Dear Roy,
I installed a 350 engine in a ’91 Chevy truck. The motor was stock except for having an RV cam.
At cold idle, it surges up and down, and then it dies. If I raise the idle a couple of hundred rpm, it seems to smooth out enough to continue running and has good response and power. This is the third 350 in Chevy trucks that I have run across with the same symptoms: two ’91 models and one ’89 model. So far I have replaced the EGR valve and solenoid, MAP sensor, IAC valve, throttle body and gasket, all with GM parts. I’ve checked body grounds, PCM and prom numbers, the knock sensor, O2 sensor, exhaust system, fuel pump and filter. I have checked for vacuum leaks with carburetor cleaner, including the brake booster. I’ve talked to every mechanic that I know, and no one has been able to shed any light on the problem. Any help you can provide would be greatly appreciated.
-From [email protected] (via e-mail)
Dear Sir:
Based on what you have done and without some further testing, I can only make suggestions based on similar problems with these vehicles. If you block off the IAC air intake in the throttle body, does it still surge? What is the oxygen sensor doing when this problem occurs? Check the distributor shaft and timer core assembly. A weak magnet or excessive shaft bushing wear will affect spark timing and idle quality.Try unplugging the Set Timing Connector so that the engine runs at base timing. If this improves the running condition to near normal, check the timer core on the distributor shaft. Check the base idle adjustment. If the throttle stop screw has been tampered with, it will idle as though it had a vacuum leak. Even though the EGR valve was replaced, I would check its operation. The negative backpressure EGR has an internal air bleed that is held closed with a small spring when there is no backpressure. At idle, with vacuum and backpressure, the small internal air bleed is open and the vacuum will not open the EGR. In order to test a negative backpressure EGR, with the engine off, apply 18 in Hg. vacuum to the EGR. It should open. Start the engine, and the EGR should close.
