Misfire not logged

[boggie]

Hi All,
My 996 Turbo has developed an occasional misfire. A short but significant misfire (across multiple cylinders) and only on full throttle/power at high revs. No engine check light at the time. I took it to Porsche to check the error codes, but there is nothing logged. Any ideas on what it could be please, and why it is not being logged?
Thanks,
Ian

 

[Dave jones]

Could be something to do with the spark plugs not firing correctly but you're still getting the pulse from the coils so no engine management light will come on

 

[Motorhead]

Ian,

A single cylinder misfire would indicate a spark plug/coil pack issue, but if you’re sure that the misfire is occurring on multiple cylinders surely that would indicate a DME issue or something in the circuit between the DME and coil packs and likely to be picked-up by a diagnostic check?

Jeff

 

[boggie]

Thanks for the replies!

Yes, it feels like multiple cylinders to me, a very noticable misfiring in a half-second or so significant loss of power before everything kicking back in again up towards redline. I was wondering if it could be fuel pump or MAF rather than a coil pack or one of the new plugs (the car has just had a major service).

I recently returned the car (a 70K miles, FSH, 2004 X50) to the road after 6 years in storage. Initially, the fuel pump did have a slight whine but this soon disappeared. I thought that potentially it was hunting fuel, as i only put a gallon in it for the first start, but the car was on a slight nose uphill slope, which should have meant the 2x pickups were submerged at the rear of the tank.

I have pulled the MAF out as I thought it could be contaminated (I have an oiled BMC filter). It looked OK but I have given it a good clean with MAF cleaner, just in case.

Cheers,
Ian

 

[Motorhead]

If the car’s been laid-up for such a long time Ian maybe it’s a fuelling/fuel contamination issue? Have you thought to change the fuel filter? I assume that you’ve now put in a decent tankful?

Jeff

 

[boggie]

Hi, thanks Jeff. The tank was drained before starting with fresh fuel and had a major service as part of recomissioning, including plugs and all filters.