I'm having trouble with the idle on my 1990 Gen I 3.0 EFI (manual trans). Basically it's too high: starts out around 2000 when I first start it. Drops down to around 1250 once it's warm, so it seems like it's running about 500 RPM high.

The deal on the vehicle: I just rebuilt the engine. So it's mostly all new stuff, i.e. spark plugs, distributor cap, fuel filter, etc. I also replaced the ISC with one of the new ones from DSMISC, so I believe that it should be good.

The FSM only lists three things to check in their troubleshooting matrix for "Incorrect Idle Speed (when cold)," they are: coolant temperature sensor, idle position sensor and ISC. I've checked and rechecked the first two and already replaced the last. Any suggestions as to what else might be causing the problem?

I has a previous saga on this forum about trying to get over major hesitation on this truck. I ended up setting the timing, but could never do it the "official" way because, for some reason the truck won't start/run when I ground the ignition timing connector. So I set it by feel (around -5 BTDC without grounding the connector); the truck seems to run pretty well (no hesitation, plenty of power) but the high idle is really annoying.

Is it possible that the ECU is causing the problem? Any way to diagnose this? Also, the FSM gives instructions for setting the factory idle speed; but I really only want to do that if it seems clear that this is the place where it needs to be adjusted.

any suggestions would be appreciated!

thanks,
steve