classic misfire

Hi,

I have a 1991 3.9 classic which has developed a misfire. The car will start up from cold and run for about 10 miles perfectly then a misfire develops and the car becomes undrivable within a mile. The problem first started while in stop/start traffic a few miles from home, when I released the foot brake the car did not creep forward as usual but nearly stalled. Pressing the gas pedal brought it back to life but with a misfire. I was able to struggle home using low gears lots of revs and left foot braking. Distributor cap was damp inside, so cleaned and sprayed with WD40 car now seemed OK. Next day same short trip, this time only managed 5 miles before misfire. Had to continue as I was picking up son from school bus, never made it home had to call out RAC. Patrol man arrived after an hour, sprayed WD40 and car then procedded to perform perfectly.

I have changed, distributor cap, plug leads, ignition amplifier, vacuum capsule, coil. Also tried a good ECU but fault still remains unchanged.

any ideas

Remove dizzy cap, pull off lead from coil to cap (Centre cap lead), hold lead in GOOD insulated pliers about 6-8mm from brass strip on rotor arm. Get someone to spin motor over, if spark jumps accross to rotor arm then rotor is shorting to ground and is s**te. Fix-replace rotor arm. Some aftermarket (Non OEM) rotor arms are crap. Got stung with this a few years ago when I had a 3.9 Vogue EFI and was off the road for over a week because of a s****y rotor arm which even the main stealers could not sort out. Worth a try!!

With modern ECU's people tend to look at the worst scenario first-is the ECU shot? Have one of the H.T. Coils burnt out? Look at the basics first, we all know that all an engine needs is a good spark (At the right time) and Petrol/air at the right mix. Usually if the engine is running rough, but not backfiring then it's being starved of the right fuel/mix. If it's backfiring then unburnt fuel is finding it's way into the exhaust and getting burnt there so it's not getting a decent (If any) spark in the cylinder. V8's will run quite well on 7 pots or less-o.k. it will be a bit lumpy but at least will keep going. The basic checks are sparks at the plugs (Good ones) and fuel. If there's plenty of pressure at the fuel rail (On Injected models)-the motor should run. After the basics have been checked then is the time to dig deeper.

I had a similar problem recently (not on my R/R though it was my petrol car).Basicly it would develop a major missfire about 1 min or so after starting,the best way i can describe the feeling is like trying to pull off in 3rd or 5th gear in a small petrol engine.In the end it just died and wouldn't start.After a few hours of checking various things I found the electric fuel pump wasn't running constantly it was cutting in and out.Replaced with a scrappy pump and away to go again.HOWEVER I also noticed while i was trying to get the car going the fuel filter! it still had the pipes connected to it with crimp clips which makes me think its never ever been changed before (its k reg) so its long overdue.When i changed it the fuel that ran out of the old filter was black so i can't help but wonder if the pump was put under strain by the blocked filter and thats why it failed.