I have an issue on my swap where upon cold startup the motor revs up to about 2k and hangs for a couple seconds, then starts to rev down, then shoots up to 2.5k, then drops to idle. It still does this when the motor is warm, just doesn't rev quite as high. It idles great with no issues once is settles, but is too high at start up & I keep getting a "RPM higher than expected" code. I have swapped TPS & IAC sensors with no change. I am running an '02 5.3 (converted to cable TB) mated to a NV4500 manual, so no VSS or brake switch hooked up to computer but they have been properly tuned out. Any ideas on the problem?

 

It sounds like you might have a vacuum leak and the IAC tries to compensate for it, which makes the idle go up and down like you described. Vacuum leaks will usually go away when the engine warms up and everything swells up. That's where I'd start.

 

Sorry for the long delayed reply, you guys know how the holidays are. I have sprayed every single inch of this motor several times trying to locate a leak and have had no luck. It only revs high the first couple seconds on start-up and then drops to a normal idle, I know the leak doesn't go away that quickly because of thermal expansion. Do you still think this could be a vacuum leak?