Troubleshooting missfire

pmc_3 · Nov 3, 2008

Hi guys,

I've had an early 944 for a couple of months now, its an 82 Lux with the 2.5 lump. I've been tidying up bits and pieces but have been trying to cure a slight missfire for a little while. Initial throttle response is good and then between 2-3k revs it is a bit wooly and you can feel it slightly kangarooing, if I hold it at 2.5k stationary I can hear it missing.

I started by changing the dizzy cap and rotor arm, then the plugs and now new leads and its not really any better. I was thinking fuel filter next. It does seem more of fueling issue as its fine under full throttle and pulls hard to 6k. What else is it likely to be, i've heard afm's can go a bit dodgy, my old man wondered if the timing might be slightly out. I've sprayed all the connectors and made sure they are all seated properly, air filter is pretty new and doesnt look manky.

Hope you can help, will make it much nicer to drive!

Fen · Nov 3, 2008

I'd say the fuel filter is unlikely to be the cause if it's fine under full throttle.

AFMs can wear around the bit of the track that gets most use - which is probably the sort of area you deascribe the problem, otherwise it could be a slightly out of adjustment or even failing flywheel sensor.

pmc_3 · Nov 3, 2008

Whats likely to be out of adjustment, can the mixture be adjusted or something?

peanut · Nov 4, 2008

if the car has been stood for any time before you bought it ,which is highly likely , then I would consider checking the injectors . It wouldn't hurt to have them cleaned and checked. I'd also check the plugs . You should be able to see which cylinder or cylinders are being effected by the misfire if you know how to read plugs.
Its a very old car so I would also carefully check all relevant wiring connections under the engine bay particularly the injector plugs.Take off the earthing straps and clean up the connections wouldn't hurt either.

Fen · Nov 4, 2008

My out of adjustment suggestion was aimed specifically at the flywheel sensor. With even the relatively crude by modern standards ECU in the 944, provided the sensors are telling the car what's happening correctly the fuelling should be correct. I'm less sure what is available in terms of ignition adjustment on a Lux.

zcacogp · Nov 4, 2008

Shooting slightly in the dark, I'd be suspecting the AFM as was suggested by Fen.

Taking it out, cleaning it and adjusting it so that the wiper wipes on a slightly different arc of track is pretty easy, and could well solve your problem. Well worth a try. Clarks Garage tells you how to do it too.

Oli.