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MG MGF Technical - MG TF Misfire help please

Hi all, I'm trying to help a friend out with a problem on her mgtf and wondered if anyone can help as I'm stumped!

This fault is on a 2004 TF. The car starts and idles ok but as soon as we start driving the car misfires badly, sound like running on 2 cylinders. When we stop it idles fine again.

I have checked the car with my ODB reader and no faults show up.

She has taken it to her regular garage who can find no fault but suggest that it may be a faulty fuel pump. Does this seem feasable?

I wondered about coil packs but nothing showed up on the fault code reader.

Any suggestions welcomed.


Andy Kenning

Has to be fuel or ignition.

Does it rev freely under no load?
Ignition systems can give problems when under load that don't show up at idle.

Would a faulty coil pack give an OBD fault code? Maybe it would, I just don't know.

If you can hear the fuel pump ticking eagerly just before starting that is a good sign.
Has the fuel filter been changed recently?

Please keep us informed if you manage to cure it!
JB Anderson

What model / engine? I had a quick look in the archives, 2 possibilities were lised, dizzy rotor on the way out or low voltage to one of the coil packs.
Leigh Reid

I believe the TFs did away with distributors and rotor arms by 2004, but the HT leads do break down and cause problems.

I would change them anyway, and plugs too.
JB Anderson

I've had issues with leads on my VVC, however the symptom was refusal to start when it was damp. You are correct though, no dizzy arms on the TF.
Leigh Reid

Once the usual simple suspects have been eliminated with a hand held diagnostics operated by a passenger in the car that provides live time readings of the system operating check the MAP sensor readings.

Expect in the region of 32 to 38kPa at natural idle on a warmed engine and then put the engine under load (driving) and expect the readings to rise to something over 65kPa with a throttle that is partially open and closer to 90 to 100kPa on a fully open throttle. Anything outside of those readings under load conditions would indicate a faulty MAP sensor or degrading electrical connections in the sensor plug.

Next look at the upstream lambda sensor reading (exhaust oxygen sensor closest to the engine) and expect a rapidly fluctuating reading of between 0 and 1 volt with the changes being at a rate of about twice a second on a fully warmed engine. The range should be quite widely fluctuation between 0 and 1v and if there is slow changes then this points to a sensor issue, or if the readings are fairly static that could also be a sensor issue or if the reading is hovering around either end of the voltage ranges then that could indicate a seriously rich or weak mixture, but then this should over time generate a 'mixture out of range fault'.

Lastly if the rev counter needle is jumping about and totally out of synch with the engine rpms, when the engine is misfiring, move to look at the crank sensor signal and expect to see a changing signal between 0 and 720 degrees, any missing signal display can indicate a sensor problem that may be nothing more than the need to clean the end of the sensor probe which being slightly magnetised will pick up small ferrous debris particles from the rubbing of the clutch and flywheel that creates a 'fir tree' cover on the probe that can mask the sensor from working properly. It could also mean a more deep rooted sensor issue.

Roger Parker

Based on MGF experience:-
Change the HT leads to good quality silicone. I did this at around 20k to cure a wet weather misfire
Change the plugs. Mine "lifetime" plugs were changed at 40k and cured a horrible misfire under load

Both are DIY jobs but a long plug spanner is needed and the plug leads should be changed one at a time to make sue they end up on the correct cylinder. There is no distributor on my VVC model, only two coils, each firing two cyclinders. I believe that the non VVC engine had a distributor.
Brian Highe

This thread was discussed between 23/07/2015 and 12/11/2015

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