Odd intermittent misfire

[MGB3405]

Hi

I still have the odd misfire problem..

4 litre pre-serp (full de-cat) 1994

When engine gets hot 85 - 90 (normally sits at 70 so only after traffic) the engine won't rev past 4000 it acts like it has hit a rev limiter (pulls like a train up to that in all gears) and clears as soon as the engine cools to 80.

Rover guage shows correct map (2) and no engine faults.

I have changed

Plugs, extenders, leads, coil, dizzy cap, rotar arm, ignition amp, coolant temp sensor, air filter.

the fault is a lot better than it was but still there

any thoughts would be appreciated

Edited by MGB3405 on Friday 2nd May 22:41

[MGB3405]

now also changed the fuel filter (pig of a job) still happened after parking....... any thoughts?

[FlipFlopGriff]

Was it like it before hand? If not swap out each item one by one. Sounds like the ignition amp but could be anything. Maybe you hgave afaulty new item.
Have you reset the ECU - unplug or take the earth off the battery for 30 seconds and reconnect.
FFG

[jamienshelly]

I have this twice and has been different faults both times.
the first time was the 100 amp fuse on the chassis broken and the 2nd time was the AFM.
I was informed that if the TPS goes wrong then that can produce a similar fault but that should throw up a fault (I think).
Check the voltage at the battery with it running, if its above battery voltage then the fuse is ok.
but as for testing the AFM im not sure as I had a loaner and just swapped it over.
I hope this helps.

[TV8]

I had a couple of your problems on my pre-serp and it was the AFM. If you are near to Bromley I have a spare you can try if you want? Also, your car may not have one of those 100 amp fuses. Mine didn't but it didn't stop me wasting a lot of time looking for it....

[MGB3405]

Thanks for the advice, problem gets steadily better as I change things but is still there. I guess a look at the AFM is next on the card. TV8 thanks very much for the offer but bromley is a bit of a trek, I have also searched in vain for the 100 amp fuse.

[MGB3405]

So the saga continues......

Thought I got the reason sussed but not the cure

noticed one cooling fan not working, so stripped and cleaned connections, cleaned the easy earths (drivers wheel arch and rear of engine) car then grieving like a dream.

Until there is a big power drain such as cooling fans and heater fans on at the same time (to the point that the passenger can control the misfire by turning the de-mister on.

this worked fine for the day and in excess of 100 miles.....

parked up for a couple of weeks, now driving really bad, perversely OK flat out but shunting all over the place at low speeds with the odd massive backfire and oddly speedo running about 20 mph slow

funds limited at the moment due to imminent gearbox re-build so any thoughts welcomed

[Chuffmeister]

Have you checked the lambda sensors?