Re: [Chrysler300] 300G Missfire

Thank you to everyone who has taken the time to respond. I apologize for the delay in my response, but here is some additional information in related to some of the inquiries:

I only use Champion plugs, RJ12YC for the 300G, this is the successor to the original plug, slightly hotter, but same reach.

First thing I asked my son when he told me the plug gap was closed was if he had dropped them when he put them in the car last year. He said no.

I don’t think any pistons hit the plug, The plugs showed no evidence of a hit. If things were that out of whack, all of the plugs would have been hit.

The compression is OK based upon the old starter, only removing 1 plug at a time to test, cooler weather and a mediocre battery.

I do not have another BB distributor.

The cam is a “Mopar Perf. Six Pack” cam, specs are similar to the original grind, not radical.

I have heard of high cylinder pressure squashing the plug, if this is the case, it will be the first time I have seen it. But again, the misfire is violent!

I had the machine shop assemble the bottom end, I assembled the rest. My biggest mistake on this was thinking that since they were famous for building alcohol Hemi’s for TA/AA and TA/FC and tractor pulling motors, that a street engine would be easy. I overlooked the fact that those other motors get pulled apart all the time and frequently. This was the one motor I built that I NEVER wanted to take apart again, looks like this will be the second time (having heads off)….

I am hoping to have my son take the plugs back out in the next few days and will advise. Good thing he likes old cars and is flexible. Doesn’t seem to bother him to have to take the front tires off, remove the ram panels just to remove the plugs…..

Thanks again for the feedback, I’ll advise when we dig into it again.

RIchard

On Mar 30, 2020, at 3:19 PM, Richard Osborne reomotorsports1@xxxxxx [Chrysler300] <Chrysler300-noreply@xxxxxxxxxxxxxxx> wrote:

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: Richard Osborne <reomotorsports1@xxxxxx>

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