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MG MGF Technical - Performance problems

My 1996 VVC has served me well over the past two years. However during the summer I visted the MG show at Silverstone and decided to give the car a make over and add some performance enhances that I have read so much about on the BB threads . Firstly I changed the spark plugs and HT leads from Browns & Gammon, then fitted a K&N induction kit and finally a Pheonix exhaust rear box system.

The car ran perfectly with a sweet noise which brought a grin to my face evey time I pressed the accelerator.

Unfortunaly of tlate the car has developed a what I can only describe as a stutter...this happens when a decelerate....for excample approaching a junction drop in to second gear the car starts to jerk back and forth.... sometimes mildly sometimes quite severly . TYhis can be quite embarasing when there are bystanders. I have had the car into the rover dealership and they carried out a testbook check reset the stepper motor and it still does it. I took it back they checked again and said every thing was fine all enging sensors are opperating correctly ! The problem I was experienceing was due to the performance parts I had fitted ie the K&N and the exhaust. The dealer said the car was struggling due to the back pressure and this would be causing the judder. They recommended that I take it to be rolling road tuned as they could not perform the work.

Could any one please let me know if the have experienced a similar problem . I know a lot of people must have fitted these parts and not had any difficulties. This is as far as I wanted to go in upgrading the car.. I realy don't want to have to revert back to the standard air filter & exhaust.

Can a VVC be tuned whith out going to great expence ?

Please help as I would not wish to be causing any dammage to my car

many thanks



I had a slight stutter on deceleration, but it was more that I would not reliably get any engine breaking.

It turned out to be a faulty throttle position sensor (well, the wire to it!!) which was causing the MEMS to not retard the engine. As it was an intermittant bad connetion I was getting stuttering.

My dealer had checked for faults and found nothing wrong with the TPS.

Others will probably have better suggestions but I hope that this helps a little.

Paul Nothard

No worries against the tiny upgrades with different exhaust or airfilter. The dealer daubts are total crap.

After checking the problem reported by Paul, I would also look for the lambda sensor then.

Dieter Koennecke

I would suspect a problem like paul suspects above

However I am not sure if the MGF Fuel Injection is self learning so there may be some truth in what the dealer is saying

I base this on my experience of motorbikes with fuel injection

On a susuki hayabusa fitting a new exhaust requires re mapping of the Fuelinjection/ignition system - many dealers told owners that the Fuel injection/ignition would self learn and remap itself -IT DIDNT resulting in poor performance and valve damamge!!

In my opinion fitting a K&N just allows for a better air flow which the FI SYSTEM detects and adjusts for just like on a bike - and results in a cleaner burn

However a new exhaust changes the resonance and back pressure which the valves rely on to operate correctly - therefore a rolling road set up would be required - especially if its a performance exhaust!!!

Thanks every one for your responses..

Paul, What you have described sounds like my cars symptoms. How did you eventually discover it was a faulty throttle position sensor wire ? I've been back to the dealers twice within a week and they are saying all is fine. How did you get it fixed ... is it a DIY job, cost etc

Dieter, Lambada sensor.. how can I check these for faults without the dealer ?

Tony ... Valve Damamge now your scaring me.. I don't want any expensive repairs.... The exhaust is a Pheonix performance system. Although I have been advised by a K-series engine buff that there is very little a rolling road set up can do for a VVC and I would just be wasting my money. Can you advise further on what this check would actually do an dwould it suit a VVC engine without any further mods ?

Many thanks in anticipation for your continued assitance.

Happy New year


Ps I would very much like to hear from any one who has fitted a Pheonix exhaust system in conbination with a K&N i filter and what you think !!!



I've got the same set up as you entirely, and haven't experianced any of the problems that you have mentioned.
I would also suspect it's a dodgy sensor and it might be worth finding another dealer to check it out......

MG Dealer Guide at:
HTH Paul Lathwell -not at my computer :o)

>throttle position sensor wire ?

assume Paul has something better to do then writing.
I may answer... Not any wire was the trouble, but the sensor itself.
... he got a second hand one from me the other year.
He did exchange of JZX 3491 by try and error and it worked for him.

The Archive (technical) contends the resistance of the Potentiometer (sensor)
Exchange is a very easy job, but the part es not cheap and can be supplied from MGR only.
Potentiometer symbol
3 position connector. The resistance between the positions at closed throttle are

1-3. 977 Ohm
1-2. 2000 Ohm
2-3 2620 Ohm

fully open
1-3. 2550 Ohm
1-2. 2000 Ohm
2-3 1004 Ohm
I daubt on a static failure. Will be a problem only when the part is moved.

Checking the lambda sensor is another case
The resistance of the
heating element can be measured using a
multimeter and should be 6 §Łat 20”Ę C. Never tried myself. Data are from the workshop manual.

PS. Cheers with another glass of nice red wine :)
Dieter Koennecke

Frank - The chances are the set up is fine and please dont get alarmed - your problem sounds like a sensor rather than fuel mix problems

however ------>>>>>>> its a performance exhaust SO !!!

CALL PHOENIX AND ASK (if they tell you the exhaust is a straight fit and no remapping is required get that in writing)! then your covered!

If the set up is the same as a bike then you will need to buy a new "programable" ECU unit and remap this to suit the new exhaust unless the manufacturer states differently

You then run the car on a rolling road adjusting the fuelling/mapping about every 200-500 rpm until you get a good mixture right across the range - this should be done proffessionally and isnt cheap (REMAP ECU £300 and add about £100 for rolling road set up)!

Just because someone else is running with no problems doesnt mean you wont suffer problems yourself so make the call! -I advise Sue to do the same as well !

A cheap method - not accurate but gives a general indication to check an incorrect fuel mix is to inspect the plugs - they should be greyish brown after a long run

If they are deformed or burnt/white the engine is running lean - not enough fuel and overheating in the cylinders

If they are black the car is running rich -too much fuel - which is preferable as it will run cooler but will lose fuel economy

My one concern is that the F engine is high compression and runs quite lean anyway giving good fuel economy

A good engine tuner wouldnt fit a performance exhaust without checking the CO2 across the rpm range in every gear and there is good reason for that - phoenix may have already done this testing and it may be fine!!!!!

This thread was discussed between 30/12/2001 and 31/12/2001

MG MGF Technical index

This thread is from the archive. The Live MG MGF Technical BBS is active now.