So I recently picked up a 1990 s5 gxl auto. It will idle just fine around 750 in park and with the ac off. The min you shift into any gear other then N it will drop and stumble then die, same thing happens when you turn the A/C on. However if you give it some gas and hold an idle with your foot it works. Any ideas what it could be?

 

I went through and check the BAC. the solenoid clicks when it gets power, I cleaned it out, and I checked its resistance it was just barely over spec. I did notice when I was cleaning it that there was a bit of the rubber lip on the top of the plunger that was tore off. Would that cause anything? Also I went through and tried to reset the TPS sensor, I got the low resistance at 1000 ohms. I checked the resistance of the high as per the factory service manual and was well over almost three times. Would that cause my idle to die when a mechanical load is put on the engine?

 

did you check for vacuum leaks? if it's mostly original, gaskets could have failed.

 

it sat for 5 year so that is a good possibility.

 

If your tps is out that would be my first priority, that may not be your problem here but is defiantly a problem that needs to be fixed before you do anything else.