Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Does your engine have AFM or DOD?
AFM engines are known for some valves not operating once the engine has oil pressure.
Are your sure it is cylinder 4 and 5?
AFM cylinders are 1,4,6,7.
Are the coils firing?
What do the spark plugs look like?
Ok, you have swapped the injectors, does it have injector pulse on those 2 cylinders?
Tested with a noid light?
Compression could still test ok, have you run the engine with valve covers off?
Could have bad cam, lifters or valve of its seat once the engine is running?
That's certainly a strange issue you have there.
Even though you have done a compression test, how about conducting a leakdown test on those two cylinders and a pair of good cylinders for comparison.
Sounds odd to pull hard on 8 cylinders, but lean misfire at idle.
Have you checked the inlet manifold bolt torque?
We have one in our range that produces P0300 misfires.
Early versions were unable to locate fault. It then became a new ECM program.
Does your misfire data count up on cylinders 4 & 5?
I've seen many engines with high carbon on the valves that cause rough idle, misfire codes etc.
I would conduct a leakdown test on those 2.
Then I would pull the intake manifold, and clean the back of the intake valves with an upper cylinder cleaner.
Subaru use this on their engines at every service.
I have used this product on many cars.