Rev ups, and drops. Please Help!!

Ok, after my car(92 honda prelude si 130,000 miles) is warm (driving about 10-15 minutes). When i come to a stoplight and push the clutch in or put it in neutral, the engine will rev up a bit and drop quickly back to idle, and do this about 10 times and quit. It doesn't just gradually go back down to idle either, it sounds like it's gonna die...like drops really fast. I have had the catalytic converter cut out with a 2.5 inch pipe going from the exhaust manifold back. could this be a messed up o2 sensor? Map sensor?Loose vacuum hose? EGR valve? Please help, no check engine light, however when i turn the ignition on, that light doesn't come on anyways so i don't think it works. Thanks.

The first thing you need to do is get the check engine light working again. Your problem sound like the IACV (intake air control valve) some people refer to it as the fast idle thermo valve. It does the same thing as a fast idle thermo valve. Get the light working then see if you have tripped any CEL's.

Do you know how i'd get the check engine light workin again? Bulb or fuse or relay?

i actually have a similiar problem. I got a 95 GSR and the revs jump from 1 to 2 while idle. But it doesnt go away. it just does it all day long and also my check engine light comes on.

Do you know how i'd get the check engine light workin again? Bulb or fuse or relay?

I would start by checking the bulb. If the other lights light up then it would not be a fuse. Check the bulb and maybe even replace it, they are cheap.




i actually have a similiar problem. I got a 95 GSR and the revs jump from 1 to 2 while idle. But it doesnt go away. it just does it all day long and also my check engine light comes on.


You need to pull the codes form the ecu. To do this you will need a paper clip. Look under the dash on the kick panel on the passenger side, you will see two harness' plugged into a Green (i think it is green or blue can't remember) rubber thing. Take the connector out that has only 2 wires, you will want to bend the paper clip so that you can connect the two wires in the connector with it. Then turn your ignition switch to "ON" not "start". Then you will see the check engine light start to blink, you need to count these blinks. Now you will have short blinks and long blinks. The short blinks have a value of 1 and the long blinks have a value of 10 so if you have 1 loing blink and 4 short blinks then you have a code 14 which is IACV. Try it and post back if you have any questions.

That might not be the problem. I'm experincing the same exact thing (randomly revs up to 1300 and then down to 700 after a while at normal temp) and I've tried the fix. It didn't do squat. I even went as far as to make a gasket to seal off the IACV valve from the TB completely. Still didn't help.
I've checked my temp sensors thinking that might be the problem, but no luck. I've tried several other things with no luck. I've got no CEL to go on either. And Honda refuses to check my car out because it's "too heavily modified" So I'm shit out of luck. It seemed to start right after I installed my nitrous, VAFC, and hi flow fuel filter.

Funny thing is it'll stop when it wants to. Once I shut off my headlights and it stopped :eek7:

It is the IAC valve. All honda's and acura's have had a bad history with them. If you run even slightly low on coolant, the IAC valve will overheat and begin to do exacly what you described. I replaed mine and made sure the coolant was running through it and haven't had a problem since. Sometimes, and this is rare, but you can take it off and clean it out with some carb cleaner and then spray it with wd-40 and it will fix the problem....i haven't personally seen this work on a honda, but it does work on a GM.

take the fast idle valve off and clean it real good inside and outside and put back together and put back on the car should work fine after that .. oh yea when you open it see if the plastic part is screw down if not there is your problem but still clean it... it is right of the ECU !

I would just get a new one but i don't feel like spending 300 for one at a honda dealer. As for the check engine light, when i have the ignition turned on, the low fuel light and the seatbelt light don't come on either. I would replace the bulb but i heard taking the dash out is a HUGE project, and i'd rather hope it was a fuse and try to replace that since there's a few out.

Replace any and all fuses that are missing then reset the ECU and try again. C'MON guy you are missing fuses and wonder why something is wrong?

No dude, not a few fuses out, a few bulbs out. the fuses under my dash all appear to be fine.

..... I would replace the bulb but i heard taking the dash out is a HUGE project, and i'd rather hope it was a fuse and try to replace that since there's a few out.

Sorry guy the way it is worded makes it sound like you were taking aboiut fuses. Anywho.

You are saying that there are a few bulbs out? Like out of the dash or burned out? Either way i would replace them still, it will be hard to find the problem if the MIL does not work.

It could be a relay in the underdash relay box also.....but i dunno how to get to those.

i'd say hook up some NOS to it, Blow it up (KaboooooooooMMMM!), then put in a new B series...hahaha.

you boys need to get on ebay and find a GOOD used TPS(throttle position sensor) and replace with olddrilling required) to fit fluctuating idle!!!

you boys need to get on ebay and find a GOOD used TPS(throttle position sensor) and replace with olddrilling required) to fit fluctuating idle!!!

So he should spend money on a part that may or may not be the problem? The IACV is the problem with the idle problems almost 90% of the time. So if he was to follow your advice he should buy a IACV instead of a TPS.

i'd say hook up some NOS to it, Blow it up (KaboooooooooMMMM!), then put in a new B series...hahaha.

If you can't contribute helpful information than please don't reply.
Also you think he should replace his H series motor with a B series motor, in a car that is heavier than the civic's and integra's that the B series came in? :screwy:

your problems will be covered here.

http://www.automotiveforums.com/vbulletin/showthread.php?t=328798

i'd say hook up some NOS to it, Blow it up (KaboooooooooMMMM!), then put in a new B series...hahaha.


YEA BLOW THOSE PISTONS THROUGH THE HEAD, HOOD AND TO THE MOON..HAHAHAHA

Ive Heard This Happen Before I Was Told It Was The Pcv (postive Crank Ventilation) Valve Id Check That