Thread: Engine Misfiring @ High RPMs

Hey all. So my engine is misfiring but only at higher rpms (around 4000+), or when I floor it and the supercharger is working (I think). It doesn't matter if I floor it from a 20mph roll or 70mph on the highway, the engine sounds like its skipping and I lose alot of power. I've been searching forums and asking my friends that are mechanics. I know there are a number of things that could be causing this; something wrong w/ sparkplugs (moisture, gap, etc..), ignition coils, fuel filter, SC belt slipping, or maybe something else. If anyone has had similar problems or thinks they know what might be going on it would be greatly appreciated. I feel as if whatever this is its an easy/inexpensive fix.

Previous owner installed; cat-back exhaust, 3.4" pulley, K&N CAI, U-bend delete, performance spark plugs, some other minor bolt-ons

put the stock pulley back on ASAP

u don't need that 3.4 yet. you probably have knock retard, kr, like crazy....get a scanner and check it. possibly pulg gap to big or coils. but i know its knocking, ur gonna blow it up....

please read the sticky right now:modding your ur 3800 safely thread. its here look for it. but read it now.. or get ready for a new engine.

Okay I will look into the safely modding thread, however I want to point out that this problem just began recently, in the last couple hundred miles, and that the car has ran well for probably 20k miles since I bought it from the previous owner who modded the car. So it's not like the smaller pulley or any of the other mods were done recently and then problems began. Could this be as simple as bad gas? I have a '98 w/ about 155k miles on it.

clogged cat. exactly what my car did. about 3-4k the check engine light would start to blink out on me.

kfish - what exactly do you mean by 3-4k miles? since you got the car or since you started having the knocking problems?

So now that I've gotten all these suggestions I'm not sure where to turn first? I will still go get it scanned at Autozone, but are there any of you guys that are more confident that one of the suggestions listed above is more likely the problem now that you've seen all that it could potentially be? Can any of them be ruled out since I ONLY get knock retard in higher rpms (4000+)?

So far the solutions suggested have been;
scan engine
knock retard due to; 3.4" pulley too soon, spark plugs bad/gap too big, ignition coils
clogged cat
lower intake manifold gasket leak

take the pulley off first and put on stock pulley. replace plugs with autolite 104;s or 605's, cheap copperplugs is all u needs..after that get wires...then lim gaskets...u gotta start somewhere--do the cheapest first...

If the LIM gasket was leaking coolant it would end up in the oil eventually. If your oil is not discolored (rusty color) then that isn’t the problem. LIM gasket is a quite a bit of work to do for nothing unless you want to do it as a preventative measure. I read that GM did f up these gaskets and they came out with a revised version. I’m just offering problems that I’ve encountered that are similar to your issue. Not saying that LIM gasket is it. I would start with the easy/cheap fixes first.

I honestly think I just got some bad gas guys. Filled up @ Shell yesterday and my problems have disappeared. Thanks for the suggestions. Atleast now I already have new plugs and a fuel filter next time I tune up.

I garauntee it's the maf sensor starting to go. If so try to clean it real good.

BTW leaking intake gaskets do not always put ur water in the oil. mine in particular had 2 very clean chambers...due to water being burnt..or passing out thru the exhaust,,every how u wanna call it.

One last note. I figured out that everything runs fine, even at high rpms, if running the engine after a cold start. However, once the engine is warm and I get into the higher rpm range I continue to have the knocking problem. Does this have any significance?

I have a similar problem with my 99 GTP 3.8SC engine. I had a reputable mechanice check it out and the gasket for the supercharger is blown. My question is was it due to another so called mechanice blocking the two exhausts pipes to check for leaks or just wear and tear? Is it ever a good idea to block the exhaust while running the engine?

If you tried to reuse it then it'd "blow"