Hot misfire..... Anyone?

[HoggyR32]

Hi chaps, got a little mk2 MX5 to be used for track days and general mucking about. However it's got a fairly serious misfire. Only starts to happen when it's warmed up tho! Runs sweet as a nut as it gets upto temperature but then starts misfiring very badly when warm. Really pops and bangs. The rev counter also goes a bit mental and bounches about. The warmer the car gets the worse it gets.

My mate has a near identical car so we've been able to try bits from his. It's also been scanned with a snap on diagnostic machine and has no codes stored.

Things we've tried, coils, leads, plugs, ecu, coolant temp sensor, new knock sensor, air mass meter, tested fuel pressure, checked all earths.

Anyone help? Really struggling now.

Cheers lads

[Spangles]

Assuming the crankshaft position sensor also runs the revcounter then I'd start there.

Well actually I'd get it scoped before doing anything.

[HoggyR32]

Spangles said:

Assuming the crankshaft position sensor also runs the revcounter then I'd start there.

Well actually I'd get it scoped before doing anything.

Ah, didn't know that. Could be a good call! Thanks mate

[TheEnd]

Yep, whatever sensor does the RPM, I know the BMW's crank sensors will start showing problems when they warm up, and they also make the rev counter needle go haywire.

[the_stoat]

I would try a different coil pack, they can be fine when cold then give up when hot.