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.

 

check coolant level in radiator, when cold. coolant in expansion bottle
means nothing if there is a leak.

 

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.

 

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.