What causes RENIX to idle high only on startup? Sporadically.

I feel like I'm beating a dead horse. Rargh.

My '89 has had an intermitant problem since I bought it. When I start it up, any time it's cold or hot, it'll randomly start and rev up to 2000 RPM's and stay there indefinitely. Sometimes I'll wait 10 seconds or so and it'll come back down to 750ish.

If I restart it one or two more times, it'll rev to 1500 and very quickly come back down to 750 RPM's - as it should.

This appears to be a very common problem on RENIX era cherokees, and I want to come to the bottom of this.

I have replaced the IAC and the TPS to no avail. Set the TPS to 0.8 volts, and to 0.69 volts (what a computer at a shop said it wanted) with the same results.

I have swapped the latch relay with no change.

I have cleaned electrical connectors. No change.

I have taken off my throttle body, doused it in solvent, then cleaned it with a tooth brush. No change.

What causes this? I can usually duplicate it if I stall it by letting the clutch out too fast and then restarting it.

Also, as a hint to the problem, sometimes when I try restarting it to fix it and it's revving up again, I can definitely hear the motor missing and it'll shake until it revs down again and it's fine. This doesn't happen on the initial rev-up.

One thing is I may have an intake vacuum leak, however if that was the case, the idle should come down very quickly but it doesn't, only after restarting the jeep a few times.

Damnit, what causes this?

I've had random high idle, because of the TPS, the TPS ground at the splice in the harness on the firewall (under the wrapping) and loose intake manifold bolts. Give the harness a good shake near the firewall bend and see what happens.
The Renix will try to correct the idle mixture to a point, then it gives up and the idle can go up suddenly. Kind of a cascade affect, along with dropping in and out of open and closed loop operation.
To be sure, you almost have to test the resistance in the ground circuit and the sensor connector to ECU connector. And even the pins on the ECU connector, I've found them partially backed out of the connector body before.
You also have to seriously clean the connectors, preferably with a good contact spray and even scrub on them a bit with a tiny brush or a Q tip. I bought a kit of contact cleaners at a computer store, they are erasers of various sizes with a handy holder.
The IAC will act up when the system voltage is low, it needs almost all the juice it can get to operate correctly. A weak battery can cause problems, shortly after startup it gets much of the available juice to refill. If it starts out a little weak and then sucks down the system, the IAC gets sticky.

Since you have adjusted the TPS, i definetely think that the root of your problem lies with a vacuum leak. I still think that it could be from the intake manifold... mine had a leak here, and the idle did not come down quickly.
Triple check those vacuum lines, and seriously consider changing your intake/exhaust manifold gasket. Also, the bolts here come loose with time, and its possible that your problems are associated to this common problem. Maybe even an exhaust leak... have you checked for that?

I really wish I could tell you what cured mine, I did an afternoons work on it, re soldered the ground wire for the TPS, tightened up the intake manifold and a few other odds and ends, like cleaning connectors and such and my hot start high idle problem disappeared as did my random high and low idle at operating temperature.
It now starts up normally, stays a little higher than normal till it reaches 140 degrees or so then settles in to it's normal idle, steady as can be. It doesn't over rev during a hot start anymore.
Sometime around the same time frame, I put in a new battery.
Are we talking the typical Renix hot start high idle here, or random high idle?
I seem to have cured both, but I sure can't tell you exactly what I did. My shotgun approach seems to have worked though.