Here's what I've got. My daughter found a decent deal on on 01 300M which looked gorgeous, but had some obvious mechanical defects. Me, having some experience with Chysler engines said go ahead and buy it - we can fix it. So she did and I immediately took it to the local dealership to have them check it out. Well the news wasn't good. The bearings were shot and on their way out.

Decided to rebuild the engine rather than purchase a new one and have been questioning that decision since I made it. The overhead cam 3.5l engine is unlike anything I had ever worked on before. I did most of the upper end work myself and I had the local machine shop take care of the lower end, including line boring, grinding the crank, boring the cylinders, new piston heads and rings and of course, new bearings.

Anyway, got the car put back together last October and aside from not being able to get the thermostat in place without it overheating, she was running beatifully. About two months ago, my daughter calls me and says she started it up and it backfired and was now idling in the 3500 rpm range. Told her to shut it down and I would get it. I did, brought it back and tinkered with it. After a few unnecessary part replacements, I found the intake manifold gasket was slightly out of line with the lower intake. Changed the gasket, put it back together and she ran fine again.

Thursday evening, it did it again. So, my question is does anyone have any ideas on what might be causing this? I can't figure out why the engine will backfire so infrequently, and in doing so take out the intake manifold gasket each time.

Any help on this would be greatly appreciated!

 

Take off the intake and check the head and intake surfaces with a straight edge. There has to be a low spot somewhere OR your torque is incorrect. The backfiring is a RESULT of the intake gasket sliding not the CAUSE (You have intake gases being sucked into the wrong cylinder at the wrong time.) What is happening to you is an issue that happens anyway when they get alot of miles on them. It has made a fool of many a chain store mechanics..lol...and some Chrysler techs. Most of the time it's not an easy find like yours with an rpm issue, which would lead one to check for a vacuum leak. It usually shows as a misfire and is much harder to find. Just another instance when it pays to take your car to a place where people work on them every day almost.

 

TNtech - I read you reply late Sunday, actually early Monday morning, and I think you are right on with the manifold surface being off a little bit. The mounting bolts are all torqued to spec, but the two times this happened, the gasket lost it's seal in about the exact same point near the 5 or 6 intake port. At about 205K, the car does have a lot of miles on it. I haven't done it yet, but I will separate the mainfolds and check the surfaces more thoroughly.

Thanks for the info...I think you saved me a ton of time and head scratching.

By the way, if the surface is out of level do you know of any alternatives other than replacement or machine shop resurfacing? I'm thinking I may need to replace the lower intake anyway because the bleed screw threading is about shot (which is why I didn't get to read your message until so late, but that is a whole nuther story). Thanks again!

 

Na, just replace it. I would get one from a salvage yard and be sure to straight edge it before you install it.

 

Na, just replace it. I would get one from a salvage yard and be sure to straight edge it before you install it. Junkyard dog is where I go online but you may have another source.