THIS POST IS FROM A HAPPY CAMPER!! After finding why my 90xj has been running on five, I replaced the bad injector plug and the lady is now running like a sewing machine at 303k. My question is: What would cause a fast idle (not bad around 1500rpm) after driving enough to reach operating temperature? When first started, it idles fine (around 700rpm). It's no big deal, I just need to know what to look at next.

Sounds like a cracked exhaust header, common around the collectors.
I had the same issue and welding up my header fixed it.

Cracked vacuum hoses too. Cleaning the throttle body and idle air controller might help. Also possible someone dorked with the throttle stop and it needs adjusted correctly. See http://cruiser54.com/?p=94

Mine did this when the TPS was going bad. I got it wet and it would randomly idle up to around 1500rpm. It was sometimes variable with temperature. Is yours still a Renix from 1990? Mine's a later HO, but I imagine it's somewhat similar.
A new TPS permanently cured it for me.

A cracked header/manifold shouldn't have any impact on idle speed.

Had the same problem and replaced the TPS. It would clear when I shut off, then restarted the engine but would soon return. TPS replacement solved that problem..

Thanks all, I guess I'll have to check out the TPS.

Complete Tips 1,3,4, and 5 on my website first.
THEN, check the TPS following the instructions on the website.