Ever since it started getting colder, my check engine light keeps coming on and the car won't idle properly. At first I replaced the Intake Air Temp Sensor located in the air cleaner, but that didn't fix it. Then I decided to replace my IAC with a different one from a different SBC I tore apart for my engine build. That seemed to fix the issue for about a week until it started happening again. I finally went and bought a brand new IAC and put that in instead. I'm still getting the check engine light every 1 out of 4 startups.

When the CEL comes on, the engine barely maintains idle at 500 rpm, and stalls whenever I come to a stop at a light.

Here's the strange part. I tried to pull the code from the OBD port by jumpering pin 1 & 2 with a paperclip, but I didn't get anything. But that's not the strange part. If I leave the paper clip in the OBD port and start the car it starts up fine and goes into high idle no problem, and the CEL goes away, I then remove the paperclip and I'm on my way.

It's now become a habit to start the car with the paperclip in the OBD port because that's the only way it'll run normally.

I ordered a new TPS Sensor because that's the only thing I can think of which might still be causing the problem. Any ideas? How is it that the car starts fine when it's running through diagnostic mode but it won't when I start it normally.

 

Sounds like s bad ecm, you don’t state but assuming your dealing with tbi.
With the engine running you can wrap on the ecm case to see if running issues appear.

 

Yes, sorry it is an '88 305 TBI. I have no reason to believe that the ECM has failed. The vehicle has sat in a garage for 12 years and the engine doesn't have an excessive amount of miles (136,000 km).

The car starts and runs fine when it's warm out, and the problem is intermittent in the cold, but below a certain temperature it's almost guaranteed to happen.

I probed the TPS wires and I did find the voltage jumped around, so it's gonna have to be replaced anyways. But the wire to the ECM is giving a strong signal, which leads me to believe the ECM isn't the issue.

Sent from the mobile client - Forum Talker

 

When you say wrap on the ECM case do you mean give it a little whack to see if its failing?

Sent from the mobile client - Forum Talker

 

Yes
Btw if needed it’s a service number 1228063 a one year only ecm.
A 1228746 can be used in its place with appropriate prom installed.
eBay is a good source if needed. Regardless of miles electronics can fail.

 

Thanks for the info! I'll report back after the TPS comes in. We'll see what happens.

Sent from the mobile client - Forum Talker

 

Try doing this set up for the IAC and idle. It's from the How-tos section on here. You need to do that rather than just stick the IAC in there
https://www.thirdgen.org/tpimod2/
hope that HELPS