I searched.

I bought a 1991 Honda Prelude 2.0 SI with 267k miles on the chassis and motor (I wasn't told otherwise). I started it up for the test drive and heard it idling up and down constantly and immediately said to myself "IACV". So I took it out and cleaned it with break cleaner (it looked great even before cleaning) but it didn't help. I ordered a used one from ebay that looked a lot dirtier. I put it in anyways and the idle didn't change. I then came to the conclusion that the IACV that was already in it was new or recently replaced. So I don't know what is causing the idle problem. When I start it up (cold or not) it was usually rev up to 3000 and stay there for 20 seconds at most. Then drop and go back up constantly. Usually around 1200 rpm to 2200 rpm. Constantly goes up and down. I'd really like this issue to stop because it is using a lot of gas. I am at a loss for what the issue could be. Ask any questions that can help you further diagnose my issue and please chime in if you have an educated answer. Thank you!

 

Somebody please chime in.

 

Something I didn't mention. The check engine light comes on after driving a couple minutes down the road. I have not checked it because I haven't gotten the chance and the fact that it's OBD0 and it's a pain.

 

Been a long time since my B20A5 days so I'm not sure if this applies:

http://www.kelownacivics.com/forum/u..._DIY#Post24001

 

Thanks for chiming in but it didn't fix it. I had high hopes for it. That 'plunger' only needed a few turns until it was completely tightened and it looks brand new.

 

I know it is 4 years later and sure you don't own this car anymore. If you still own it and haven't found out what was causing the problem here is your answer. It is the ITS(Intake Temp Sensor). It is located in the middle of the throttle body on the driver's side. If it is dirty or old and worn out it will cause these problems. It is very common with preludes of this age. My '90 2.0 Si had the same problem. It took me almost a year to track it back to that. It will run about $27 at Autozone or around $5 at a junkyard for this sensor. It is a pain to replace due to its plastic base and the 2 Phillips head bolts that hold it in. You will need a socket with that head on it to remove it. Just don't over tighten or you could break the plastic base and be right back where you started.