Thread: Misfire under Boost please help!

Hello everyone,
I have a real head scratcher here. I have a misfire only under when I demand boost. I have replaced plugs wires and coils but it still misfires like a bastard, I have lost alot of fuel mileage and lost some boost as well. Usually pumps about 14-15 psi of boost but has dropped to about 11psi. If I floor it it will misfire and then go and my Check engine light will blink then go off. I do have a code right now but its an evap code for a vaccum leak not sure if that's the cause or not. Not sure if it's low compression in a cylinder or a fuel delivery problem, If anyone has Ideas on where to start that would be great.

Blaine

14-15 PSI means is modded or very broken.

Mod list?

What brand/product line ignition products did you use?

EVAP leak and vacuum leaks are two different things.

If you have a vacuum leak that will cause fueling issues (see misfires) and/or lost boost depending on location of vacuum line.

I have a upgraded ECU, pulley, headers, CAI, full exhaust system with no CAT, but i have a chip for the no CAT and cooling system for headers. It says vaccum leak, to be specific. Yeah it sucks! it's a round about right now

No cat rules that out.

Most end up running around 10-11 psi with those modifications.

Still need to know what ignition products you used. If they were used MSD coils, that would account for the high load misfires.

Side note: there is no chip tuning available for your PCM AFAIK. Flash tuning is what's used. DHP or HPT being the main two pieces of software/hardware to do it.

Most of the modding was done before I got it so I'm going off what I know to be honest, the ignition products were OEM and delco plugs and wires

ICM can cause high load misfires.

Fuel pressure could be falling and resulting in lean misfires. What did the old plugs look like?

honestly no idea i had a garage do them cause I wasn't doing them in the freezing cold! I just have read other people with threads and it could be so many things, my car was misfiring at cylinder 5 and 6 and the code went away but its still doing it at high rpms

Did you request the work or ask them to fix the misfire?

I asked them to fix the misfire and had them do plugs wires and coils cause I read that those are the most common reasons why it happens

They said the misfire code went away which it did but still misfires when I start to give it boost or went I just floor it

Does the SES light flash when it's misfiring? My car did the same thing, ran fine but as soon as it added boost it misfired. I bought a used OEM coil pack from a junk yard for CHEAP and went through and swapped it out. Everytime I swapped one out I'd take it out and drive it, if it didn't fix it I put original coil pack back in and moved to the next one. It ended up being the last coil pack I swapped out, no more misfire and lots of power!

You may try using an HEI ignition tester to put load on your wires, coils and ICM. Here's what an ignition tester is, Part 1 -HEI Spark Tester: How it Works. Make sure it is an HEI one. Some autoparts stores carry them or order online at Amazon for around $10.

Good luck

Hey bro, im not overly experienced with super chargers, but i do know a fair bit: does your exhaust sound different than usually? you might have a clogged cat causing there to be too much exhaust being forced back into the engine. i would HIGHLY recommend stop flooring it, drive it very cautiously with rpms, you do not want to much presssure in the engine it could cause: engine blowing (worst case), seals broken, head gasket blowing, cam misfunction, valve seal forced the wrong way, and youre probably sucking way to much fuel.

Yes as pointed out above I don't have a CAT it's a straight 3' down pipe to the y pipe then flowmaster mufflers, so you can rule out the cat, The exhaust actually helps being so loud cause you can hear when its misfiring, Also when Im in park I will rev the RPMS up to about 4000 right before the supercharger starts to kick in and you can feel and hear the car vibrate and studder so I dont know if that gives any one else a clue of what it could be. I just replaced the ICM as well so the whole ignition system has been replaced now I'm just itching my head of what else....

And yes to joann the SES light does flash when misfiring, also stays on and has a vaccum leak code.