Thread: Mystery misfire

Is the coil mounted on its side ,if it is & has lost some of the oil it can short inside . My father had a RR that did this , also when I worked for BMC we had a few Minis with this same problem.
With penrite oil when I had my business we had a fleet of Ford Couriers & we started using Penrite oil in them a couple developed a rattle from the lifters on start up untill they wormed up . We changed them off the Penrite & fixed the problem.

PLR, The temp is good. No change towing on the highway up hill etc. No coolant lose either. But you could be onto something. My sister had a misfires at idle before from a head gasket leak, so I have been monitoring the coolant. I need to fix my compression gauge and test all cylinders.

1950landy, Yes the coil is mounted on it's side with no visible oil leak. My gut (and my wallet ) is telling me that it is the engine oil as it all started a short time after swapping to Penrite synthetic (Penrite worked wonders in my other car). I bought some 20w50 mineral oil and filter this morning and will try and change it all tonight. I am going away in it for the weekend, so there will be no rest for it this weekend.

My fathers RR was only 6months old when it started , & no sign of oil leak at coil . JRA could not find the problem went rite through Carbys . distributor & replaced valley cover gasket . I was lucky enough to hear the cracking noise in side the coil . I mounted the coil vertical until Rover replaced the coil with out any problems. Just a thought.

Thanks 1950landy. I might just fit a new Bosch coil to remove that from the equation.

You can run a Bosch 717 transformer coil which alleviates these problems.
Be careful not to buy a "GT 40" which are designed for points, although now they may have changed.
And you probably need a 7 volt coil. Check that you have a resistor wire to the coil ie you should have 2 wires to the positive side.
Regards Philip A

Originally Posted by PhilipA

You can run a Bosch 717 transformer coil which alleviates these problems.
Be careful not to buy a "GT 40" which are designed for points, although now they may have changed.
And you probably need a 7 volt coil. Check that you have a resistor wire to the coil ie you should have 2 wires to the positive side.
Regards Philip A

LR didn't fit resistor wire to any vehicle with reluctor based ("constant energy" in LR speak) electronic ignition. Not sure which coils are safe to use on the Lucas module but I'd suggest getting a good used OEM Bosch coil to try out if needed. Bosch list a BIC290 which may be suitable although all new Bosch coils I've seen lately all look like cheap Chinese made crap. A single cylinder misfiring is hardly likely to be the coil though.

I have changed the oil (20W50 mineral oil) and filter to remove that as a possible cause. Maybe it is me being hopeful, but I think the misfire has reduced slightly since the oil change. Friday is usually the worst day for the misfire (it gets progressively worse over the 5 days of use) and I think today it is more like a Wednesday. I am going camping this weekend, so will report back with any changes next week.

No Good news to report. On the weekend I had a low glow/flickering oil light develop. After some track side diagnostics I was fairly confident that it was either the oil pressure sensor (only 3 months old) or some other electrical issue as if I turned the engine off and straight back on again the light would go out for 10 minutes. So after many stop / starts I made it home. Come Monday morning the oil light would not turn on at all. I tested the oil pressure sensor and it is stuffed. I carried out an oil pressure test while I had it all apart and at idle I have 28psi and @ 2400 I have 42 psi. All within the specs of the WSM. So now I can eliminate oil pressure as the cause of the misfire.

The misfire seems to be gone for 5-10 seconds (hard to tell as it is fast idling during that period) after sitting overnight before coming back. I still think that the 20W50 has improved it slightly.

1950Landy, you were sort of on the money about a cracking sound at the coil. I could hear a zap zap zap sound with every fire. I went to pull out the coil but found the condenser cracked. Now the noise is gone. I will get a new one when I can.