Have a 2004 Monte Carlo SS with the L67 engine. Have had a consistent issue where the engine will misfire on most to all cylinders when the boost actuator closes to allow boost to build. No codes other than some O2 codes (which seemed to have developed from me trying to fix this for so long) and random misfire P0300.

I have replaced plugs with AutoLite 606s, wires, all vacuum lines, boost actuator, boost solenoid, fuel filter, LIM gasket, and supercharger gasket.

I have tested the ICM and it passes according to Oreillys. I have tested fuel pressure at the rail and get 40-45 at idle. It will raise to around 50 when giving heavy throttle. When taking the vacuum line off the FPR it will sit at 50.

The misfire only happens when the engine would start building boost. If I hold the Boost Actuator open it will not misfire. I have even gone as far as tying it open and going for a drive. Never broke up or misfired while it was being held open.
Recently, after replacing every vacuum line (the first time I replaced them I did not replace the original rubber T's at the FPR and LIM) I have developed a misfire on Cylinder 3 at idle. I checked and swapped plugs and swapped coils around and it did not fix it.

From what I can see, KR is at 0 degrees, spark is around 20-26 degrees when warmed up. I can also see that the car never seems to go into closed loop. This might be a point in the right direction because it feels like the car doesn't misfire as bad when cold. Not sure.

A couple of issues that car has that I don't know would cause this issue:
MAF metal screen is damaged. It isn't missing any material, but it is damaged. Not sure if this would cause my issue. Has an exhaust leak about where the Resonator would be. It doesn't have a resonator as it seems the previous owner deleted it, but there is a sizable leak in that area.

Ideas? Thanks.

 

First thought, did you verify the gap on the new spark plugs was correct? Even if they are pre-gapped, I always check (and I have found they sometimes need adjusted).

The resonator delete should not be related and I doubt the exhaust leak being past/behind the cat should not be an issue.

The MAF screen could be related. If it is damaged in a way that it skews the results the MAF is getting, the MAF sensor is not properly doing it's job based on the change in air flow. I have heard different theories running with and without a MAF screen. One theory for the screen is that GM designed the car to have air enter the throttle body in a linear fashion, the screen promotes that and thus it helps what the MAF sensor wants to see.

I would assume you could safely remove the screen temporarily and see if the results change.

As for holding the boost actuator open, perhaps that is a part of this issue. I am not sure how to properly test that to ensure it's functioning as it should.

 

As you probably know, the actuator arm pushes down to close a valve to allow boost to build. This only happens when the manifold pressure changed from a vacuum, to equal pressure, to positive pressure. If I tie the arm to remain in it's upward position, as if there was no actuator to begin with (basically running NA) then it does not misfire at all.

Edit: I'm pretty sure the boost actuator isn't the issue considering the result was the same when I replaced it with a new one from ZZP.

Edit2: I did check the gap a few days ago. They all seemed correct. Regapped just in case.

 

AH, never mind on the boost actuator. I was thinking something else.

 

Hey guys I'm having the same issue with a 2000 Buick Regal GS was running fine previously I'd sat for a little while and then I purchased it from my body and him and I are getting it ready for a safety we had a blown brake line and the fuel lines were pretty tough so brake lines are now nickel copper and the fuel lines are replaced front to back with the neoprene PVC or whatever you want to call it fuel line and a new filter we trained what little fuel was left in the tank as it didn't smell very volatile anymore and I filled a Jerry can. 91 took it out have similar issues engine was up to temperature it's partially clear but still there once you get into a boost situation vehicle has 144000 km and was one owner since new and maintained that the dealership I have plugs and wires as we are unsure when they were done last and what is in it for plugs bought the Denso iridium plugs and NGK wires the fuel line is 3/8 inside diameter which is the same as what Bastille lines were so we shouldn't be starving for fuel why do we get on Boost but that is what we are thinking it may be the problem wondering if anyone has come to a solution any help is greatly appreciated thanks in advance

 

I did end up finding out what the issue was. I did a leak down test and found that I had a leaking intake valve. I took the heads off and found the valve seats were cracked. I got new heads and everything was good.

I'd suggest getting the ICM tested, changing coils, doing a compression test, and doing a leak down test. My advantage was I had access to a Tech 2 scanner so I could see exactly what cylinder(s) had the misfire issue and diagnose from there (swapping injectors, etc). My issue was a slight misfire under boost. If you're having a bigger miss, it could be the coils. I had a bigger miss under load and it ended up being a coil.

 

Thanks! We are going to tackle the rear leaking valve cover and plugs and wires once I'm done work Wednesday night. It ran fine before and has no modifications. My friend helping is a gm mechanic and believes that it's probably plugs and it's become more noticeable since we freshened up the fuel system. It sat for about 6 to 8 months and in that time he did take it out for a drive up and down the road the shop is on and was fine then and that was a few months ago. He's also going to bring his scan tool to Do some live data checks. There is no misfire at idle or easy throttle. It's almost acting as though there is a fuel restriction but all lines are clear and no kinks. I'll update with our findings as we dig deeper. I'm assuming the old fuel fouled the plugs. The misfire is only when you try to give any load and clears up in the higher rpm.

 

Yeah definitely change your plugs. Could also check fuel pressure and even run some fuel cleaner through. Injectors might me a little clogged from sitting. Though, if it clears in the higher RPMs it doesn't seem like a fueling issue. You might be able to take the ICM to an auto parts store and test it. We have Oreilly's out here and they test it for free.

 

So i was having a similar issue. Hooked up the tech 2 and got these readings.
Turned out that because the previous filter before we did the lines and such was partially plugged the vehicle had learn new fuel trims over it's course to keep running properly. Now that the lines were more free flowing and the filter fresh it was causing the misfire. We reset the fuel trims and all seems good. Pulls as it should with no hesitation. We are still going to do plugs and wires for piece of mind but glad that it wasn't anything more serious.