3.9 intermittent engine failure

Hello all

I wonder if any experts out there have any idea / suggestions about a problem I'm having please?

I have a issue with 3.9 where by intermittently the engine will cut in and out rapidly and violently.

Its always happens within 5ish miles so a cold or just warming engine, always about the same speed at about 45mph and 2000ish rpm - it will suddenly nose dive and recover as if I am lifting on and off the right pedal rapidly but without me moving the pedal at all. When it happens I have tried dropping it a gear or 2 (auto) the revs increase but its still doing the same violent accelerate decelerate action, the rev counter rising and falling rapidly. The only way to get past it is to accelerate and if you put your foot down it will pull hard as it should and get past the problem and continue to drive normally.

So to me it seems to be speed related and temperature related. It is not one cylinder failing causing rough running but a total power loss / on off sensation.

Otherwise its a very healthy engine, mechanically good - compressions / general ware etc

I've been over the whole engine, with top end off, tappets lifters are all good, plenum gaskets etc all good and clean, new HT leads, new maf, All sensors relating to engine management have been checked with multimeter inc Lamda / temp / maf / earths / tps, and also have roverguage
The problem was there before and after the work so nothing has made a difference
I doubt a fuel starvation issue as it has no problem pulling past the problem if you floor it
I have never had it happen with a fully warm engine.

Any one had a similar problem?
Thanks

I've had a number of mystery misfires and running maladies over the years. I think you might want to swap out the ignition amplifier and see what happens. The other question is whether or not you have any pattern ignition parts? Genuine Lucas is the way to go - trust me on that.

classic kev said:

I've had a number of mystery misfires and running maladies over the years. I think you might want to swap out the and see what happens. The other question is whether or not you have any pattern ignition parts? Genuine Lucas is the way to go - trust me on that.

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My first thought was coil or distributor, and as you say the ignition amplifier is probably a good place to investigate first. Far as I can see all the ingnition parts are orriginal aside from the leads I just fitted (bearmach)
Thanks