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MG MGF Technical - [MGF] Engine stumbles and stutters

I have a MGF 75th LE 1999 with 60.000 km.

Suddenly, and without knowing why, my car started to have a problem. Here its description:

I drive at 2500 rpm, in second or third gear. I release the gas throttle completely and I suddenly push down the throttle. The car accelerates but with great jolts: engine stumbles and stutters. It makes only when accelerating with low rpm and only when I completely release the throttle (if I open gases slightly and suddenly push down the throttle nothing wrong occurs).

Problem also appears if I drive on the motorway in fifth gear. If a release the throttle the car gives a jolt. If I press the throttle the car gives another jolt.

And when the outside temperature is high (in summer for example), the idle speed fails to return to its 875rpm idle speed and stay in the 1200-1500rpm range.

I read a lot of threads on this forum relating this problem and here the points I checked/changed but did not solve the problem:

-Change of the spark plugs
-Change of the ignition leads
-Change of the rotor distributor head
-Change of the water temperature sensor
-Change of the air filter
-Clutch check (no problem)
-MEMS ECU check with dealer (no error)
-TPS check and reset (no problem)
-Throttle body dont seems to be dirty or sticky (but I didnt remove it)

Do you have any idea where this problem may come from?

The next thing I want to change is the throttle body with an alloy one. Is it a good idea?

Thanks for your answers.

Best regards,


PS : Sorry for my poor English but I'm French.



Bob Millar

>> -Change of the rotor distributor head <<

Well, this was my immediate thought when I read this! You've replaced both the rotor arm AND the distributor head?

I think that the throttle body is unlikely to be the cause here Benjy. The areas I'd investigate next are:
1. fuel pump/ fuel pressure regulator: are you getting adequate fuel to the injectors?
2. Crank position sensor - these can and do fail - NeilW most recently had a similar problem which turned out to the down to this sensor
3. Are you getting an adequate spark? Might be a problem with the coil.

Hope this helps

PS your English is excellent! :o)
Rob Bell

Yes maybe I don't have check it.

But there is no idle speed problem except when outside temperature is above 20įC and the engine bay is hot. Maybe a distortion of the throttle body due to heat ? Because mine is in plastic.

But I'll look at it anyway...

I was talking about IACV in my previous post.

Thanks for your answer Rob.

Concerning the "rotor distributor head", I have to check what exactly the mechanic changed and Iíll give you a good translation.

Fuel pump/fuel pressure regulator and crank position sensor: Iíll investigate in this way...

I bought my sparks in my MGR dealer. They are exactly the same as the old ones. Nothing changed when I replaced them.

Okay, good luck Benjy :o)

PS when you fitted your spark plugs, did you check the electrode gap?
Rob Bell

Yes I checked the electrode gap.

For information the machanic changed :

Cap distributor (NJD10010)
Rotor distributor (NJE100070)
HT leads (NGC000090)

Okay - so that really leaves fuelling or an ignition fault be that with the coil or with the MEMS (most usually the crank position sensor).

Keep us posted on how you get on Benjy :o)
Rob Bell

Don't think it's anything with IACV. IMO a faulty IACV will only trouble near idle mode.

Anything blocked with vacuum system ?
I hope for you it's not the pressure sensor inside the MEMS ECU.
Not sure whether Testbook shows any analogue signal of the sensor (manifold pressure)

How's the idle when the engine is in cold and warm condition ?

Good luck
Dieter K.

Thanks Rob and Dieter.

When engine is in cold, idle is higher (as it always did) and progressively get down as engine became hot to stabilise at 875 rpm.

But car have more jolts when engine is cold.

The only weird thing is that idle takes a few seconds to stabilize when I give a short and quick gas throttle when the car is stopped.

The mechanic did an adjustment for the close throttle from a screw located on top of the throttle body with the computer plugged on the car. But this changes nothing except idle which is now at 900rpm instead of 875rpm before.

Next week-end Iíll change the body throttle with an alloy one and clean/check all air intake system (ICAV, vacuum, tubes connected to body throttle etc.)

Just a question: Is it an air intake temperature sensor? To inform MEMS if air flow is cold (more oxygen) or warm (less oxygen)?



This thread was discussed between 17/02/2004 and 19/02/2004

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