Misfire at idle

Question

I have a 97 Montero sport with 2.5k miles on the engine(it’s remanufactured). I’ve been chasing a p0305 for 2 years but can’t find the problem. I know I have a possible vacuum leak since the fuel trims go to zero while I’m driving.
I have changed spark plugs, wires, tried moving fuel injectors. I have sprayed the entire intake system with brake cleaner but haven’t found a leak. Would appreciate any help.
Fuel trims at idle:

LTFT bank 1:-8.6%
STFT bank 1:7.8%
LTFT bank 2:12.5%
STFT bank 2:-3.1%

Carguy · Answer

Assuming cylinder #5 is on bank 2 (which is typical), I'd check the compression of all cylinders and compare them with cylinder #5.  Bank 2 fuel trim is much more positive than bank 1 (LTFT), suggesting either a sizable vacuum leak or poor combustion.  If there were a vacuum leak, you would likely get a code indicating a lean bank.  In your case, the code points to just one cylinder.  You've already checked the injectors and ignition.
Given that the engine is rebuilt, with such low miles, I'd start with the compression test.  I purchased a rebuild myself, several decades ago, and it had 10-20 lbs. weak compression in two adjacent cylinders since the first day.  There were no OBD codes back then, so I just lived with the problem, drove the car 50k miles, then sold it when the compression suddenly dropped much further.

Misfire at idle

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