I have a 86 prelude 2.0fi that surges at an idle what might cause this

Air/idle control sensor

cold engine idle surge. (problem)
RPM range of surge not stated
300-700 or 800 to 1500? (range is critical to diagnosis)

bad fuel
running summer fuel in winter (running old fuel?)
needs a tune up bad, see the list in the operator guide.
idle speed controller,sticking or EGR valve same. (clean them)
location not stated, so cant guess about local air temps.?
no other symptoms, lack of engine power? etc?
was it ok last week, or better, when was the last time it was OK?
1week, 1year?
surge , causes, on a tuned engine might be:
1: too lean. so lean the ISC can not control idle. rpm low, stalling
2: too rich, flooding, stalling, spark plugs foul (look at them)
3: weak compression in 1 or more cylinders.
4: bad spark.
5: air leaks, in the plenum causing , ISC to lose control.
why high RPM?

especially?
well that means it fails hot too.
and there is a simple tests. for hot surge.
is engine fully hot? 180F and holds solid there?
say rpm is about 800
and you turn on head lights, heater fan and (not AC) and defrost.
the RPM must hold or the ISC is failing.
ISC -= idle speed controls. (all EFI has this function)

run fault codes and look for problems in the MAF sensor--- accelerator position sensor--- and idle speed sensor.. Have a pressure test done on the fuel system to check for other faults that will cause this problem

vaccum leak

This 86 Honda Prelude does not have a link connector, code's are read from the computer it's self, the computer is under the passenger seat or under the carpet on the passenger side, you count led light flashes and on the computer panel will have the trouble code's refernce.

this sounds like normal warm up no other problems i have an 87 integra and its similar when cool

If the ignition timing is incorrect and or someone has turned on the idle screw without bypassing the PCM. Otherwise the IAC(idle air control motor) is defective. Possible vacuum leak also.

There is a warm up valve on the throttle body(should have 2 small coolant hoses). remove the valve and have someone start the car while you block the port to the valve on the intake. If that clears up the problem you can cut a piece of cardboard to mount in between the valve and intake.should straighten it out.

Best way is to call a salvage yard and ask how new a motor you can use. Just tell them your looking for one.

sounds like you have a vacumn leak some where need to find that first