Bouncy Idle

I've been wondering lately why my 1990 Prelude has a bouncy idle, along with a strange high-RPM idle while in motion. (Shift into neutral and coast; regardless of speed, it will "idle" at 2000-2500RPM until I come
to a complete stop. Then it will start bouncing) When the idle bounces, it usually bounces between 500-750RPMs, and does this rhythmically about every second or two. I had read on www.prelude3g.com that the fast-idle valve can contribute to bouncy idle, and the car can function without the device. (Granted, you have to plug the holes, which I did. I checked, and they are air-tight) I also checked the vacuum lines that go from the control box to the throttle body, and they are all connected correctly. This leaves the EACV as the culprit, unless the wealth of Honda knowledge out there can enlighten me as to what else could be the cause of this annoyance. Any ideas and/or suggestions will be appreciated.
Thanks in advance! :-)
Jon
P.S. My engine is the JDM B20A; manual transmission.
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BigJon wrote:

check coolant level in radiator, when cold. coolant in expansion bottle means nothing if there is a leak.
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isn't there a coolant air bleed port on the B20 block? There was on my B18 GS-R motor.
I also had a coolant temp sensor go open.
--
Jim Yanik
jyanik
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Check level of coolant in reservoir. With the engine cold, top off the coolant in the radiator. Use OEM coolant (or possibly orange Havoline, or whatever's currently in the cooling system).
The EACV, for one, is cooled by the engine cooling system. It actually has a port in it through which anti-freeze from the radiator, water pump, etc. flow. If air got into the cooling system, then the EACV may not work properly. I know on my 91 Civic, the EACV is a high point, and air may accumulate in its coolant's port.
So you want to top off the cooling system and monitor its level. Hopefully there is no leak. The car does lose coolant slowly over time, due to evaporation etc.
Supposedly an air purge is not necessary, because the system will by itself draw coolant from the reservoir, and expel air, by design. Still, there is a procedure for purging air, and you can try this too. See the free online manuals or your owner's manual. If you have neither, ask.
My 1991 Civic had problems like yours a few years ago. I took apart the EACV and found no coolant in it. I topped off the system, and the problem was fixed.

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