Intermittent 2000 RPM idle on warm/hot CIS-E engine

[Chris Cappuccio]

I have an 89 80q that seems to 'stick' at 2000 RPM but usually only when the
engine is mostly or all the way warmed up.  The problem actually gets worse
(or it sticks/surges longer) as the engine is warmer.

If you are at a stop, around 900 RPM and touch the gas a little, and it goes
up to 1500 RPM, then it might just stay at 1500 for a while.  If you are 
slowing down and passing 2000, you might just stay at 2000 for a while. 
Later the RPM will drop to 900 and maybe some times it will surge up to 2000
for a while as you idle.  This usually only happens on warm/hot, though.

As an aside, the engine seems to run better when its cold, slightly better,
more power.  I thought that perhaps it is running a bit rich.  When it
is hot and has been driven for several hours, the check engine light will
come on and stay on during idle, until I hit the gas.  Since the 1989 model
doesn't have the fuel pump relay slot, I haven't yet pulled codes.  But the
symptom seems fairly specific, if you have already had this problem, I'm
curious how people have solved this problem before.  Same for the RPM issue.  
(Maybe they're the same problem? :)

My first guesses are coolant temp sensor and maybe even oxygen sensor.  The
rig does have 200,000 miles, after all.  But it's in very pretty shape for 200k.

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