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MG MGF Technical - Running really rich and lumpy with no power.

I bought the car about a month ago and all was ok. but suddenly the car started sounding really rough and lumpy and is running really rich. I changed the sparke plugs and ht leads but nothings changed. can anyone help or no what it could be?
A Synnott

does it run slightly fast when warm - if so it may be the engine temp sensor, if so then you will also get one of the two following symptoms EITHER

The front fan will run all the time the engine is running (air con switched off if you have it fitted)

OR

The fan will not come on at all allowing the temp gauge to raise above the central position once the car has been sat idling for a long time (switch off or start moving immediatly if this happens)
Will Munns

no its not getting hot just sounding rough!!
A Synnott

Is your car a vvc or mpi?
Chris

if the temp sensor has failed it will not run hot, it will try to compensate _in case_ it is getting hot
Will Munns

its the 1.8 vvc on a r plate 1998.
A Synnott

Worthwhile trying the reset trick with the accelerator maybe?
Leigh

I think would be tempted to change the distributor cap.
Steve Ratledge

He says its a vvc, so it wont have a distributor cap to change.. ;)
Dave

It could be that one of the VVC solenoids has stuck and so the engine is permanently running on the high-rev cam. Try running the engine up the rev range and if it revs freely through to the rev limiter with no lumpiness after about 3.5-4K revs then this is your answer. I had this on my '98 VVC once.
Dave Livingstone

So how would you fix it dave, is it an expensive garage job?
Leigh

Check the vacuum tube from your inlet manifold to your ecu,it may be split.

Regards, Simon
SCJ Connolly

VVC of course uses a direct to inlet MAP sensor so no tubes to split or detach, and no sensitive sensor on the ECU circuit board to become damaged by fuel vapours. You can see if there is any difference to idel if the sensor plug is removed. It will of course generate a fault code within the ECU memory but not the light once the car has gone through a complete igniiton power down sequence (30 second plus ign off) and restart.

Most VVC engine have a metal filter screen around the intake ports of the hydraulic control unit (the alloy tower off the inlet side of the cylinder head). I have seen one where there was no screen and a minute piece of debris got jammed and locked out the control rod so no oil flow variation was possible so the VVC timing was held where it was when it was jammed, which in the case of this car was at around 280 degrees. As with any conventional engine using a wide overlap cam the engine took on a very 'loping' up and down idle and lost much low rpm performance, but then flew above 4000 as would be expected for such overlaps. It did not make for a rich mixture though just a bad idle.

Coolant sensor for the ECU (separate from the gauge and engine bay fan) is the most common as Will mentions. A clue here is good cold starts and initial running but going off after a couple of miles as the temp of the engine rises and the cold running enrichment provided through the temp sensor information is reduced, but if the sensor is faulty it is not reduced so excess fuel when not needed makes the idle and light throttle running lumpy.

This sensor is one that any competent DIY person can do at home. Checking the VVC operation initially needs diagnostic equipment that reads VVC cam dwell, which is much more idely available now than it was 10 years ago, but really only committed owners have this (yes I'm one). Most garages now have diagnostic equipment and many of the newer modules for 'Rover' vehicles now have VVC monitoring function so a small charge at a garage would be in order. The same equipment can do a wide range of other functions aside from the simplistic fault code reading and clearing, and this may offer a short cut to pointing towards the problem or problems.

Rog
Roger Parker

This thread was discussed between 10/04/2008 and 24/04/2008

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