sorry-1993 chevy 350 tbi. all new sensors, new computer, new vacuum lines and intake manifold gasket, tbi gasket, no cat, new injectors, fuel pump and relay, fuel lines, possibly more can't remember. Vacuum readings seem ok at 20 in hg. When it is cold, it idles slightly high-I imagine to warm up. After it is driven, it runs high (1000 rpm) and the iac opens every 10-15 seconds to surge it up to 1200-1300 rpms. If the truck is shut off and restarted, it drops down to a nice healthy 700 as if nothing is wrong. My feeling is that it runs fine when warm in open loop, but not in closed for whatever reason. However, if I unplug the 02 sensor while it is running and surging, it makes no difference. Same deal with the temp sensor. Other than the idling issue, there are no other symptoms, but it is very annoying when stopped at a light. Also, from time to time over long idle periods with some throttle usage at slow speeds (i.e. pulling out bushes etc)-after restart so idle is at 700 or so- it will drop out to almost stalling, then catch itself, then again eventually stalling out. My thought is that something is telling the computer to surge the iac, but I have no idea what. Is it possible to get a programmed chip that will change idle in such a way as to steady it? Any thoughts are welcome-

 

Could it be timing, and maybe when the engine is cold, the computer retards the timing to eliminate knock, then advances it when warm causing high idle? Is it possible to pull the computer and have it tested-prom included?

 

just in case anyone was losing sleep over this the problem was a bad computer chip. Strange-they never go bad, except apparently in my case. Running great now though, considering it's got all brand new sensors, performance chip, etc, etc, etc haha.