hey guys so ive been working on a 98 gmc sierra with a 5.7. it has a code p0300 and my scan tool shows me 2&8 are missing hard and bad but the rest are fine. fuel trims show me the passenger bank with the misfires are running lean with a +20-24% fuel trim. the entire fuel system has been replaced for a previous lean code. the spider injectors were just replaced due to leaking lines. its had a full tune up with a new egr valve as well. ive checked compression getting 185 psi on each cylinder and they're all getting spark. the intake gaskets are all new due to again another previous issue. ive also checked for arcing in the ignition system and found nothing. i hooked up a timing light to the misfiring cylinders and it seems to have pauses in the spark while the other cylinders are steady telling me its an ignition issue but the fuel trims tell me otherwise. i hooked up the timing light right off the ignition coil as well and can see that there are pauses and breaks in the spark there. the ignition coil is new as well as the ignitor. which im stuck at either a pcm or mechanical failure like a bent push rod or maybe a pulled stud causing the valve to possibly not be opening enough causing a lean issue. but i just wanna see if anyone has experienced such a bizarre issue before i tear into the truck anymore all input appreciated.
98 sierra misfires at a loss
- Thread starter vince_454
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Supercharged111
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Be more specific when you say spider injectors have been replaced. The poppets are very sensitive to pressure and on my truck would only misfire at idle. At least enough to cause issues.
east302
I'm Awesome
I would almost use a spark tester so you can see how strong the spark is.
A quick fuel pressure leakdown test will probably rule out a leaking injector, but you wouldn't be the first one to report a leaking injector on a new spider assembly. Though two leaking injectors would seem unlikely, I'd do a test.
How is your coolant level? #2 and 8 are in the corners where you have to overlap the lip on the intake gasket with a small amount of RTV along the front and back edges of the block. Your oil looks good, no coolant in it?
It sounds like you ruled out compression in the air/fuel/spark equation if you're at 185-psi on all cylinders. If you had a low cylinder, adding oil through the spark plug hole would tell you if you had a valve issue...the compression would not improve on successive strokes.
I'd start with a spark tester and fuel pressure test.
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A quick fuel pressure leakdown test will probably rule out a leaking injector, but you wouldn't be the first one to report a leaking injector on a new spider assembly. Though two leaking injectors would seem unlikely, I'd do a test.
How is your coolant level? #2 and 8 are in the corners where you have to overlap the lip on the intake gasket with a small amount of RTV along the front and back edges of the block. Your oil looks good, no coolant in it?
It sounds like you ruled out compression in the air/fuel/spark equation if you're at 185-psi on all cylinders. If you had a low cylinder, adding oil through the spark plug hole would tell you if you had a valve issue...the compression would not improve on successive strokes.
I'd start with a spark tester and fuel pressure test.
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Last edited:
redfishsc
Tired of fixing lousy engineering.
It sounds to me like something electrical is not happy. I just corrected a strange "shaky idle" and a #5 misfire under acceleration, due to a bad (6 month old) distributor. Was even one with brass posts instead of aluminum.
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