So my mom has a 2007 H3 with 200k miles on it. She's been plagued with a P0300 engine misfire detected for months. I replaced all the coils and plugs with new ACDelco ones but that didn't help. It's most cylinder 2 that is misfiring but all of them do it but not as much, I took it to a shop and they told me the cause was low compression. They said cylinder 2 was the worst. I did a compression test and got the following readings: C1-150, C2-120, C3-130, C4-150 and C5-150. Does anyone know if those are normal readings. If those readings are too low would that cause the misfire and if so does that mean the head needs to be rebuilt?

 

I wouldn't necessarily say those are low compression readings but it should be a bit closer in reality.
I've seen Jeep Cherokee (XJ's) with the inline 6 run with 20Lbs. or less on each piston lol!
H@ll its not uncommon for them to get down to single digits.
I'd be more incline to think that starting with a MAF, TB cleaning as well as to check all wiring going to and from the injectors, plus replacing the injectors. Like wise Id make certain to check both my cam and crank readings with a scan tool or oscilloscope.
Battery and battery terminals. Have an analysis of the exhaust gases as well as test the radiator fluid and oil for leakage, contamination.
If all other things that can be tested are exhausted then and only then would it be a head related issue. It could be a faulty injector or incorrect signal from the computer due to a failing component/s.

 

Sticking a compression tester in an open spark plug hole and bumping the starter is only a tiny bit of a compression test. For it to be diagnostic, takes 3-5 compression strokes and watching for change. You use the CT to determine if valves or rings is the culprit. With 200K miles on an I5 I would be looking for about 150-160 as close to normal, and your #2 & 3 are not that bad. I've seen worse on newer lower mile motors.

I think crud on your valve seating surfaces may be a possible issue. Give it a fuel injection an induction cleaning, see if that helps.

Check it out in the link. H3 L5-3.7L (2007) Compression Check

 

PIP5029F

 

Actually today I just bumped on that
I don't know if it helps. Because I have same problem, but it not persistent

 

Did you ever figure out what the problem was I'm having same problem. Rpms dropping at idle only.

 

Yes, we told you, re-read this OLD Thread and you will see... Give it a fuel injection an induction cleaning.

There is already another thread at the front of the board with a similar discussion. LOOK: 2006 H3 3.5 Cylinder #2 65% Leak

Avoid digging up Old Threads to tack on a question, just ask your question in a new thread without the Necro-Bump. READ: Old Threads/'Necro-Bumping'