Chasing Random Misfre
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Teching In
Joined: May 2006
Posts: 23
Likes: 1
From: Delaware
Chasing Random Misfre I have a 1998 Z28 with 250xxx miles. I have replaced the Spark plugs and plug wires but am still chasing random misfires. I have gotten misfires on cylinder 3, 5, 7 and 8, but now it's just P0300. Could the coil packs be on the way out or do I need to have to injectors cleaned? I have done no internal work to the engine, the only mods are SLP exhaust and long tube headers. If some one could offer any insight to what the issue may be I would be appreciative.
TECH Fanatic
Joined: May 2016
Posts: 1,292
Likes: 436
From: St. Louis
This is the internal engine section a p0300 code is not something so terrifying that we should just pour things in our gas tank and "make a wish".
We troubleshoot and solve the problem.
Op, try these things first and report back
Check your fuel pressure at the rail 55-60psi is normal. If it is less Replace the fuel filter and see if that fixes it. If it doesn't then you probably need a fuel pump.
the funny thing is, the fuel filter costs less than the bottle of crap the clown above recommended
Make sure you're EGR valve isn't stuck open That's another thing to check before you go crazy buying bottles of wish juice at the parts store.
There are plenty of write-ups here and online everywhere to follow when you get 0300 code. It's typical of a modified car but yours is not modified so the standard troubleshooting methods will easily apply to your situation. Just search "LS1 p0300 troubleshooting guide" find one you like and follow along.
No need to get everyone riled up around here sifting through the additive books trying to figure out which one might magically solve your problem.
Last edited by stockA4; Jan 4, 2023 at 03:56 PM.
LS1-check-engine-code-p0300-problem is a decent thread with some details to check.
When trouble shooting I'd include checking the MAF and seeing if it's dirty or has a broken wire.
When trouble shooting I'd include checking the MAF and seeing if it's dirty or has a broken wire.
IMO very unlikely your coils are all going out at the same time. As mentioned, check fuel pressure and MAF, also check fuel trims if you can. I think good rule of thumb for the MAF is about 5-6 grams/sec of air at idle for a 5.7l engine. Normal fuel trims are "typically" single digits +/- for both long and short.
Since you're stock, I think you technically could unplug your MAF and your car will fall back to a backup strategy using the MAP, see if your misfires go away. Obviously you'll get a MAF code but that test could show a misreporting MAF. I know you have a 98 and maybe it's different not sure.
Does the problem happen only at idle or when running hard? Do you feel the misfire or are you just going by the computer telling you that there was a misfire?
If only at idle could be one the many things that cause vac leaks like evap open or EGR or intake gasket leak or vac hose broken/disconnected.
If misfires are mostly while running hard I'd think less likely air/vac related and I'd look closer at fuel pressure and trims AT THE TIME the problem happens if possible. For example, checking pressure at idle means nothing if the problem happens when running hard. I think less likely injector/electrical/coils/plugs/wires as those tend to show up one at a time and not many different cylinders at once. Note that running it too hard with blocked filter causing low fuel pressure could damage the engine, I suspect that's what took out my original near-stock HIGH MILEAGE LS1.
Since you're stock, I think you technically could unplug your MAF and your car will fall back to a backup strategy using the MAP, see if your misfires go away. Obviously you'll get a MAF code but that test could show a misreporting MAF. I know you have a 98 and maybe it's different not sure.
Does the problem happen only at idle or when running hard? Do you feel the misfire or are you just going by the computer telling you that there was a misfire?
If only at idle could be one the many things that cause vac leaks like evap open or EGR or intake gasket leak or vac hose broken/disconnected.
If misfires are mostly while running hard I'd think less likely air/vac related and I'd look closer at fuel pressure and trims AT THE TIME the problem happens if possible. For example, checking pressure at idle means nothing if the problem happens when running hard. I think less likely injector/electrical/coils/plugs/wires as those tend to show up one at a time and not many different cylinders at once. Note that running it too hard with blocked filter causing low fuel pressure could damage the engine, I suspect that's what took out my original near-stock HIGH MILEAGE LS1.
Last edited by mk3cn4; Jan 5, 2023 at 09:49 AM.
