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MG MGF Technical - Mis-firing developing after short drive.

Hi, Can you help, my 1995 MGF 1.8 MPi has suddenly developed a bad miss. The engine starts from cold and idles with no immediate sign of a problem, other than a small, random but infrequent little skip in the engine beat. But after a relatively short drive and with the engine up to full operating temperature it suddenly starts to mis-fire and keeps approaching a stall , then suddenly picks-up resulting in me having to nurse a juddering and spluttering car back to the garage.

The car has been a refurb project (excluding the engine) so off the road for sometime, so I initially changed the fuel filter, fitted new plugs, plug leads and distributor cap but these failed to cure the problem. I then tried a substitute HT coil still with no success.

My suspicions then turned to the engine management system. Having a spare low mileage 1.6 engine (and having checked compatibility) I have progressively swopped the various sensors and at the same time checked wiring continuity to the ECU. The only exception was to fit a brand new Oxygen (Lambda) sensor in the exhaust manifold.

But the mis-fire is still occurring after a short drive. Any ideas/suggestions please?
S Cotton

Did you renew the rotor arm?
See Rob Bells site for info:- http://www.mgf.ultimatemg.com/group2/common_problems/distributor/stuttering.htm

Jeff
J Lennon

Hi Jeff,

Thanks for the rapid response. Did in fact replace the rotor arm at the same time as the other ignition electrics, sorry just forgot to include this in my original description. But many thanks for the idea.

Steve.
S Cotton

Also check the oxygen sensor condition and harness before you buy a new one.
See the "How to test:* here (scroll down to the section)
http://www.mgfcar.de/sensor/index.htm
Dieter

Can you try a second brand new distributor cap and more importantly, rotor arm. As Jeff said, those symptoms shout rotor arm failure. The arm contains a resistor which can become intermittent as the engine warms up.
Charless

Hi, Thanks to you all, normal service restored!!!

Took onboard your advice concerning the rotor arm and as a starter for 10 recovered the arm from the spare engine I have, cleaned it up, installed it and bingo the mis-firing had disappeared, even the random minor missing on idle.

So the recently purchased CI (alias Quinton Hazell) produced rotor arm and distributor cap obtained from a local Motor Factors were instantly de-commissioned. Today purchased a new Lucas produced rotor arm and distributor cap from ‘Abingdon Parts 4 MG Triumph Cars’ and on inspection the improvement in build quality was obvious. And guess what, half the price charged by the Motor Factor outlet.

The Lucas rotor arm has a integral metal bush through which the securing screw goes and is a very close tolerance fit to the camshaft. The CI produced arm had no metal bush so you were tightening directly onto the plastic moulding and was also a very slack fit on the camshaft prior to securing (which could result in a poor rotor arm to distributor cap electrode alignment!).

The Lucas produced distributor cap has a much longer carbon centre electrode (the main incoming HT from the coil) and more importantly I felt, a stronger spring pushing this electrode down harder onto the rotor arm, the CI supplied cap had a shorter electrode and the spring tension was much less.

Once installed the running was even smoother than with the recovered rotor arm and on the test drive the improvement was incredible. No lag from idle, excellent acceleration from low revs and most important NO MIS-FIRE……yippee!!

Having only recently purchased the CI produced rotor arm and distributor cap I had just eliminated these as being the cause of the ongoing problem. Hopes this update may help others who may experience a similar problem. Just goes to show a poor component build quality can result in an early life failure scenario such as this.

Guys, thank you so much once again for dragging me back to basics.

Regards: Steve
S Cotton

This thread was discussed between 02/05/2011 and 04/05/2011

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