Normal noises at idle?

[sawood12]

Has anyone out there had problems with the engine idling? Where my car idles has suddenly dropped from a nice smooth consistent idle at around 750rpm to a laboured almost about to stall idle at somwhere just above 500rpm (although it doesn't stall). When the engine is cold it idles perfectly as it always has but when the engine warms up the idle revs drop.

I immediately suspected I had an air leak downstream of the air flow meter so have been focussing on trying to detect a sucking sound. I can hear a sucking or rush of air sound coming from where the big rubber hose attatches to the airflow meter but this quickly disappears as you open the throttle. If there was a leak here I would expect this air rush/sucking noise to get louder as the throttle opened.

Is this sound normal at idle - should I be looking elsewhere for a problem?
Thanks.

 

[John Sims]

If there was a leak here I would expect this air rush/sucking noise to get louder as the throttle opened.

The Air Flow Meter has a big flap in it which causes a restriction to the air flow. As revs rise the flap is pushed back by the engine sucking in more air. At idle there may be insufficient air flow to move the flap so a split in the hose could offer less resistance to incomming air and hence suck via this route.

This is just a theory, but may explain why you get sucking at low revs but apparently none as revs rise.

 

[Fen]

I imagine it's the idle control valve that's the fault. My cab did something similar when I got it but a clean with carb cleaner had it right again and lasted 40-odd thousand miles.

 

[jr2015]

Check the auxilliary air valve with an A/Vo meter(correct value in the workshop manual) or try having the potentiometer in the throttle body recalibrated.
jr.

 

[richthomas]

I had a similar problem and it turned out to be the throttle position switch, might be worth a check.

 

[sawood12]

After much poking and looking and seeking advice I think the problem has been discovered.

Throttle position switch checked out OK as did all the various vacuum pipes and air hoses. Apparently the sucking/air rushing noise I was hearing is normal. Finally I took it to a local MOT garage where a proble was stuck up the rear end and the engine is running very lean (reading should be 1.3 or something like that, it was reading 0.5) which certainly would cause idling problems. Upon following attempts to adjust the mixture the adjusting screws are seized solid in the housing. Booked in next week for a new Air Flow Meter (£65 2nd hand).

Seems like a probe up the rear end can tell you alot.