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MG MGF Technical - Air Control Valve

Has anyone else have had this problem,whilst driving home from work on Friday my car started reving up when I was stationery at traffic lights. Also when I was slowing down at a roundabout the car was reving instead of slowing down. I know its probably to do with the electronics. Anyway my husband changed the plugs and put a new air filter on and since then its been going alright. If you put new valve in do you have to have it checked out at a dealership or is it as straight forward as just taking the old one out and replacing it with a new one. Your comments would be appreciated.
Jeanne Dyson

First, the IACV is probebly doing the right job!
The IACV misbehaving is usually a symptom rather than a cause.

When you say 'rev up' do you mean you had the car in gear right down to 1000 rpm and when you decluched the revs jumped up to 2k, then settled to 1500, then slowly fell?

If so then it's probebly a faulty temp sensor telling the ECU the engine is cold, this could be dirry contacts (clean with contact cleaner for 2.50) or a faulty temp sensor itself (unbolt, lose a little coolant and refit a new sensor for 20)

Alternativly you could have a sticky throttle cable, which is a case of oiling and working the cable.

If you are sure its the IACV then it does indeed bolt off, you can 'clean' it with carb cleaner as apparently they can get gummy - but try the above first!
Will Munns

I was stationery at the traffic lights, infact I had my foot on the brakes, and when I approached the roundabout I had stopped to give way to traffic when the engine was reving up to high revs.
JEANNE DYSON

so engine out of gear, foot on the brake, car stationary, revs stationary @900, then suddenly revs up to ?(well over 2000)

Sorry then I don't know with this one, still quite unlightly to be the IACV as they get gummy and 'stick', but the temp sensor should only lead you to around 1200, the throttle sensor should only take you to around 1500. in fact the IACV can only take you so far.

I think you have another problem - possibly an air leak, is it possible that the MAP sensor has got loose - and how are the brakes?
Will Munns

Hi Will

The brakes are fine, you could be right about a air leak as my husband fitted a new radiator after Christmas.
Jeanne Dyson

>The brakes are fine, you could be right about a air
>leak as my husband fitted a new radiator after
>Christmas.

I mean an air leak into the inlet manifold, if this was in the vacume tube from the brake servo then the brakes would be heavy. If it happens again try taking your foot off the brakes (if stationary!!) to see if it's the servo itself at fault.
Will Munns

I don't think that we can rule out any of the usual candidates that cause a high idle speed.

The ECU temperature sensor that Will mentions right at the beginning would be the first place I'd look at too. If this were at fault, the revs would remain high whether the engine was hot or cold (would remain about the same). Moreover, the radiator fan would not kick in either: if you leave the engine running for about 45 minutes, if the ECU temp sensor is at fault, the engine temperature gauge will start to rise into the red, and you have your answer.

What air filter has your car got, and what type of retainer is there to the throttle body? If a jubilee clip has been used here, then the is a good chance that the throttle body has warped - particularly if the throttlebody is of plastic construction (readily identified - it'll be black). In this case, replacement is the best option - and alloy replacements are readily available.

The IACV can cause problems. Often just muck (oil from the oil breather). As Will says - carb cleaner will do the job of cleaning this out.

Time to don the dearstalker hat Jeanne, and do some detective work! :o) Good luck
Rob Bell

I have found this can be caused by wide shoes hitting the accelarator and brake at the same time...
Rich Moods

It could be the air stepper motor needs a clean. Used to happen on my MG Maestro from time to time.
Blue Pocket Rocket

This thread was discussed on 24/01/2005

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