Since my rebuild, my tuner has been sending back my logs telling me there is a vacuum leak. I used carb spray and monitored fuel trims, nothing. I then built a smoker using mineral oil and resistance wire and did find a tiny leak around a cracked MAF mounting bolt. However, it did not fix the fuel trims. At idle, the long term trims are steady at about 8-10% (both banks match within 1%). Under high throttle, they go down quite a bit, but if I release the throttle they snap up to 25% and stay there. I'm running out of things to check, so reaching out to the masses here for help.

The PCM is also pulling timing 8° due to knock.

Mods that could be relevant:
- Catch can with 2 vacuum sources (intake and just before the throttle body) with check valves
- Have a "clean air separator" oil cap which feeds MAF metered air into the crankcase via the oil cap
- The catch can was installed pre-rebuild and never had a problem before
- Heads were milled .035", intake seems to seal just fine, no indication of leak at idle


I'll post the EFIlive graph & attach the log file.

 

The idea of sending MAF metered air into the crankcase has got to be a typing error. I can't possibly imagine any other explanation. Now that that nonsense is out of the way, plug the vacuum lines going to your catch cans. This will tell you if the check valves are leaking air. Since the bulk of your problem is at idle/low load, this can be checked without causing any problems.

 

The factory system sends MAF metered air to the crankcase does it not? It pulls from the throttle bottle for a vent and the PCV system sucks it into the intake. If the crankcase was open to amotsphere wouldnt it throw the fuel trims off? Since unmetered air is now being fed into the intake? I don't see how that is that absurd of a statement.

I do appreciate the advice on plugging the checkvalves, plan to isolate the catch can completely tomorrow to check it.