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MG MGF Technical - Alternator or Dis Cap (again)???? HELP!


OK I need advice. There's a bit of history to it as well.

Basically my car flattened it battery, last night we jump started it - evrything went fine. Went to put petrol in it (5 minute drive) and on the way back it displayed the classic symptoms of a soaking distributor cap.

Problem is this is the third time it's happened in a year! first it happened last march, got it fixed (at an mg specialist). problem happened again in July, new dis cap & Rotor arm. Now it's happened again 6 months after it was last done!

Can it really 3 times in a year? are the replacement dis caps really that bad quality? I was thinking of simply drying the dis cap and covering it in damp start, would this help?

I was also worrying a bit about the alternator. can this contribute to this sort of problem? If the alternator is having problem it would explain kind of why it was draining the battery and also why the dash lights go a bit brighter as you rev the engine, although the battery light does not come on when driving.

Any ideas welcome

Hi Jon,

Even when the alternator output has dropped to below the level that the ignition circuits demand, the warning light will not come on. Has the output ever been tested?

Mike Hankin

No it hasn't.

Could a dodgey alternator cause almost total loss of power at low revs when you press the throttle past 25%?

Hmm, hard to say. When you say you took it to an MG specialist, was it one with the Testbook kit? That's probably the only way of determining what the ECU is sensing. Not an area I'm too familiar with, but MEMS has inbuilt safeguards against unburnt fuel reaching the catalytic convertor, this may be playing a part.

Mike Hankin

This sounds like a poor spark problem - of which one cause is the distributor and rotor arm. It isn't the only cause of course.

When were the ignition leads last changed? Have you checked the spark plugs? (Despite the spark plugs being long life items, they don't usually survive the full 60k miles that they are advertised to last)

If the leads and spark plugs are new (and the spark plugs gapped correctly), check the coil and its connections.

Good luck!
Rob Bell

Yes as part of the debacle last summer I replaced leads and plugs.
jr ainsley

Check the alternator next - but also consider the possibility that the spark is being mis-timed - i.e. the crank position sensor could be failing... (not unheard of)
Rob Bell

So I need to check Dis cap & rotor, then alternator then cam sensor?

How much is an alternator to replace on average?
jr ainsley

sorry i'm the same Jon, Slough that originally posted the question, i just logged in! (don't live in leicestershire anymore either ;-) )
jr ainsley

New / reconditioned alternators can be found for around 70-80, and an hour should be enough to fit it. I'd advise budgeting for a new battery too, they're never 100% after a complete discharge.
Mike Hankin


Had a go at this problem this afternoon. I removed the Dis cap (what a PITA job!) and it obviously had condensation inside. It looked like the inside of a window on a cold day, although it seemed slightly greasy.

I dried the inside and we're about to jump start it again. Four questions though.

-Why and how would condensation get into it for the third time in a year?

- Where is the grease coming from?

- Will the problem improve when I start using it as daily transport in three weeks time? i.e. will the sustained use burn away the damp, as its only used once a week at the moment and is stored outside.

- Finally if I sprayed damp start on it(or even in it) would this help the damp problems?

Any thoughts welcome, thanks



any thoughts anyone?

Ist it a good idea to spray damp start inside the distributor cap?


Shouldn't be necessary Jon. If the cap remains at fault, then I would suspect that there is a crack that is enabling current to bleed away.

If the flash guard damaged? Very easy to do in my experience (!)
Rob Bell

Everything looks intact, although I will have a close look at the flash guard tonight.

I plan to just try and dry things up and protect it for now, and if it recurs (again :-( ) then think about more drastic measure. I am a bit loathed to keep changing these parts...

Thanks for all your help guys

This thread was discussed between 17/02/2006 and 20/02/2006

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