It has a good fuel pressure, it has no air leak, and also when you rev the truck it doesnt do the misfire, it runs good, and does the misfire only on low idle. The customer says that he bought the truck like that, and it looks like the previous owner change the pcm, bolt cat, spark plugs. There is no other codes but the p0300.
any advice will help thanks!
Do you have any misfire data or any other eng data ? Do you have any scope captures of ign or inj?
I would run a compression test first. If fuel trims are in range I would think there should be some misfires on the data pids.
When testing for a misfire, start with the easy stuff first. Scan tool data is priceless, especially the fuel trim & misfire data. Then move on to a relative compression test with your scope. Now you know exactly which cylinder and most likely the cause of the misfire.
Is it running in open or closed loop?
Looks like the truck is modified. I see shorty headers and possibly a mild cam?? Can you do a VE test to see if it has any issues there? What are MAP, MAF, an 02 sensor reading at idle. I want to see cam an crank signal waveforms also. Do you actually feel a misfire?
Can have a lifter roller with pitting. That can cause the lifter to bounce on the cam and vary cam timing and cause misfires. Need to do compression waveform on the suspected cylinder and look for variations from event to event. Or a cam going flat could cause a low rpm misfire as well. Good Luck!
In my experience misfire at idle only(sometimes accompanied by low idle) and no noticeable mis under a load(it may be missing,be it cant really be felt, like say a bad coil) is usually a valve train issue. The logic is simple enough,the airflow at idle is much slower than under a load and the combustion process is more affected by turbulence or disturbance to this flow. These problems typically…