My GTA has been sitting in storage for about 15 yrs. Recently replaced the fuel pump\filter and injectors. The car finally runs, but the idle is all over the place.

With the original MAF, there's a high idle around 1200-1500 RPMs and a VSS code, and the idle drops every few seconds and then jumps back up. What's weird is the MAF values sit around 6grams (at 1400 RPMs), which seems normal (maybe a tad low). With the MAF unplugged, the car runs perfect (default\failsafe MAF value is exactly 4grams at idle ~700RPMs.)

I bought an el cheapo MAF on Amazon, and the car runs much better, however it runs VERY rich (enough to foul the plugs). With the Amazon MAF, values are about 12grams (seems a little high) at idle (700 RPMs), but the idle is more stable (though it still searches...) But the VSS code is gone.

I put the original MAF back in and took a closer look at the scan data, and what I noticed is that the injector base pulse width keeps dropping to 0, which is causing the RPMs to drop and jump back up... When the pulse width drops to 0, the IAC opens to compensate and then closes...

What would be causing the injector base pulse width to drop to 0 at idle? I thought that really only happened during deceleration?

I attached a screenshot of the scan data.

You'll see (in green) the pulse width dropping to 0, right before the IAC (orange) jumps up. Whereas the MAF values (red) stay more or less constant.

Any thoughts on what else to check or what might be going on?

 

Is this in closed loop? If so, what is the CTS at scan time? Stock PROM?

 

Vader, happy to see that you're still in the community! I used to frequent these boards several times a day back in the early 2000s before I had to put my GTA in storage for 15 yrs. You name brings back some good memories.

It does do this in closed loop as well. CTS is 188* according to the data logging. It is NOT a stock PROM, I don't recall the details of what was changed, I had a fellow TX member burn me a custom chip back in the day. I've got a programmer coming hopefully today so I can reset it back to stock and go from there.

One thing I'm wondering... with a MAF reading at 6grams at 1300 RPMS... I'm wondering if such a low MAF reading is what's causing the idle speed to be so high? IAC is at 85. (going to unplug the IAC later and see what the MAF reading is at lower RPM. I'm hoping MAF reading so low is what's causing all these issues. I've got a new MAF coming at the end of the week that's actually decent quality and not an amazon special.)

 

You may be onto a root cause. If the MAF data is below threshold, it may be confusing/scaling the target idle RPM. As that calculates into the IAC steps the commands out could get FUBARed a little bit.