How are you adding fuel as a diagnostic?

 

Simply down the intake. On it’s own, the exhaust is very smoky and reeks of running rich. Spraying ether or brake cleaner into the intake as additional fuel makes the engine want to stall. Spraying it on every possible vacuum line/connect gives no changes .

 

What are the front O2 readings / fuel trims? Generally air leaks post-MAF cause a lean condition (as the pcm hasn't measured that air coming in, and hasn't added fuel to match it). Certainly the pcm will adjust via trims over time, but it shouldn't go rich as its just adding fuel until the fuel trims drag the AFR back to 14.7.

Id also do a deeper dive on the IAC. I can't think of anything else that would gradually change with time causing rpm to ramp up if youre not touching anything. Perhaps measure the signal coming from the pcm to see if the pcm is opening it for some reason (read it when the idle is lower, then monitor until idle goes way up).

Id also probably try unplugging the MAF to force it into running off the MAP sensor, then take off the intake tube and try blocking off the intake inlet slowly. If you get to the point where its 100% sealed off, but is still running- you know its sucking air in elsewhere downstream (but should now be at a much higher vacuum level, so you might be able to find it with smoke).

Does anything look odd with the sensor readings between when its at low and high idle?

 

I can get some readings O2 and trims soon.

As for the IAC I have tried two different new units. It’s been awhile since I’ve read anything about them but I feel I’ve heard here and there that they are adjustable. I never had any luck trying to adjust it and I don’t recall where I read that. Will check the signal to it.

Unplugging the MAF instantly causes the engine to die. Not sure what to do about that. In the past I did pinch off the intake tube which did slow the engine down. However, no MAF at all always seemed to shut it off.

I’ll see about getting a live data video and posting it where I can link here.

 

I do not think the IAC is adjustable, I could be wrong. Ive disassembled them a few times and they always readjust themselves after plugging them back in. Should be able to read info to see if its in the correct position.
Another thing, ive been thru TOO MANY MAFs, generic and name brand (ACDelco)... Every time ive unplugged them and the car would die, it was a bad MAF combined with a different bad sensor. Twice it was a combo of bad MAP and MAF.

 

May be a dead end, but it would be worth a look to see if anything gets screwy from low idle to high idle. Maybe that coincides with going to closed loop, etc -just hoping there may be a clue in the data.

My thought here was making sure this is something the pcm is trying to do (raise idle). If the pcm really is commanding more idle, then we'll need to focus on investigating why.

What about starting the car with the MAF unplugged? Will it start? I'm not sure if the engine is able to switch between fueling off the two sensors on the fly - so killing the engine while unplugging MAF may be normal, I've just never tried it.

I'm not thinking the MAF is bad, but I'd like to see what blocking off the intake does (say with a piece of cardboard or something). If its still running at all, then its clearly pulling air from elsewhere and just making up for it in the trims.

 

Got it to run without the intake tube on it so the maf was plugged in but not reading anything. However it seems the IAC goes to its normal position when the key is turned on and for a moment after cranking up, then I can hear it opening way up? We let the IAC go to its normal position with the key on, unplugged it, then started the car and it ran much better than before. What would be telling it to open up so much? It’s a good unit as I remember trying it on another vehicle.

 

That's a good sign as at least you've narrowed it to a single cause (IAC being told to open too far). Thatll drastically reduce the amount of diagnostic work left.

As for why, I'd think it would have to boil down to one of 3 things:
-issue with the IAC wiring (signal pcm is sending is getting altered)
-bad pcm (pcm sending wrong signal to start)
-pcm receiving bad input (perhaps engine speed sensor is not giving out correct info making the engine think its going to die, so it spikes the IAC)

 

Okay, is there a specific signal the IAC should receive? I suppose I can check it at the PCM and at the valve itself. I’ll check the engine speed sensor as well, though it does seem to be reading the correct RPM according to the live data and tach.

 

Does your scan tool have an option for reading IAC counts? If I remember right, this represents what the pcm is trying to output to the IAC. I believe the range is 0 (closed) to 255 (full open, although on many TBs it fully uncovers the entire passage well before that).

If so, it would be useful to monitor that from startup and see if the increased idle corresponds to a spike in counts. It should start fairly high but slowly begin closing. If it spikes up when your idle jumps, then you've eliminated one of the three possibilities as you will know its a pcm commanded spike.