Hey everyone! I have an 88 Corvette that I have been driving daily for a year and with heavy driving has come some worn-out parts. Back in April I lost all fuel pressure and ended up replacing the fuel pump, regulator and pulsator. With fuel pressure back the car was running great and noticeably better than it had been. After driving it for a few days, I had what seemed like some misses and overall rougher engine performance. Next, I noticed that the engine would completely lose a steady idle once it hit around 155F. If I let the engine continue running with no throttling it will soar and dip from 300-900 rpm. This behavior usually throws a code 44 (lean exhaust), but this code has been present the whole time I have owned the car (even when it seems to be running well). The base idle is normally 700. It has died at traffic stops several times so I have not been driving it much since April. Finally having the time to work on it, I ended up replacing IAC, TPS, cleaning the throttle body, O2 sensor, tried a new MAF sensor but returned it after no improvement. Today I did a smoke test and found a vacuum leak in the intake cone, throttle body shaft, and EGR valve. After the smoke test, I removed the vacuum line going to the EGR valve and plugged it, and started the car. It had a smooth idle even after fully warming up. This was the first time I have been able to achieve a real idle. I then turned the car off, hooked the EGR valve back up, and started the car. Immediately the idle soared and dipped after running at a normal starting idle for a few seconds.
The current theory is that I have a bad EGR valve that is both sticking and sucking outside air. Has anyone else had this issue? Am I on the right track?
Thanks everyone.

*Edit* After letting the car cool down and starting again with EGR disconnected, the rough idle returned at 153F again. I am wondering now if the leak I found in the throttle body shaft is enough to be causing the issue.

 

When you're chasing a problem like this you really have to have a scan tool that gives you live data. The fact that it idles fine until warmed up-that's when it goes closed loop. If it has an air leak at the exhaust manifold it will tell the O2 sensor it's lean and it will keep trying to dump more fuel in. On live data you'd see the injector pulsewidth suddenly increase right when it goes closed loop if that's the case.

 

Yea, I figured that needs to be the next move. There is a local Corvette shop close to me I could call to see if they have an obd1 scanner. I didn't want to go that route immediately because with these ECMs I have seen that when you hook up a scanner the computer goes into "field service mode" and it idles the car up which wouldn't help much in diagnosing my problem.