I have been fighting a P305 engine code (cylinder 5 misfire) for months now. I have new plugs and new coils that were done for free. I was still having the problem, so I took it in and they said my valve o-ring was cracked and letting too much air in so that was replaced. Later I found that my PCM was failing and that was replaced. The jeep ran great for about 2 weeks.

None of the mechanics did an engine compression test, so I decided to do one myself. Cylinder 5 did great. For various reasons, I ended up doing the test cold (I had the engine warmed up, but I then had to run to the store to get a different plug socket because mine didn't fit. After I got everything taken apart, the gauge I had didn't fit so I had to run back to the store again. By the time I did the test the engine wasn't very warm).

Here were my results: (I recorded the first time it turned over and then the max compression)
Cylinder 1: 90/120
Cylinder 2: 90/115
Cylinder 3: 70/105
Cylinder 4: 85/125
Cylinder 5: 95/122
Cylinder 6: 85/120

Cylinder 5 did great, but I didn't like Cylinder 3. Maybe I have a cracked 0331 head, ugh. I'm not looking forward to that.

I put everything back together and drove to work (less than 3 miles). Usually my engine gets to about 180 on the way to work. Today, it got to 222. I have a new thermostat as of 3 months ago, the hoses coming out of the engine were hot so I assume coolant is flowing. I also have enough coolant in the radiator.

The main question is if any of you think I could have done something to the coolant system when I was doing my compression test? A bunch of the hoses run right over the coil pack. I took a look at the hoses, but they seem fine and nothing is kinked...

 

Don't think pressure from one stroke is very valid.

Has it got hot/overheated again? Or just the one time?

 

The second number is after multiple stokes which is the typical number on a compression test.

It hasn't overheated again and I have driven it a few times around town. My temp stabalized around 195 like normal on a short drive. I'll watch the temp, and if this keeps happening I will probably change the water pump. I bought the jeep about 8 months ago, and I have the feeling the previous owner didn't do much with the coolant system, it looked like the coolant system had never been flushed when I cleaned it out.

 

"I will probably change the water pump".........r u suspecting the water pump 1st?..........why?

 

Not sure if it is the water pump, but on that drive it overheaded at low speeds, the temp climbed much more rapidly compared to normal, and there is a tiny leak at the water pump. I don't know a ton about coolant systems, but to my limited knowledge I would consider the water pump as an issue. But it is working great now so I am going to watch and see what happens.