Weird Idle when cold (1994 prelude VTEC)

Hello all. I need opinions. Car is a 1994 prelude VTEC.

On cold starts, idle climbs to 2 to 2.5K RPM. All is well it seems.

Then after a while, when the engine starts to warm up, the RPM will go
down, then up, down then up (you get the picture).

When the car is fully warmed up, RPM stays at the same place normally.

Opinions?

Thanks!


Ben

 

check coolant level inside radiator. must be full to the top. if
diluting antifreeze, use distilled water.

 

Thanks for your reply!

I did check the coolant level IN the radiator. Full to the top, but
the expansion tank was empty. So I added some in there.

My "instincts" tell me that the RPM "dance" is not related to the
radiator. Perhaps something about fast idle valve when cold or
something? (I don't know much about cars, more motorcycles). It seems
the car cannot transition well between cold and warm status. Know what
I mean?

Cheers and thanks again for any information!!!!!


Ben

 

Bendit wrote:


. . . . . . .but

=========================

Expansion tank has to be kept at MAX so there's a RESERVE to replace the
AIR that's going to burp out next time the engine warms up.. Computer
gets confused if the sensors aren't immersed, idle goes up and down.
Been covered a million times.

'Curly'

 

Thanks for the information. Filling the expansion tank did not fix my
problem (even though it needed to be done).

Any other ideas?

(keep in mind idle maintains itself just fine once the car has reached
operating temperature)

Thanks again!

 

If the expansion tank was empty, did you also fill the radiator itself
(which most likely is also low)? The problem you are trying to fix is to
get rid of the air bubble in the cooling system that is isolating the
temperature sensors and making the engine computer think the temp of the
engine is something other than what it actually is.

 

Thanks for your reply. Yes, I did check the level of coolant inside
the radiator. It seemed to be full (all the way to the top). Perhaps
it will take time to get rid of the air then. I do not currently drive
this car every day (but will start on the 23rd of this month when I
return my leased car). I will post back progress once I do (and see if
the air problem fixed itself).

Question: where exactly is the temperature sensor(s) ?

Thanks again guys!

 

yeah, my guess too. don't know preludes, but the civic uses a small
idle air valve that gets opened and closed to regulate idle speed (as
opposed to other cars which have a little motor to open and close
things) which gets gunked up, and might benefit from attention/
replacement.

 

Problem solved. I have replaced both temperature sensors and now all
is well.