89 GMC Suburban revs high

We have an '89 GMC Suburban with a 7.4L 454 engine. Engine idles at 1800 rpm when engine warms up and stays there until you turn it off. When you turn the engine off and back on it idles normal until you drive again then it idles high again when you stop the vehicle. We have new throttle body intake gaskets, throttle position sensor, tried new computer twice, new oxygen sensor. Nothing reads out on scope to the problem. Reworked vacuum lines. Also, another separate problem? Transmission when cold doesn't shift from second to high but does when it warms up. Then shifts fine when it warms up. Is this from sitting? And/or seals? We've had this to several mechanics and noone seems to be able to fix? Everyone is stumpted. Please, any feedback appreciated!

I guess the clue here is the word "sitting", if the thing has been sitting a while, and there are no computer codes, then the problem must be a mechanical one (an actual moving part) somewhere. Clean, lubricate and adjust everything from the gas pedal to the throttle body butterfly.

If that doiesn't work try adjusting the timing a bit.

Change the tranny filter and replace the fluid, it wouldn't hurt to dump in some tranny treatment (slick 50) as well.

You might also check your temperature sensors, check the connectors and make sure they are not corroded or loose. Sounds like the computer may not always know the correct temperature and runs the engine to warm it up. Just a thought.
Best,
tomic

could be the computer inside the distributor cap. you need to take off the cap and there is a computer module screwed on the backside. you can take it off and take it down to auto zone and they will test it for free, if it is junk they have them there and they will run you about 25 bucks. it sometimes gets cindensation inside the cap and makes the computer inside the cap go haywire. hope this helps i also have an 89.