Sounds like a Diesel

First off thanks to Billy Baker and Mdub in the Arizona forum for giving me some ideas to look for.

At first it sounded like a rod knock but I was not convinced so I set off to eliminate everything else. First thing I tried was changing the oil and adding some Slick 50 and that did nothing for the noise. Second I made sure the torque converter bolts and exhaust was tight and they were so that did nothing. Then the last thing I tried was to decarbon the combustion chamber and whala!!! Sounds like a Jeep now instead of a Cummins. I have to admit I liked the sound but knew something was wrong.

All I did was take a bottle of Sea Foam from Auto Zone and take off a vacuum line close to the throttle body and added 3 feet of rubber line. Then with the engine at normal temp and running I allowed it to suck in the Sea Foam into the engine through the intake manifold. I had to slightly pinch off the rubber line so it would only suck a little in and keep running. Then I allowed the Jeep to sit with the engine off for 20 minutes then started it up and run it around the block at high RPM until the smoke quit coming out of the exhaust. When I pulled it into the driveway the knock was gone. I repeated the process one more time to make sure it was nice and clean.

So the bottom line was that there was so much carbon build up inside the combustion chamber that it was causing it to knock at idle and just above idle. This was a known issue on the old 258 motors in the CJ's and early Wranglers but I have not run into it on the 4.0.

Anyhow I hope this helps someone else out because I almost put a new engine in this and for 10 dollars I saved 1000.

Good info, thanks.

Oh realllllly...I would love my 93 to get a little quieter. I hate that noise! Thanks for the tip!

Has anyone tried just using a little water in the same fashion? I believe water will clean up some carbon as well. I've never tried it, just heard about it in an engine class I took quite a few years ago.

hmmm... definitely useful info! My 91 has developed a fairly loud knock at idle recently, I figured it was either piston slap or my bearings going south and was already thinking over the logistics of getting my spare engine out of storage without a working truck to carry it with.

Water Injection: http://www.rallycars.com/Cars/WaterInjection.html

As alluded to in the article during WWII some aircraft engines had water injection. Normally, in fighter aircraft equipped with the water injection, the throttle had a "seal" on it that limited its travel. When in a desperate emergency situation they could jam the throttle past the seal and would gain substantial HP increase. This could only be sustained for a short duration without damage to the engine.

Interesting personal note: during the mid-80s we had some limited production "special" Volvos come through our dealership that had TWO injection systems--the CIS mechanical injection and the EFI. These were built and sold to qualify the model for "production" class racing. If my failing brain cells are still correct the CIS was fuel and the EFI for water.

I think I'll try some water first. Maybe even window washer fluid -- turbo guys run this stuff in their water/alcohol injection kits because it's basically pre-mixed water and alcohol. Should clean things up nicely. If that doesn't work, I'll pick up some SeaFoam or Mopar stuff.