Civic erratic idling

I replaced my timing belt, belt tensioner, and water pump this
weekend on my 88 civic SE and I used all Honda parts. After everything
was put back together, I started to refill the cooling system by
following the directions in the Honda factory manual. The book says to
loosen the bleeder bolt, fill the radiator up to the bottom of the
filler neck and tighten the bleeder bolt when antifreeze begins to
stream from the bleeder bolt. Next, the book says to allow the car run
the car with the rad cap off until the rad fan comes on at least
twice. The fan took forever to come on (20 + minutes) and it was a
very hot day. So I decided to give the car a bit of gas and bring up
the rpm's a bit (around 2000) in an effort to speed up the process.
After a minute or so at 2000 rpm, the rpm's began fluctuating wildly.
They would repeatedly and consistently bounce from 1500-900 rpm until
I released the gas pedal.
Other than this crazy behavior, the car runs very well and it
hasn't done it while driving yet. I rechecked all connections and
hoses and everything appears to be ok. Also, the car is not
registering any trouble codes (i.e. the check engine light is not
coming on). Any ideas what may be causing this?

TIA,
Curt

 

It may have been air bubbles going by the temperature sensor. Check the
oxygen sensor if you can still cause the fluctuations now that the air
is out.

 

The engine rpms only oscillated when you were trying to purge the air from
the cooling system?... but it's OK now? Is this fuel injected? The ECU
can do strange things if you throw "abnormal" behavior at it. One thing
which can affect idle speed like that is the evaporative emissions control
- if the valve opens to purge the charcoal canister. If it hasn't repeated
under normal driving, I wouldn't worry about it.

BTW I find that repeatedly blipping the throttle briefly is more effective
at purging air out of the cooling system - it comes out in belches.

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??