AW: [Chrysler300] 300G Missfire

Hello Richard,

just a few cents from across the atlantic.

Before getting into a lot of wild guessing, it would probably best to check if the spark plugs are bent again. To be honest it would be pretty irritating that the piston would hit a spark plug, but wont hit a valve.

There is possibly something in your cylinder 3+5 which causes the bent plugs tips. It might also cause the (exhaust) valve to stick or not close properly which than would explain the fumes near the intake.

As it appeas to two cylinders my guessing would be parts of the intake manifold gasket. When changing the spark plugs and they are bent again use a bore scope to see if there is something in the cylinder, or if the piston top has damage.

If the spark plugs are not bent, check for your petronix. In general i´m happy with them, but they can go bad too. Had problems with one after two years of flwless driving. Found out I was running a coil with wrong ohm resistance, killed the electronics. But it took a few thousand miles.

As said make an effort to check a few things and isolate the problem – than we might all get closer to the problem.

Good luck

Lars

PS: if you dont hear any knocks your rod bearings should be ok. If the compression is ok, the likelyhood of having a broken piston ring in your cylinder is very small. Also chances would even be reduced as it apears in two cylinders at same time.

Von: Chrysler300@xxxxxxxxxxxxxxx <Chrysler300@xxxxxxxxxxxxxxx> Im Auftrag von Richard Osborne reomotorsports1@xxxxxx [Chrysler300]
Gesendet: Montag, 30. März 2020 21:19
An: 300 Club Group <Chrysler300@xxxxxxxxxxxxxxx>
Betreff: [Chrysler300] 300G Missfire

Hello,

My G engine is mostly stock. I have a Pertronix ignition and a Mopar Performance Cam (Which I degreed to the ORIGINAL Ram engine specs, not the Mopar Perf specs) and lifters. Stock pushrods and rocker arms. The block was over-bored .040 therefore it had new pistons installed. This engine was built close to 20 years ago and I am guessing I have put around 20-30,000 miles on it since. Otherwise, engine is stock.

Last Fall, my son and I drove the car approx 120 miles to a car show. On the way home, it developed a miss, a cylinder or cylinders not firing. It seemed to get worse as we continued home and the freeway.

The car was parked and only taken out of the garage once or twice for very short drives since then. The miss seemed to be still there, but not as bad as it was on the freeway. A few weeks ago, we got it out to take my son to his Confirmation. On the way to church, the miss turn into a serious miss-fire. It was violent. It rattled EVERYTHING. clearly not a miss, but cylinders not firing when they should. I did not notice any backfiring thru exhaust or intakes.

When replacing the spark plugs. we found #3 and #5 electrodes squashed and therefore zero gap. There was no evidence of mechanical interference (broken valve,piston…). Also, I have always run the car hard enough that it is difficult for me to believe there is any carbon buildup which could’ve broken free and caused the spark plugs being compressed. We ran a compression check and found each of the cylinders very close and good (115-120). We also replaced the cap and rotor. Plug wires were not replaced as I don’t have an extra set and the wires on the car appear OK.

The car fired OK and seemed to run good, there was a very slight hint of a miss, but otherwise seemed OK. I let it idle and get up to temp. On the test drive, it started to miss-fire again, and again it was violent, massive vibrations throughout the car. We brought the car home and parked it. I noticed the front face of the intake manifold on the drives side (in front of the master cylinder) looks to have exhaust gas residue on it. I have yet to re-fire the car and feel for exhaust leaks, nor have we removed the spark plugs, but I am suspecting there will be issues with #3 and #5 agains as they are positioned close to the intake manifold where the exhaust gas residue appears. While the car is at idle, no exhaust leak can be heard.

I'm asking for suggestions in trying to diagnose what may be happening, prior to taking the intake and valve cover off? I’m more inclined to believe this is a mechanical problem vs an ignition issue. Could this be wiped lobe(s) on the cam? Could a lifter collapsed? Bent pushrod(s)? What else?

Thank you,

Richard Osborne
Columbus, OH

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Posted by: <lars.kirschke@xxxxxxxx>

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