Misfire at 5800-6000 rpm

[Guest]

Before I book my car in for its annual service, has anyone else suffered the same problem, and shed some light on the likely scale of my 1999 3.4 C2's fault?
In first and second, I am able to accelerate up to the rev limiter smoothly, unless pointing up a steep road, or anything less than flat throttle. In third and above the engine simply stops accelerating at around 5750, crawls up to about 6000-ish and then seems to clear itself. At 140 in top, it refuses to go any quicker, and you must change down and try and creep through (obviously, I need to do this regulalrly in South London, which is why I have left it for months to sort out!) The load on the engine is clearly a factor, I reckon.

Could be a plug, lead or coil, I guess.

Any clues, please?

 

[Ricardo]

I've had a kind of similar problem. Misfire at 4200 and then crawling slowly through to red line. Trying to save money I went to local independant who could not fix it. A few weeks ago I went to local OPC who diagnosed a faulty air flow mass meter. Apparently, this is a very common fault and in Purely Porsche this month there's a letter that describes similar problems and they suggest it could be air flow mass meter.

So, I'd be willing to bet that this is the problem. All in cost to fix was <£300 and it TRANSFORMED the performance.


Let me know how yiou get on

 

[Guest]

Sounds like Ricardo has hit the nail on the head.

I have a similar fault, but it does not seem as bad as yours.
My car accelerates really smoothly until I get to around 5000-5200. At this point the acceleration has a blip in it for about a second and then is fine all the way after. Happens in all gears.
I took it to Paragon and they put the diagnostics on it, which was all OK, they also took it for a test drive and found no fault, but I expect they did not get the chance to get the revs up enough to notice.

I have since found out that the car is Ruf chipped to 320bhp, so this may have something to do with the problem I am experiencing. Any feedback on that will be greatfully received.

Bruce

98 996 C2 Coupe manual

 

[Guest]

Funnily enough, I spoke to Mick @ Paragon today, to book it in for a service and check out the misfire. My problem does not sound exactly the same, as it occurs at higher revs, and only under load (that is, not at all in first, and only in second uphill, or on partial throttle).
I have read other threads about the 4000-5000 'flat spot' which has been (a) air mass metering (b) variocam sender unit (but probably mis-diagnosed by a muppet technician) and even (c) misaligned cam timing (the result of allowing the same muppet technician to change a cambelt?)!

Anyway, Mick seemed to think my problem was probably trivial (spark plug etc.). Famous last words!

 

[Ricardo]

I hope it's something less expensive than air flow mass meter - that'll save money. Apparently, the variances on settings for fault diagnosis are so wide that a diagnostic test does not flag an air flow mass meter fault.

I know I sound like an air flow mass meter zealot - I was just so impressed with the difference it made & that I was not going mad - there was definetly a hesitation at 4200 & nobody believed me!

 

[Grant]

As an aside whilst talking air flow meter's, it also appears that they are sensitive to the increased dirt an aftermarket air filter can allow through, it's ironic don't you think that in searching for extra performance from a free flow filter you can actually end up reducing performance!!!
Anyway I hope it is simply a spark plug rather than the variocam on the twitch!!!

 

[Guest]

Yes, me too! Thanks.
Booked in next Wednesday - so watch this space.

 

[Ricardo]

Good point Grant. Coincidentally (?) I fitted an induction kit and sports filter probably around 1000 miles prior to suffering problems. Who knows eh!

ORIGINAL: Grant

As an aside whilst talking air flow meter's, it also appears that they are sensitive to the increased dirt an aftermarket air filter can allow through, it's ironic don't you think that in searching for extra performance from a free flow filter you can actually end up reducing performance!!!
Anyway I hope it is simply a spark plug rather than the variocam on the twitch!!!

 

[Ricardo]

Andrew,

Did u get the misfire sorted??

 

[Guest]

I think so. The car seemed okay on the blast back from Five Ashes to Shirley, in any case. The diagnostics did not report any faults, but, as you say, this does neccesarily mean there isn't one!
New plugs were fitted as part of the 24k service and the technician reckoned that the throttle body was 'very dirty', so stripped it. Funny how these parts, and mass sensors etc. seem to 'get dirty' despite being upstream of the air cleaner.

I would have thought that a 'sports filter' was something like a K&N and supposed to be 'cleaner' than paper elements (but only when oiled properly).
Upgrading this part is normally the first thing anyone does to improve the performance of any engine, but from the sounds of it I might leave well alone.

Whilst on the subject of performance upgrades, Paragon have a very cheap alternative. They lend you a Nissan Almeira for 24 hours, and when you get back in your 996 and pull away from the forecourt you could swear they added another 200 bhp!

So next time your car feels a bit sluggish, hire a sh*t car for a day - a much cheaper alternative than chips, exhausts, superchargers...................

 

[Ricardo]

Excellent! Now, what do we think of superchargers....there was a report in one of the porsche magazines a few months back & I'd be sorely tempted if I did not have heart set on a GT3...