Weird MAF issue
 

My 99 Jimmy was running rough and I tracked it down to a bad MAF, I replaced the MAF and now is still runs rough with the MAF plugged in but fine with it unplugged, if idling a bit high.
Any ideas out there?

Any help would be appreciated.

What were the symptoms/codes/measurements that lead you to indict the old MAF?


George

Unplugging the MAF causes the engine computer to change how it computes fuel delivery so if the engine runs better it can be the MAF, its connector or its wiring but it can be other things as well. Since the engine is idling high your problem can be an unmetered air leak downstream of the MAF in the intake path or sau a vacuum leak. There can be other sensors that are having problems as well which are involved in calculating fuel delivery. Do you have a scanner with live data capability to see sensor values and fuel trims? An example is that high positive trims at idle (lean condition) that get better at 2500 rpm is an indication of a vacuum leak. You can also look at other sensor values to make sure that they make sense: MAP, TPS, O2, ECT, fuel delivery, ...


George

Yes, I do have a scanner with live data, scope all that.
I will run some tests on MAP, TPS, o2 upstream/downstream, etc. Not sure what values are normal, so I'll record and post

Thank you in advance for the help

P.S.
Brand new ACDelco MAF, it came with a different connector and an adapter to the original harness.

Why did you condemn the first MAF, what were the symptoms/readings?

Start by plugging the MAF back in and getting all 4 fuel trim numbers at idle and again at 2500 rpms. That STFT1, LTFT1, STFT2, LTFT2 being mindful of sign (+/-) . Then unplug the MAF and do them again.

What are the MAF values at those 2 rpms?

On a cold engine do the MAP and ECT values agree roughly with the ambient conditions?

What do your 2 upstream O2 sensors look like on a live graph with an idling warm engine, MAF plugged in?

That will get you started, we will discuss after you get the numbers.

George