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MG MGF Technical - VVC Loss of power :-(

Hi, Iíve got a 97 VVC which has developed a strange problem. It run fine for normal driving around but if I really put my foot down it does not pull as well as it should and at just over 6,000rpm it starts to loose power whereas before it used to really start to pull at that point. So basically has anyone got any ideas what could be causing it? Is it worth having rover check it out with their test book thing. Its very annoying because apart from the loss of power at high revs there is nothing else apparently wrong!


James King

air filter? Restricting flow at high revs? Perhaps it needs cleaned/replaced.

Possibly but I would have thought that if it was the air filter it would have gradually got worse, whereas this literally just happened half way through a short journey, one minute it was fine the next it wasnít!
I will check the air filter though. I also thought about replacing the HT leads and spark plugs but I went to Halfords and they donít have them listed in there books!
James King

Plugs and leads usually,

I checked the air filter today and although it is grubby its certainly no dirty enough to cause the performance loss. I havenít checked the plugs and leads yet, however it has been a bit difficult to start on damp mornings so it could well be a cause. One other thing that I noticed today is that the oil temperature is a lot warmer then normal, could this also be a symptom?


James King

Whilst plugs and leads are degrading items that are often left in sevice too long for best performance in this case I suspect the problem is more deep rooted.

The symptoms described are classic for two specific faults, which I have yet to see on an MGF, I have certainly seen them on a number of other cars from Ford, VW and a couple of Rovers.

One is a faulty Lambda sensor (oxygen sensor in the exhaust) This usually has the ability to display it's presence in other areas of the engine operation, such as acceleration before the engine becomes 'congested'.

Second is a physical blocking of the cat, which is a problem that acts just like a rev restricter. Get to the same revs in whatever gear and you run into a wall stopping you progressing further.

The simplest diagnostic check for the Lambda sensor is to use a Dealer. However many good garages will have the gear to actually test and replace the sensor without the need to see the dealer or his spare parts.

Checking the cat is a simple afair, except for accessing the sensor hidden as it is at the back of the exhaust manifold, next to the engine block. IN this case the sensor is removed and a special pressure tester adapter screwed into it's place. I have such a device as I anticipate this problem getting more frequent. The engine is then run at varying rpms and the pressure in the exhaust read on a very accurate gauge.

Another way of checking the cat is blocked or not is to use a cat replacement pipe as this will remove any restriction. However don't bother taking the car for an emission test as it will be way out of accepted limits.

Replacement cats are usually associated with dealer and yet here there is also the option of using a non dealer part, which can be supplied new by a number of companies at significantly reduced prices.

Roger Parker

had this too: solution was replacing spark plugs (one was almost not working anymore, causing my F to run on 3,5 cylinders)

Once had a similar problem on an Astra GTE, and it has not been mentioned yet, so just in case.....

I had real problems getting the car over about 100/105 mph (on the Autobahns of Munich of course), and the car would sometimes splutter as if it was about to stop and die, then it would be OK again (happened only very occassionally). The culprit was a heavily congested fuel filter. When I replaced it, there were no further probs, and top speed was easily attainable!
Andrew Hay

Thanks for the replies, being the paranoid type I was also thinking that there might be a problem with the VVC mechanism but I guess the car would not run if that was the case. I have given the car to the dealers to have a look at it, Iíll post the results tomorrow (hopefully!)


James King

If the VVC mechanism is not operative you will find that the engine peak revs are reduced to around 6700, but under that the car remains sharp and doesn't demonstrate any real problem until hitting the premature rev ceiling.

The fuel filter tip is a good one that can be checked by replacement at low cost. Time wise it wouldn't hurt and knowing the disturbance some filling station storage tanks have suffered in the last 6 months there is every possibility that a much larger volume of dirt and debris has been passed into your tank.

Roger Parker

Thanks for all the comments (again!). Just to let you know it was the cat which I have just had to have replaced :-(
At least I should get my car back later today!


James King

It's the sensor which detects the cam advance/retard - it happened a few years back on the Gaydon Team spirit car which ran in the Phoenix race series. Without a signal the ECU restricts the variable cam timing and revs to 'safe' parameters.

They had changed some of the wiring and the connection on to the sensor wasn't making proper contact. Apparently the sensor's can fail, but more often it's crap getting onto the connector. It's located on the side of the cam carrier (inlet side) I beleive.

Alan B


All in (parts+labour) how much was your cat to replace?
paul weatherill

Unfortunately I need my car first thing tomorrow so I had to let the Rover dealer fit it as ATS could not get one for a couple of days so I had to pay £439 all in :-( although that did include the initial diagnostic. If anyone else needs a cat replacement then try ATS as they quoted me a far more reasonable £250 for it.
James King

For future reference...
If the oil temp sensor is dead, the VVC unit thinks the oil is cold and refuses to cut in.
Thus a slow VVC car.

Paul Nothard

This thread was discussed between 24/02/2001 and 28/02/2001

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