I have a 92 z here that is 100% bone stock. Car came in with vacuum leaks, missing egr solenoid with manifold vacuum ran straight to the EGR valve, base timing incorrect. I am sure there are other things but it's been here a while.

The problem has been: with minimum air rate set correctly, confirmed proper IAC operation, confirmed TPS values the car no matter what anywhere below 700RPM would die like you literally turned off the ignition switch. I have high speed datalogging equipment for OBD1 and all that and still couldn't put my finger on what it was actually doing. Even went to the point of having the car on the dyno with dual trace RPM setup matching what the ECM saw, the pickup coil transmitted rpm and what the optical RPM sensor on the dyno (part of the dyno utilities) was receiving including oscilloscope graphs of inputs and outputs of the ignition module. All this to no avail, all the data was clean and nothing didn't matchup as the RPM decreased when the engine would die.

After much testing and trial and error what I have discovered is the throttle body shaft bore is worn out. I can have the car idling at 750 RPM locked out (AKA IAC disconnected). When I drop it in gear the throttle shaft (very visibly) moves forward and kills the engine. In other words at 750 RPM the engine's vacuum will hold the blades back and against the throttle adjustment screw, when the vacuum drops the return spring rotates the throttle shut when the blades move away from the plenum. What is so messed up is I went back and reviewed high speed datalogs and increased the precision on the TPS voltage and I can see the TPS voltage drop when this occurs but it is in the .001 and occasionally in the .01 ranges when most diagnostic tools only read in .1 (tenths of a volt.

I know there has to be tons of people dealing with this same problem so I just had to share.

 

So what did you do to fix it? Replace throttle body?

 

Yes ultimately. I did tune around the issue and got it stop dying like it was but it was a total hack by removing most of the IAC response delay and commanding the IAC to drastically over respond compared to what should be necessary. I mainly just did that to prove to myself I could though.

 

Glad you were able to fix it. I like keeping track of issue so down the road if I ever run into any problems that resemble what I have read it pops back in my brain.