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MG MGF Technical - Idle high, intermittant sticky purge valve.? No

Thread was: *Final answer to running problems*
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*Idle high, intermittant sticky purge valve*

Some of you may recall. It was two month or 3000km ago. Stu seams to having the same mad problem, sometimes.

Problem didn't get back since two month until yesterday while the long home ride from Belgium.
Engine revs high with 1500 per minute, realised after 100 miles at any junction where to gear down..
Throttle isn't it and nothing else I investigated ...

Had the spare purge valve in the boot when it happened and as well I had the advantage of the ready installed electric boot opener. So I just pulled aside, opened the boot, removed the engine grill and pulled off the valve while running engine.
Dooh, surprise. It was closed and not sticky as suspected.

I increased revs 'by hand' above 1800 revs. The valve started intermittant *farting*, as it should. (low noise *pht, pht, pht and underpressure at the conmection).
No needs to swap the valve.

I went home and parked the car in the garage (still high reving, before shut down)
This morning, Start from cold, as far as I can speak from cold under current ambient temperatures.

Surprse, surprise reving still high with 1500. Then I did the IACV reset *on the fly* while driving and it was OK then with 800 revs.

Damn, this funny problem really makes me curious. ;)

I think tomorrow I start from new with slight opened butterfly. Will have some measues of voltages at the TPS connections.

Speak to you soon.
If anyone can find the official output voltage in idle of the TPS, this would be nice. I only have self measured resistances.

PS. old page, pix and mpg

Dieter K.

Woman driver who knows nothing about engines. Any ideas. Occasionally my MGF(nearly 4 years old) will rev between 2.5 and 3.5k coming to a stand still. It is more likely to happen when engine is cold. Sometimes I can feel it happening whilst driving i.e.if I remove foot from accelerator the car will not immediately start to slow down, and braking is needed. This problem has only just started and happens about once a day.
Barbara Bolt

So the mystery only deepens Dieter :o(

Unfortunately I haven't got the diagnostic information on TPS voltages. Not sure where you'd get these from: Testbook perhaps?
Might it be easier to change the TPS and see what happens? I'm sure someone near you will have a spare (after all, who hasn't changed their throttle body recently??? ;o))

I assume that the sensor looms all check out?

Hi Barbara,

sorry to hear that you too are suffering from the "sticking throttle" problem. Commonest cause of this is with the black plastic throttle body. If the air filter hose is attached with a jubilee clip, the throttlebody can warp - and cause the throttle to stick (especially when warm - which I know is not the same pattern that you describe). Other problems can occur with the Idle Air Control Valve (acts rather like the old fashioned choke). A checklist of things to check can be found at

Hope this helps!
Rob Bell


it didn't happen today... err
Yep, I have a spare TPS, but just swapping is no *solution*. I want to know the reason. I read about the output voltage anywhere, can't remember where. Must have been a newer electrical library or desription. I'll look myself and report.

@ Barabara,
we are chatting of course about a strange problem (revs stay at 1500 per minute. Sounds to complicate for the readers of the BBS who didn't follow up the last 7 years ;) and our experiance with the involved parts.
What Rob said is almost the reason.
Though, might be worse checking the easier failures as well:

- the footweel carpet is crumpled under the pedal
- Sticky accelerator pedal
- sticky and/or kinked bowden cable
- throttle body itself does the problem, as written by Rob.

PS. thinking about a new strategy to get behind the problem at my car (minor prob of course)
Dieter K.

Hi Dieter,

have you checked that there is no additional air entering via a partly faulty brake-booster, (servo) ? It is not unknown on other cars that the valve that should build up a steady vacum in the booster can stick open at regulare intervalls and give enough bypass air to increase the revīs. Try to block off that inlet with a thin sheet from a plastic bag for a test. NOTE: There will be NO brakeboost during this test so be careful as much more applied force to the pedal is needed!!

BR, Carl.
Carl Blom

Hi Carl,

checked this all the last time it happened. You gave good reasonabel instructions :)

I think the stepper IACV will have lost some steps or things like that. *why ever*,
But I wonder why it still was there in wrong location (if so) after the engine had cooled down over night.

When I get a grip to the IA hose (big hose at the upper input to the TB) and restrict airflow, then the idle decreases. The more I restrict, the more idle goes down and reverse. I think this is evidence enough for no other air leaks. Removed as well the brake vacuum pipe at the Inlet manifold the last time. Idle increases but not so much.

That's why I though to give more *initial* opening gap to the butterfly (paper adhered on the bolt) when I tried to overcome the problem the last time.

However, at first the TPS voltage rated value and some more measures. Still didn't look for it. *to many other jobs*

No worries, it's interesting only :)
Funny failure, but to hot outside to put my head into the engine bay. ;)

Dieter K.

Strange it happened again... my problem did happen two days ago.

Would pushing the brake pedal during the phenomenon (sp?) change the revs slightly ?

just trying to catch up

Sounds like IACV to me, I had a similar problem. We fixed it by replacing the valve, but until the part arrived, we used a temporary solution. You can fit a piece of heater mose to the manifold, and insert a "Gate" valve in the air supply tube. I used one from a garden hose, it was cheap and easy. Close the valve off until you get an acceptable idle. Be warned that the cold start circuit then won't work as well.
The results are remarkable, you don't realise how much the IACV hampers smooth driving! My temporary fix has become permanent, and the car is much more fun to drive, as revs will drop immediately you come off the throttle. Just a few weeks ago I thought "I don't need this now" and took the "air strangler" off, drove a couple of kilometers and it was so bad I put it straight back on. Anyone who wants a bit of "Throttle response" must try this, it rocks.
Grant Vowles

>>The results are remarkable, you don't realise how much the IACV hampers smooth driving! My temporary fix has become permanent, and the car is much more fun to drive, as revs will drop immediately you come off the throttle.<<

Interesting - might have to look at this - and I don't have a problem with the IACV!
Rob Bell

I can really reccomend it Rob, when you try it, you won't go back, it is a remarkable difference.
Grant Vowles

On todays "electonically controlled" cars that old thing of engine breaking when letting off the accelerator is totally gone... Emission control killed that useful thing that made tight traffic so much more drivable due to the absence of flickering brake-lights... Grantīs alteration might be something to test,at least worth a test by us doing sprints,trackdays etc.

BR, Carl.
Carl Blom

Rather than have an external restrictor, I was thinking of something inside the pipe (so all looks standard). Might screw up the standard idle control though - but I guess this is what a little experimentation is all about! LOL

Hmm - how about a electromechanical restrictor that is operated when the EPAS is switched off? ;o) Sorry, day-dreaming again! LOL
Rob Bell

Hey Rob, I got there ahead of you! Thought of that and also an Electromechanical or cable operated valve I could control from the cabin. These were options to replacing the expensive IACV, but I couldn't source parts small or unobtrusive enough (particularly in 12 Volt form). Suprisingly, the best options came from home air conditioning systems.... I find that I have adjusted the valve and basically left it there, the only time it's a problem is when the car is stone cold and the accellerator needs blipping a few times to get a steady cold idle. You could get the same effect with a restictor in the piping with a small (maybe 1/4") hole drilled in it, you don't even need an adjustable valve.
Grant Vowles

Great minds think alike Grant! ;o)

I'll look into this later in the year (got plenty of other car - and house - projects on the go as it is! LOL)
Rob Bell

This thread was discussed between 04/08/2003 and 13/08/2003

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