I picked up an 86 tpi 5.0 TA. It’s actually the first GM I’ve ever owned, so I’m not too familiar with them. It has a hypertech chip and egr is disconnected. Light wasn’t on when I got it and it runs good except for a fluctuating idle I’ve been trying to cure. Trying to lower the idle, I found where the PO had cut the ground wire to the MAF so that the idle would stabilize. Now bounces around between 700 and 1200 rpm. Sometimes it stalls or cranks and dies immediately. Mostly just idles like trash.

the wiring is a mess and I had a parasitic draw that I was searching for so I disconnected a lot of sensors and fuses. I also smoke tested the intake. After that the light stays on, but I only get a code 12 when jumping the ALDL.

anybody know what conditions would turn on the light, but not throw codes? I’ve gone over all the connectors and I think I’m good there.

 

It should be throwing a code if the SES light is on. You should be able to capture it with an ALDU1 or scanner from 1320electronics hooked up to a phone or laptop. The only system you can disconnect without affecting drivability (significantly) is the AIR system, so you'll either need to reconnect the EGR or get a tune that never activates it. If all you did was mess with the wires, and it's now running differently, you either uncovered an existing issue that wasn't fixed correctly or you missed something.

 

Sorry, smog is deleted. EGR is still intact.

It’s not running any different than before I started messing around with it. The idle started being erratic when I put the ground back on the maf relay a few weeks ago. The service light is new and probably is related to something I did searching for the wiring problem.

Are there any systems on these cars that turn on the light but don’t always throw a code?

 

At the ecm ground a9 white/blk wire . If the mil doesn’t flash I’d say it’s bad.
the lack of communication from the ecm is a sign of a badecm. Can take a screw driver handle and tap on ecm case while running and see if running changes this flexes the circuit board another test to see if ecm is bad.