MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGF Technical - Throttle Surge

Hello chaps and chapesses. Happy New Year!

Have an unusual one for you to ponder.

I have a 1997 MGF vvc 1.8. Currently runing standard everything including sometimes sticky plastic throttle body. I've got used to the 1500 - 2000 tickover and blipping the throttle to clear it but the last few times I've taken the car out the throttle seems to over react to the input, especially from a standing start.

I'm too old to be considered a boy racer but find that setting off from a junction or a stop is very difficult without the revs going berserk and me frantically dipping or slipping the clutch to control the resultant wheelspin.

It literally feels like the throttle pedal is being drawn down off my foot by a poltergeist and it is an intermittent issue. Every so often with only light pedal pressure of 2k ish revs it pulls away spinning the wheels and giving oncoming traffic a major coronary, not to mention your truly.(so have seen some very worried loking faces approaching me in other cars)

Have tried the potentiometer reset mentioned in the tech pages but if this is working it is very temporary.

Basically, on anything else other than dry open roads, the car can be unmanageable.......daren't take it near town.

Appreciate any thoughts you have.
Cheers
andy t
Andy Tyson

Is it only happening when pulling away from stationary? Things I'd check for are

1. Excessive engine movement - if a mount has let go, the engine may be pulling against the throttle cable causing it to open further.
2. Failing connections on ECU coolant temperature sender (the brown one) which would cause the ECU to increase the fuelling
3. Failing connections on the throttle position sensor, confusing the ECU and causing it to increase fuelling as a protective measure

Probably worth pulling off every plug connector and sluicing with electrical contact spray & cleaning up with sandpaper if there's visible corrosion. I'd also undo the inlet to the throttle body and check there is no free play in the butterfly, and whilst you're at it check for sticky deposits in the housing & clean out with carburettor cleaner spray.

Good luck, the worst kind of faults are the 'intermittent' ones...
bandit

Thanks Bandit.
Really helpful info.
Will de-thaw the car and check the things you listed at the weekend.
It does just seem to be from a standing start that the problem occurs.
The main thing that worried me was the fact that the throttle pedal was being drawn down off the sole of my foot. Even with me lifting off the throttle once the fault stated to take over, the revs still increased and the car carried on regardless.
Scary stuff !!!!
Thanks for the tips.........will let you know.
Andy
Andy Tyson

Hi Andy,
Check that the outer sheath of the throttle cable is fully located at each end and that there is no large movements of the outer sheath as this will vary the position of the throttle to pedal relationship.
HTH
willyphixitt
W A Nixson

Full marks to Bandit for the first option.
Checked the engine position only to find the remote filler hard up against the bulkhead.
Also the throttle cable and other pipework had been dislodged from their mounts.

Not sure which mount has let go yet. Could just be gone soft or could be the upper bolts sheared, either way, not a cheap or easy fix!!!

I jacked the car up and repositioned the engine. there's about 4 inches of movement at the top. As soon as I set off.......thump!!! the engine shifted backwards under the rotational force and hit the bulkhead.

Will post more when I have it fixed on wednesday.
cheers
andy t
Andy Tyson

Sounds like the upper mount, the two monster bolts were often over-torqued in the factory and the stress eventually takes its toll...

Mine snapped at full chat on a track day, and that was very messy indeed, so count yourself lucky ;o) I snapped two stud extractors trying to extract the broken bolt ends so if you're DIYing I'd favour swapping the complete mount assembly as the better option, IIRC around £20 from a breakers yard, and actually not too difficult to fit.
bandit

This thread was discussed between 06/01/2009 and 12/01/2009

MG MGF Technical index

This thread is from the archives. Join the live MG MGF Technical BBS now