Misfire issues...

[draggin95niss]

Ive recently purchased a 98 excab c1500 w/5.7 it taps upon startup and goes away in roughly 5-10 minutes, its also thrown the p0300 & p0302, so ive checked spark and its there, so i figured fuel so i replaced the whole spider to no evail. The previous owner told me he had to change the intake gasker cause of a coolant leak and claims thats when it developed the misfire, it has new plugs, wires, cap & rotor aswell as a new fuel pump...i pulled the #2 plug and its got a red tinge on the ceramic around the electrode...Ive checked compression and its good, ive also held a towel at the exhaust to see if it would suck in and nuthin....im at my witts end with this thing...can anyone iffer any other things to test???

 

[draggin95niss]

Just an update from the drive to work today...now after being stopped at a light i go to take off and it falls flat on its face then takes off...yesterday after the battery being unplugged the codes are cleared and not comin back...i figured since theres still a problem then they would pop back on before i made it to work but not yet...

 

[SAATR]

Did you replace the cap and rotor yourself, or did the PO? Crappy caps, even new, can have cross firing between conductors in the crab style caps. You can't see it, and it doesn't show up on a multimeter because it's an arc and not a direct short. I would replace the cap and rotor with a QUALITY new unit before going further. I chased the same issue with my Suburban trying to find a problem in the fuel system that wasn't really there. It was my year old distributor cap...

Sent from my XT1254 using Tapatalk

 

[draggin95niss]

The PO changed it...said the miss developed after an intake gasket change...but ill swap them and see...

 

[draggin95niss]

Ok the #2 plug is clean and the ceramic has a red tint around the electrode, i pulled others and they all look black and normal, so im thinkin im not gettin enuff fuel or any at all...but the spider is new so what could i check for the fuel now? Also i put a spark tester on the #2 wire at the plug and its got fire...

 

[SAATR]

You really need a scan tool that shows live data and misfire counters to diagnose this properly. Your codes show a cylinder two misfire AND a random, multiple misfire which means it is counting misfires on more than one cylinder. Without the scan tool data that allows you to see that, it's all just guessing. Personally, I would lay money on it being a crossfire issue ( meaning that spark meant for one cylinder is reaching another) caused by a faulty cap. You will still get spark on those cylinders, not always at the right time. A spark checker won't allow you to see that. Replace the cap and rotor with good quality parts ACDelco, Delphi, Standard Motor Products, etc and see if it helps. Otherwise you're pulling the lower manifold to see if there's a small vacuum leak at the number 2 intake port. You can have vacuum leaks on EFI and not throw a lean code, if the ECM can add enough fuel trim to compensate.

Sent from my XT1254 using Tapatalk

 

[draggin95niss]

I got a buddy with a matco scanner to read the live data yesterday, its getting fuel and spark, he disabled the #2 injector and it ran worse...hes thinkin its possibly in the valvetrain...but if that were the case wouldnt that have affected the compression reading? It had just under 150 on the #2...also the random misfire code is gone and hasnt came back...i read sumwhere there was a pcm ground on the back of the motor but all i see is the braided strap goin to the frame....the tap is on what seems to be the #1 cylinder...for the life of me i cant understand that if its tapping and they changed the intake gaskets why not fix the tap while ur in there...

 

[SAATR]

A tap that goes away is generally piston slap or a lifter that is slow to prime up. Neither should cause a misfire. Did you look at the misfire counters?

Sent from my XT1254 using Tapatalk

 

[draggin95niss]

The tap is on the opposite side of the motor...the otger cylinders were consistant zero while the #2 fluctuated but zeroed out at elevated rpm's....i also did a cap & rotor with no success...

 

[SAATR]

Did you happen to look at the fuel trims? They're divided into Bank 1 and 2, long term and short term, usually abbreviated something like B1STFT and B1LTFT. If you have a large positive trim percentage on Bank 2, which is where cylinder 2 is located, you may well have a vacuum leak at the manifold on that bank. You have essentially eliminated the other variables, so it stands to reason that if the fuel system, ignition, and compression are sound that you have an issue in the intake or head. And since the PO said it started when the manifold gaskets were done, it's possible that they botched the job.

Sent from my XT1254 using Tapatalk