Several months ago I picked up a 1994 GMC K2500 burb with the 7.4/4L80e. Only 97k miles on the clock, super faded and cracked dark blue metallic paint but a very nice tan SLE interior. Was used for the past 2 decades as a boat hauler so didn't see much winter salt either. The frame, underbody and brackets/lines all look fairly decent for 27+ years old.
It was idling odd and had a SES light showing the code 36. I found almost every vacuum line rotten and even managed to brake the nipple off of the unicorn also known as the idle speed control valve vacuum solenoid. After a short mess of hunting down a solenoid (AC Delco stopped making the 10233505 years ago and the Standard Motor Products AC437 will work, but you have to rip off the sponge and replace it with a cap) and replacing all the vacuum lines the SES light went off and it idled much smoother.
I've since replaced the distributor with a NOS AC Delco unit (resetting the base timing to 4* BTC), AC Delco plug wires and CR43TS plugs. I also replaced the MAP and TPS sensors along with the IAC valve (Also, all AC Delco).
I have a new Bosch O2 sensor sitting here beside me but I have yet to put it on as mine looks so rough I almost know it's not going to come out. I'm waiting until I'm ready to have a new exhaust system put on since I figure I'll be having a new bung welded in also.
Anyways, now that I've rambled on... I have an ALDL to USB cable in the mail, on it's way and I've downloaded the TunerPro software. Be ready to see me asking lots of questions as I learn to how datalog and diagnose problems with these old OBD1 systems. I still can't figure out why it wants to idle so high and overdrive isn't acting properly... but it's time we see real data before I throw more parts at it with nothing more than hope.
Thanks for all the info here, this place is truly a treasure trove for OBS knowledge.
Dave
It was idling odd and had a SES light showing the code 36. I found almost every vacuum line rotten and even managed to brake the nipple off of the unicorn also known as the idle speed control valve vacuum solenoid. After a short mess of hunting down a solenoid (AC Delco stopped making the 10233505 years ago and the Standard Motor Products AC437 will work, but you have to rip off the sponge and replace it with a cap) and replacing all the vacuum lines the SES light went off and it idled much smoother.
I've since replaced the distributor with a NOS AC Delco unit (resetting the base timing to 4* BTC), AC Delco plug wires and CR43TS plugs. I also replaced the MAP and TPS sensors along with the IAC valve (Also, all AC Delco).
I have a new Bosch O2 sensor sitting here beside me but I have yet to put it on as mine looks so rough I almost know it's not going to come out. I'm waiting until I'm ready to have a new exhaust system put on since I figure I'll be having a new bung welded in also.
Anyways, now that I've rambled on... I have an ALDL to USB cable in the mail, on it's way and I've downloaded the TunerPro software. Be ready to see me asking lots of questions as I learn to how datalog and diagnose problems with these old OBD1 systems. I still can't figure out why it wants to idle so high and overdrive isn't acting properly... but it's time we see real data before I throw more parts at it with nothing more than hope.
Thanks for all the info here, this place is truly a treasure trove for OBS knowledge.
Dave