My 1989 K1500 with stock 5.7 and a Getrag 5 speed suddenly will not idle. I installed a new GM IAC and checked the timing. It seems to be getting worse. It has no power and if you try to accelerate it will start missing. At times it is so bad that I have to downshift to 4th to maintain speed.
Any ideas?

 

Air and fuel filters?

Allan

 

I'd make sure your getting good spark...

 

Thanks Allan in NE and hicksvilleshick for your response.
The engine surges at idle and missies when under load, but will run perfectly without load as long as it is off idle.