Civic idle revs fluctuating

kikel

Hi,
My car is currently off the road. For the weekend i've got a mates 99 civic saloon insured. The civic hasn't being driven for 6 months.
When the car is first started the engine idles around 1900 revs. Once the engine gets warm the revs are constantly changing without touching the accelerator. The engine keeps fluctuating between 1 and 2 thousand revs. This only seems to happen at idle because if i push the accelerator and hold the revs at 2.5 thousand there doesn't seem to be an change.
Anyone shed some light on whats going on?
Kikel

AMurphy

You mean it slows down to 1000 remains there for a while, minute or two, then suddenly revs op to 2000 and hangs out there for a while, some time later to return to 1000, ad infinatum.

Or see-saws up and down between 1 and 2000 rpm like you were pumping the accelerator and not fixing at any set speed for any duration.
What else does this car have , Air-Con?.
EFI, O2 sensors, CAT and all that stuff Y/N?.

kikel

more the second. but i'm not putting my foot on the accelerator.

AMurphy

I don't know what sort of Air sensor your specific engine/model has.
But I have come across this see-saw problem on other cars that have had the 90Deg vane in the airstream sensing element.
These sensors have an internal bypass passage and metering screw. bloccage of this internal bypass can cause this see-sawing, however, I have not seen it as bad as you describe, more in the 200rpm fluctuation range.

What sort of auto Idle adjustment in this vehicle.
May as well give the full inverntory of EFI and Ignition equipment on this engine. make life a little easier.

crosstownk

Could it possibly be an O2 sensor fault - or maybe something that is causing the lambda voltage to fluctuate erratically?

Get a diagnosis done with a Honda diagnosis computer. A Honda dealer wouuld be the easiest but most expensive option.

AMurphy

Once the engine gets warm the revs are constantly changing without touching the accelerator.

could be, the O2 sensor is not used to start or during the heat up phase. It switches in at the end of hte warm up phase, when this problem is reported to initiate.

Easy check, (first how many wires on this O2 sensor), if one easy... just disconnect it and ground the signal wire to the batt gnd or frame. That will latch it to one setting or another.
If more than 1 wire yu have to figure out which is the signal... in which case just disconnect it when heated up and see what happens (apart from the check engine light, which you can ignore).

crosstownk

kikel wrote:

This only seems to happen at idle because if i push the accelerator and hold the revs at 2.5 thousand there doesn't seem to be an change.
Anyone shed some light on whats going on?
Kikel

I'm beginning to doubt my initial diagnosis. Could it be an Air flow meter problem or possibly a throttle body problem? If its only fluctuating at idle...............?