jsjs45

hello, I am at a loss and hope a smart feller can give me some insight. Sorry for the long explanation but I feel its necessary.
2000 Jeep Grand Cherokee, v8 4.7L - 120k miles - Never have I had issues with my Jeep running badly in the 15 years that I have owned it.

I was told by an auto shop that I needed new coils and plugs due to misfire when my Jeep all of a sudden started running very rough and check engine like came on. They quoted me a ridiculous price so I called around and had another shop do the work for me. They replaced the parts as I requested, they test drove and it ran normal according to them and I picked it up and it did run perfectly, I drove about 5 miles to the store and when I got back in it to drive home, I started it and it started running rough again and light came back on. I took it back to him so he could diagnose. He pulled codes p0308 / p0208 cylinder 8. He checked all the coils and they were all firing. He also tested fuel injectors.

he then noticed a ground strap at passenger side rear of engine was not on one side so he secured it and test drove for about and hour / 37 miles and jeep ran great and no check engine light. He went back to shop and started it up again for another drive and the p0208 code reappeared and light came back on. Did more research and thought it was the throttle position sensor causing wrong codes, he replaced, test drove numerous times and jeep ran great according to him. When I picked it up, the same exact thing happened to me, drove to store, ran fine, got back in and same issue reappeared.

Next, he ran compression test. #8 was 75 psi, #2 was at 175. He added trans fluid and the pressure went up to 230. "did leak down test and 20% leak down #8 but could be due to oil that was added for wet compression test." Lastly, I asked him to remove valve cover to see if he could see anything and no, so asked him to put cover back on with new seal, new oil, filter and not open the motor due to lack of money and lack of belief that the engine could be bad or need new seating, or what ever due to the random nature of the way it ran after the work was done each time.

Now after some time, and not feeling confidence in this mechanic, I had another mechanic diagnose and he claimed #8 and #2 injectors where bad, he said he could not hear fuel going through them. I replaced all fuel injectors and there was no change, still running badly.

Here I am stuck and no money left. Been searching and searching.
My main question is, if there is low compression in one cylinder, can the Jeep suddenly run great after some work was done, then suddenly run bad again? Seems strange to me; if there is an engine issue, wouldn't it just run badly consistently?

so here are my ideas based on things I have read, any feedback would be GREATLY APPRECIATED:
1. I read about cleaning all your grounds because if they get corrosion, all kinds of strange things can happen, plus I think my battery drains. I don't drive it everyday and it sits for a week sometimes. I have not yet done this. I have also lived near the beach a lot of years so I would not be surprised if many components are corroded.
2. Computer, perhaps dealership needs to re-program it or its bad? Maybe its not recognizing the new parts? or its still re-learning?
3. I read something about having to drive it for like 50 miles to retrain the computer?? I am afraid to cause more damage, like to the cat conv.
4. I have put SeaFoam in my tank and filled up with non-ethonal gas, but have only driven it for about 30 miles.
5. I bought some CRC electrical cleaner and sprayed/cleaned all the plugs to the coils.
6. about 100 miles prior to this issue, I had the radiator replaced, it had low leak but I am sure the engine never over heated. I don't drive much and I keep the fluid filled. Could there be an issue with the way it was installed?
7. should I try adding can of engine repair lube to the oil and see if that helps the low compression or is that bad?


Thank you for reading!

dave1123

For what it's worth, I'm going to throw this scenario at you. If the rings are worn enough, blowby compression leakage can cause oil vapor to pass thru the PCV/CCV system and get ingested by the engine's intake system, building carbon on the back of the intake valves, pistons and rings. Enough carbon on the valves and they don't seal right, the plugs can foul, and the rings can stick in the pistons. Eventually this carbon can plug the converters.

Seafoam is good for this but not really great. My 4.0 has been using a lot of oil lately because it's got 235K miles on it. I used a Mr. Gasket product called Cataclean. You put it in the fuel tank when it's about 1/4 full, drive it about 20 minutes, then fill it up. I noticed a marked improvement in idle and throttle response in that 20 minutes! After burning 1/2 tank of the diluted mixture it was like a different engine entirely! Smoother acceleration, snappier throttle response, and quiet idle. If you feel it's worth $29.99 to try it, you can buy it on Amazon. You'll have to burn the Seafoam first. Take it on a road trip not on the highway.

jsjs45

THANK YOU for your input Dave, its worth a try I guess but will running it while the gas is not correctly combusting in the engine cause more troubles? like the cat. conver? my Jeep never lost or burned oil and only has 120k miles on it so I am not sure that my issue is the same. I am at a loss at how an engine can very suddenly (literally very suddenly) start running bad, change plugs and coils, run great for test drive, then suddenly run the same (bad) as before, get a ground wire fixed, run great for 40 miles and numerous test drives, then suddenly run bad again next time driven, replace throttle position sensor, run great again, then start up Jeep up again and run badly again, exactly as before. It just seems like wiring, computer, corroded/loose ground wire, or ummmm who knows? I read in my Jeep repair book that only the dealership can properly reset the computer. Maybe that is my next step. I am just not knowledgable in any of this stuff so I was hoping to get some experienced Jeep owners input the code its showing now is P0208.