Top end missfire

[brummiewedge]

Hi guys, my wedge is on the road! Passed the MOT yesterday!
However this has brought to light a top end missfire at 4000rpm plus, it pops and bangs but is ok below these revs.
I thought that the ignition was advancing too hard and crossfiring but i dont think it is. I have 2 ECU's standard and JE 5 litre, they both behave the same.
I want to put some miles on the car before i take the 4.0 litre out and slip the 4.6 in.
The school kids and white van men love the car though!!

[rev-erend]

I'd put my money on the ingition breaking down somewhere - try new cap & rotor arm (if not new) - try substituting a cap & leads from some elses car (if theirs is know to be OK) - new plugs (if not new) .. also try coil swap..

Swaping is a cheap way of fault finding

[brummiewedge]

I bought thr car as a non-runner, but it looks like someone else has tried to sort this before, because the cap, leads, rotor arm and coil look very recent, but you are right i will try my mates stuff off his disco or TR8.

[number 7]

I can't tell if its a hotwire or not - if it is, they tended not to have the artillery soundtrack of the flappers. Possibly the ignition amp going - can't remember where they are on the 400, but if on the side of the dizzy, the heat eventually gets to them.

7.

[brummiewedge]

Its hotwire, and the pack on the side of the dizzy does look as if it is original, so it could be worth a go, whole dizzy swap i think.

[chunder]

Had exactly the same problem - ignition leads breaking down to start with and then the coil wasn't man enough to drive the new set at high revs.

Maybe try measuring the resistance of the coil against a new one - can't remember what figure it should be but posted recently so a search would find it - or swap to prove it if you have access to one you know is ok.

[wedg1e]

When I had a high-rev misfire, it was the pickup coil in the distributor, and not the ignition amp, that had failed. Could well be that they are all at 'that age'...

[tinker-27]

sounds like your cam has a exhaust lobe missing ?

[hamish400]

Paul,

An alternative possible cause for you.
My 400SE suffered similarly when the fuel pump was a bit worn.
Simple test was with a full tank of fuel the engine would run to higher revs than with a low fuel tank level. Obviously the head of fuel to the pump inlet was affecting its efficiency when the pump was a bit worn. With your change to 4.6 litre engine the pump that worked OK with 4 litre engine may now be marginal. If you have recently fitted a new fuel pump then this cause is unlikely.


Rgds
Hamish

[dickymint]

If your happy with plugs (change them anyway), leads, timing etc. I would look at changeing your coolant temp sensor (cheap at about £12) then look at your stepper motor.

[jack valiant]

Had a similar problem on mine and it turned out to be the cam shaft after we had exhausted all the other areas mentioned already. Good luck

Chris

[brummiewedge]

This was sorted weeks ago, dizzy was just free to do what it liked. But thanks guys!