I have a Honda it has a standard 4cyl b21a1 1990 Honda prelude (should have specified was in a rush) I recently changed the head gasket(it went bad would puff out white smoke when I cranked on the key and even in the cold of night my radiator hose would be pressurized without driving or even starting it). a fairly simple operation but when I went to start it had lacked power, I would go just as fast if I had the pedal a quarter of the way down as I would with it being full throttle but it was still functional and I was content with it. The big issue is, recently while I was coasting along I noticed my car would drive itself it felt as I had just clicked on the cruise control, I was able to work around it because of my manual but when in gear it will take off especially when u are in the lower gears. and when I sipped it into neutral it would rev up at a constant rate from 1.0k-2.8k being very loud and burning tons of gas in sure. this just happened as I was driving down the road from my slow but still running vehicle im. When I go to start it it instantly shoots up to 3k rpms then jumps between the 1-2.8rpms again and it is constant. Anyone got a good guess? it also smells very rich in gas but my ignition timing was spot on. I changed the head gasket it went bad would puff out white smoke when I cranked on the key and even in the cold of night my radiator hose would be pressurized without driving or even starting it.

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    I would suggest you have a vacuum leak, but don't know. It seems that if you are getting unmetered air into your system, you're going to experience a lot (if not all) of the symptoms you are getting. Also, what was the reason for the change of head gasket? You really need to flesh out your question with model/year of your vehicle. This will help us give you better answers (thinking a 1975 Honda CVCC is a lot different than a 2005 Honda Civic). Oct 27, 2014 at 21:08
  • it is a 1990 prelude b21a1, head gasket went bad due to overheating, car doesn't overheat since the procedure. still not really sure what the problem is but i have checked the vacuum lines that go from the black box in the left hand passenger side (not 100% sure what i would call it). i can control the revs to be normal at lights by keeping the clutch at about half mass but i really have to work the break so it doesn't go leaping forward and im also positive that it isn't doing the vehicle any good. im open to any suggestion, and will check on anything anyone has even a hunch that it might b Nov 2, 2014 at 1:59
  • I'm not sure what the problem is with the revs, but you REALLY should not use the clutch to slow the engine down. All you are doing is frying your clutch. You will in short order cause your flywheel to overheat and cause heat cracks, then you'll have another expense of replacing it and the clutch itself. Definitely not a good thing to be doing. Since this is a FI engine, I'm going back to my original point of unmetered air. This would not be a small vacuum line, but a leak around the throttle body or some such. Nov 2, 2014 at 11:16

2 Answers 2


I had a similar problem with my 1998 Honda CRV. It turned out to be the idle air control valve (IACV). I cleaned it a couple of times but the symptom returned and my check engine light came on. I replaced it with a new one and it now runs fine. It should be located mid-engine, fire wall side, attached to the intake manifold, held in place with two bolts.


A surging idle indicates a vacuum leak on the intake.

You could have a leaking vacuum line (did you forget to plug something in?) or your intake gasket may not be sealing.

Like Vince pointed out an hour ago, a bad Idle Air Control Valve (IACV) could also cause this. Since it was not doing it before your engine work, my guess is the IACV is fine.

With the engine running, spray some short bursts of starter fluid around the various components. If the engine speed increases, the leak is coming from the area you just sprayed. If it does not increase, let that area dry before moving to a different area. You don't want a lot of starter fluid on your engine. If it ignites, you would be in big trouble.

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