Hello all - I've got a 1988 GTA, 305 TPI, automatic, Borg Warner rear. Fresh block and heads, just replaced. Since the replacement, the car is rough at low rpm. Will idle in park at about 1000 RPM, in drive 500 RPM. Park is not terrible - car is noticeably rough, but bearable. In drive at 500 RPM, car will rock back and forth and sounds rough; RPM gauge jumping up and down one bar (digital dash, so maybe 100 RPM). Car is also rough when cruising at low RPMs - letting off the gas at say 40 MPH, which is just over 1000 RPM, and the car has a noticeable jerk to it. When I'm on the gas, it seems normal, but off the gas it's not quite right.

I've done the IAC/Minimum Air/TPS adjustment - the odd thing is, that it worked. Adjusted the screw, adjusted the TPS to .540, turned the car on - started rough, but smoothed out, and was normal. Had a slight jerk at speed, but bearable (car has a hotter cam in it with the new block). Did not bog down at low RPMs, and never sounded like it was going to stall. Also, ran at the same RPMs - 500 in drive, 1000 in park, no issues. Drove the car for about 60 miles, all good.

After letting the car sit overnight, turned it on today, it was back to the way it was before the adjustment - incredibly rough at low speeds. What might cause the adjustment to "reset" itself? Timing is at TDC; engine is in extremely bad shape at any other setting. The issue is both with cold engine and after the car has had a chance to warm up. The car has also been heavily tinkered with prior to my owning it - AC/Heater Delete, Heddman Headers and a straight exhaust, no cat, with an o2 sensor welded in just past the headers. Prior to the block replacement, though, the car ran normally across the board. Any help would be greatly appreciated!

 

Update: So I figured out why the adjustment wouldn't stick. Apparently on my car, the correct two pins to jump are 5 and 6, not 6 and 12. After adjusting the minimum air, I was able to get it to run smooth at idle, however it was a bit on the fast side (1100-1200 RPM park, 650-700 drive) and would accelerate up to about 35 without touching the gas. Also, when off of the throttle at 40+MPH, the car would still jerk and shake.

I noticed when adjusting the idle on the car, with the IAC and EST disconnected, it did not have any of these problems. With the IAC and EST disconnected, the car works perfectly normal: solid idle, doesn't sound like it's going to stall when shifting from park, and the jerking is no longer present at speed. The problem is, I'm not too sure what that information indicates. Something wrong with the Distributor/Coil/ECM? Is there any harm in driving the car with the EST disconnected until I narrow down the problem?

Also: the only code I've gotten off the car has been Code 36 which is intermittent - MAF is known to be working, so I'm figuring that will just be a new relay; I don't believe that code could cause my problems, but I may be mistaken. Again, thank you all in advance for any help/suggestions!