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MG MGF Technical - Help sought for engine speed problem

Hello,

Here's the first of two problems that currently have
two local MG dealers beat.

My car is a 70k unmodified VVC (60k service was at 58k
miles).

For several thousand miles, I have seen a problem in
which the engine speed drops right off to zero, before
immediately picking up again. It feels very much like
it is about to stall.

It happens in neutral when idling, and also when I am
driving, although usually only when I am driving in
town (low revs?) --- never when on the motorway.

In the last 500 miles, I have also seen, when in
neutral, the idle speed go up by a few hundred rpm,
stay there for a second or two, and then come back
down again.

The idle speed is usually about 600 rpm according to
the tachometer; I am invariably told that the device
is not accurate. However, I *think* I remember that
it used to show 900 rpm at idle, and it certainly still
does 70mph in 5th at 3200 rpm, so I don't think it's
totally wrong.

A couple of garages now have had a brief look at this
problem as part of other work, but the last report I
had was, "We could take the car in to look at this
problem but I doubt we'll work out what it is."

So, needless to say, if anyone has any suggestions that
I or a garage could try then I'd be most grateful!

Regards,

Robert
Robert Harris

My first thoughts are to check out the throttle potentiometer. It may have a dirty or worn track. Not easy without the right equipment. The only way you could do it is to replace it with a good one and reset the idle speed.

To do this you turn on the ignition but don't start the engine. Floor the throttle five times in quick succession (within 10 seconds) and turn the ignition off. The idle should now be reset.

If you're patient enough I am coming down to Hampshire (Romsey) at the end of the month and I can lend you a throttle pot to try.

cheers

Bruce
Bruce Caldwell

>>Here's the first of two problems that currently have
two local MG dealers beat.<<
What's the second problem that's got them beat ?

Have the dealers plugged in testbook (their diagnostic computer) and do you have the results from this ?

Dealers are notoriously poor at solving electrical problems; look through yellow pages for an auto electrician.

Your tachometer is accurate enough for this sort of work - and 600 RPM is too slow. Idle speed is controlled by the computer, using a special air valve (throttle should be closed) and close ignition advance/retard control. Bruce's 2nd paragraph describes how to reset this.
I guess you're not using idle on the motorway ...

Also, check the throttle action - can you rotate the throttle butterfly easily ? (is the computer trying to compensate for a 'dirty' throttle?)

Steve
Steve

I do hope you problem is not similar to mine.

My car is and has been at the garage for over 4 weeks solidly plus weeks before with a intermitant fault which the only signs I had apart from problems driving it was the rev counter going up and down and with the car dying. At the end this was happening whether I would be idling, going slow or motorway cruising.
When idling it would be at the correct speed when it was running correctly.

Mine has done 72000 miles and is modified but only air filter and exhaust and is a 1.8i.

I am wondering whether there is a design fault as there has been in the last few months more and more cars with similar symptons apearing. Mine has had all the wireloom replaced and am currently waiting for the ecu to be changed as the feeling is the wire loom shorted it out.

See the thread "2 weeks in the garage and the verdict is"

Tom
Tom Randell

Robert

I had similar problems to you, but also when driving, the car juddered every now and again. I also had slow starting issues The problems were cured when I replaced the battery.

Stefan
Stefan Gibney

Hi Robert,

if you think it might be the throttle Pot, then I have an old throttle body from my old F, also a VVC, which had about 35K on it when I replaced the body with a trophy one.

As you are also in Hampshire, you could collect. It's fairly easy to change.

regards

Paul
Paul

Try cleaning the stepper motor valve, Greygate can be used for this,lol,
Mike.
mike

This thread was discussed between 08/09/2002 and 14/09/2002

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