires at all since. Starts much nicer and everything.

**fst** · 08-18-2017

Moozery im curious what your engine coolant temps were prior to you flushing the old coolant out and what temp is at now at operating temp?

**Moozery** · 08-18-2017

Its only a couple degrees cooler while driving at highway speed. Im running a 180 thermostat and now when it first reaches operating temp and starts to cycle it stays at about 175 for a little while. Once it gets fully warmed up and im either driving or running fans it stays at a stable 180 with little fluctuation.

I think the problem with the oil coating the system is it doesnt allow the coolant to pull the heat off the engine in the first place, therefore the coolant stays cool yet the engine gets hot.

**coolone** · 08-18-2017

Crank position sensor I might say, maybe, in some cases with oil leaks and such, dirt and such could build up on mag pickup and create issues, but you low mileage discounts that, but the intermittent nature of the misfires leads me to a break in a wire somewhere... And that sucks! You'll have to check the wiring harness.

Have a 2001 SE and had similar problems but to another extent where the engine would stumble, always kept up on maintenance so didn't run through the litany of replacing all you have shown here. Car would run great but then next time, it would almost stall at a light, etc. When I hit the proverbial brick wall when tracking things down, so I took it to a buddy. He uses some diagnostic system (for mechanical shops) that he pays for and they have a database that tells you probability of problem based on diagnosis. It said a short in wiring loom by AC condenser. So Lo&behold, once it was opened up there were two wires that were kinda melted from sitting on the condenser, think they were from the crank circuit. Fixed that and never had the issue resurface.

You may very well have the same issue but the low mileage may not have compromised the wiring as much as mine had been. Think my SE was at 180k then and today is still running very well at 265+k.

Check out that spot... And if you get tired of that SE with 50+k on it, drop me a line ; )

**gingerdude** · 08-25-2017

Moozery, when I got the car it had sat for awhile so I also did a vigorous coolant flush. Had the nasty Dexcool sludge. I used the flush GM recommended and left it in for a few days. Then I rinsed with a ton of water until it came out mostly clear. Also, I've replaced the HGs and intakes so I know it was at least decently clean inside the water passages. I put green in.

Green97, I have not done a CASE relearn because I am using the ECU that came with it. Not sure if it would help, but I don't want to pay the dealer for it. I have another ECU from the same module year and features, the engine runs the exact same with it in as well. Same misfire pattern.

Fst, I've only half assed checked for a vacuum leak by going around the engine quickly with a small propane torch. I have replaced all gaskets and seals when I redid the heads up and it had no effect, so I have kind of ruled this out. I don't have a good place to make/use a smoker, but it is something I am considering when I have the chance.

Coolone you wouldn't happen to know which side the wiring loom you're talking about is on would you? Because honestly that is literally the last thing I think it could be. Something keeps saying CPS. It gets really bad after I have been on a long drive. I would imagine a failing CPS would produce these symptoms as it warms up.

**fst** · 08-25-2017

wild shot but have you looked at the exhaust manifolds if they're clogged? also do you by chance smell something similar to rotten eggs inside the car?

**waupachino** · 08-25-2017

Today's fuels are terrible after sitting and can cause the injectors to stick, which would cause a misfire. I would run a tank of premium with some Seafoam or Techron injector cleaning through it to see if it clears up.