996 Turbo: Misfire Not Logged

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

Very possible a coil is breaking down under hard throttle .. the early systems tend not to record misfires on wot .. or close to that .. ie a fast depression of the throttle . A swop out of the coils is the first thing i would do before moving onto fuel pressure checks .

Thanks for the reply deMort. 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... 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. Oh, and as part of the return to use, the car had a major service, including new plugs too. Cheers, Ian

In Essence .. WOT .. Wide Open Throttle .. or in this case a fast depress of the accelerator pedal .. Injection time will go from approx 2.4 Ms to 11 Ms .. there is a massive load on the engine which can be seen on a tester .. 70 plus load value .. normally that figure would indicate a seized ancillary but not in this instance . With WOT on the early cars there is no lambda control and there seems to be no misfire monitoring .. all the car cares about is dumping as much fuel as possible into the cylinders along with a wide open throttle .. A coil fires a spark in a compression environment .. the higher the compression ( pressure ) the more power the coil needs to generate to overcome this and allow the spark to jump the gap . The end result .. misfires under a hard throttle input with no recorded misfires . Coils is what i would start with , i would literally get a set from parts , fit them and test .. quick and easy check .. A removal of plugs and an inspection for plug " colour " is another indication .. yes it can be other things .. maf , fuel pressure .. but we need to start somewhere when there is no fault code .. Fuel pressure and high rpm only is what i would expect .. maf will give a rich or a lean mixture . The other thing i'd like to point out .. I had this exact issue on a car ( not turbo ) last week and it was X2 coils breaking down under a load ( a high pressure environment ) .

Ah, yes. That's why my coil tester has a 10 - 15mm air gap for HT to jump at atmospheric pressure. Yes, the issue happens with the throttle wide open for a long period of time (doesn't seem to happen on short full throttle bursts), i.e. taking the engine up through the rev range. It pulls strong and clean then (occasionally) bogs down briefly at full throttle and mid to high RPM, then recovers to continue to rev through towards redline. I guess if I am working a process of elimination (and assuming my MAF clean has not resolved the issue, which now sounds unlikely) then swapping out a set of coil packs is cheaper than a pump. As I don't have any lying around and have to buy a set for the test. Any recommendations for good non-Porsche items for the test please? Or, should I try the chaps at 9Apart to see if they have a good used set?

Just go on eBay and buy a set. They are cheap as chips. You can't tell if a maf is ok just by looking at it. As DeMort said start with coil packs.

It could be coils and that is something that needs to be ruled out .. a second hand set as mentioned is preferable .. for me it's obviously different in that i can just fit a set to test . Plugs if not done in a few years would be worth doing at the same time .. Turbos are a bit more involved so killing 2 birds with one stone so to speak .. it may not fix the issue but its a job that wont need doing for another 4 years . Fuel pressure at high end revs / throttle could well be the issue .. a pressure test with a gauge won't really help so its a case of eliminating other more likely faults before changing the pump . I would like to point you to a recent thread of mine .. http://911uk.com/viewtopic.php?t=150462 I have no idea if yours has a filter or not but it's obvious on removal and i can say on this car the engine power was a lot crisper with pump and lvl sender unit replaced ( lvl being the filter if applicable ). This isn't cheap though so we work through a process of eliminating items to see what we are left with ... easy for me to say i know but it's how i work an issue .

Thanks Chaps, just ordered a set of six OEM Beru coil packs. The MAF clean made no difference at all, so that is off the list. Let's hope it's not fuel pump (or worse)