I'm working on a 1999 S10 Blazer 4.3 vortec. It starts but runs wild at about 2500 rpm then will drop back to 4-500 and almost die then back up to 2500 up and down.
It doesn't have codes and I've changed throttle body and intake tube sensors and all with ones from another Blazer I have for parts. It carrys 60 psi or better fuel pressure and I dropped the exhaust incase the converter might of been plugged and changed out the pcm still up and down and if it dies it diesels back through the intake.
This Blazer has one of the blow me boxes for a drunk driver. Could it be a problem? last I knew they only shut down the engine. ANY ideas please and thank you! joaa

 

Can you try another pcm? What has changed? Was anything done on the engine prior to this?

 

I Have three blazers for parts and two daily runners but this is a customers truck and I've changed the pcm with 2 different ones already no change. It sometimes wouldn't start and I changed the fuel pump then 4 months later this starts