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MG MGF Technical - TF160 Misfire

I've just had the head gasket replaced on my TF160 (VVC). It was a very small cylinder/water leak, not water/oil. On the way home I felt a hesitation at light throttle. Revs fine, will hit the red line, managed 60mph in 7s ish on the watch - so obviously nothing major amiss.

This morning (raining) I took the car to work (trying to get some miles on the clock to check that there is no water loss) and there is a chronic misfire. Only happens with light throttle; it's fine on the overrun, and fine if I give it a bootful. Slight jerk when I lift off the accelerator. Now again it's absolutely fine, but then the misfire starts...

Plugs were replaced 2,000 miles ago, but not leads. Question is, I suppose, is this just a coincidence and I should bite the bullet and replace the leads - or should I take it back to the garage? Presumably plug leads are easy enough to change? Any thoughts? COuld it be a loose connector - if so, any thoughts on which ones to check?

Neil
Neil22

The 160 VVC engine has 2 coil packs, not HT leads in the traditional sense. Each coil pack is mounted directly above one of the plugs & feeds one other plug via a very short HT lead. It is possible that the short HT lead is failing, or perhaps one of the coil packs isn't performing as well as it should. Can you make the engine misfire by blipping the throttle? If so you could try unplugging each of the short leads in turn and seeing what effect it has.

Also worth taking out the plugs and inspecting them - either for dirt, poor combustion evidence, or it's not impossible that one was dropped during the gasket change and the gap is no longer correct.
bandit

Bandit,

No, it revs fine with no load, revs fine with foot to the floor, misfires if engine lightly loaded with small throttle openings.

Brought the car home this morning, went in the company car...

Home again now, went to move car from drive and it feels like an old car would feel without the choke. Doesn't really pick up, but okay once the revs are there - say 2000 rpm.

Neil
Neil22

Hmmm, is this the MAP sensor... and would not having it connected make a difference. I think I know the answer to the second question (!) can anyone answer the first please????

Neil

Neil22

Okay, so I know what the problem is. That is the MAP sensor; I've found the wiring plug that goes on it, but it will not reach - presumably trapped somewhere.

Manual says "if the MAP signal is missing, the ECM will substitute a default reading... reduced driveability... not immediately apparent to the driver." I think that the last bit is code for saying that it will not cause the engine management light to go on?

As I've not got triple jointed fingers, I'll need to get the engine cover off to have a proper look. Who would knowingly not reconnect a sensor because the wiring will not reach? Answers on a postcard...

N
Neil22

Hadn't spotted your other post - seems you've nailed the issue.

Agreed, a bit worrying that it escaped re-connection by the mechanic, hopefully a simple re-routing will fix it, wrong side of a coolant hose perhaps. Rest of system must be in fine health if you can still do scalded cat impressions...
bandit

This thread was discussed on 29/03/2010

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