Mis-fire at low revs

Seems I've joined the ongoing bad weather casualty list. My C2 is now a weekend car until this arctic weather improves, but still gets a decent Sunday drive. Today I noticed that it's mis-firing at low revs, coming out of roundabouts etc. Clears once you get past about 2k revs, but is there well after warming-up to 85degC It's a 2001 C2 on 37k and is due a service this month anyway, but I would appreciate a few likely causes? Trust Lloyd to depart when I need his know-how !

Coil packs? The casings split and then water gets in and causes a misfire. At higher revs / temperatures, the water evaporates and the misfire goes away.

Thanks Wattie, had a look around some of the other doom and gloom sites. It could well be coil packs, but I haven't had any engine warning light come on...... I guess next question, change all 6 during the service ? I read that have they are about £30 each from Eurocar Parts and once one goes, they seem to fail one after the other.

I had a similar situation just before Christmas, with the exact same symptons. It turns out that some of my valves had become corroded, but the Porsche expert I took it too had never seen anything like it before, so I guess that chances are you're not going to have the same problem. Good luck getting it sorted anyway!

Is it a proper misfire or just a hesitancy when accelerating? I had (have) the latter when applying part throttle and pulling away at c2k rpm and have just had all 6 coil packs changed (2 were cracked but the others fine but I thought it best to replace them all). Suffice it to say my problem hasn't been cured by the coil packs.

I'm still here... going no where. They will have to drag me away kicking and screaming!! As said already, it sounds like coil packs. Has the Orange light come on yet? It doesn't always but it should be throwing a P0300 & one or more P030* codes, these will identify the cylinders. To get a more certain answer get it scanned for codes (pref PIWIS or Durametric as these will show everything whereas Snap-on Solis, etc will only read stored codes, the EML needs to have come on for this) Another way is to jack up the rear until rear tyre is off the floor. Look under rear arch and peer through the gaps in the manifold. You will see a metal heat deflector plate about 4 x 10" held on by 2 M6 (10mm spanner) bolts at either side. Remove this to expose coil pack heads. Take a good look at the coils. The common sign of failure is split open plastic. This allows water ingress causing miss-fire. I have mentioned this enough times for folk to be clear but use Silicone Grease on the new coils, preferably change all 6 (do spark plugs at same time if required as you are 90% into that job also) totally plaster each coil in SG, up the end in the connector to loom, all over the yube etc... more the merrier. DO NOT over tighten the torx screws, this is where MANY go wrong. Get them finger tight + half a turn. Over tightened coil packs causes splitting.. back to square one!

I have exactly the same problem. If you're just under 2000 rpm with no throttle, and you then gently get on the throttle to accelerate progressively, you get this odd double-take of go, hesitate, go again. It makes smooth driving quite difficult. It's not the tip thinking about changing down, because it happens in M mode too, and the amount of throttle is not enough to prompt that anyway. It's definitely not coil packs: I had it before and still have it after changing all six (because one died this winter). It doesn't feel like a misfire anyway, it feels more like a mild version of the hesitate you used to get in the old days to remind you to push the choke back in once you didn't need it any more. (Yes, I'm that old) If anyone knows the fix, I'd love to hear it too. H

Your fault sounds like MAFS or throttle body adaption. TB adaption is free, to do this: (reading another topic you may have tried this) Turn ignition on until lights all appear on dash... wait 60 seconds Turn off ignition but leave key in place Wait 10 seconds Now start car. If still no joy give Richard a call on 07966 295 339 (RSR Autotech) Richard is one of the best 996 technicians in the UK, fully Porsche trained pretty much from school and the best bit... his workshop is in Claines, Worcester opposite The Raven pub.. just down the road from you. Richard has Durametric, this will acurately identify the fault

Thanks Lloyd I've been meaning to do a TBR so I'll definitely give this a go. Harry, I'll let you know if it makes any difference. Have you tried it already?

Lloyd, you top man :worship: I did not know of RSR, will certainly look him up, thanks. I did spot that thread on the TBR a couple weeks back, but Richard H said air intake temp has to be at least 10 degrees C which has not been the case for a while! But looking out the window, I reckon I'm going to go do it now. Cheers H PS 40Anni, you can find that thread at http://www.911uk.com/viewtopic.php?t=51494

Harry, Anni That's exactly how it is, mis-fire is perhaps the wrong description - more like hesitating/holding back, until you pull past about 2k then fine, accelerates well. Lloyd, nice one - thanks for the info. No orange fault light, but what is TB conversion? I will try it tomorrow as the temp has dropped below 10degC now. Can you drive on with faulty coil packs? I live about 30 miles from Wrightune and I don't want my local garage to start picking through the possibilities, will end up with more problems than I started with.

They're fine to drive with. If you haven't got a fault light they're only marginal anyway. Just don't expect changing them to fix your problem if it's the same as mine!

It is TB adaption, not conversion... it is only relevant to 2000 on cars with fly-by-wire throttle bodies. This is the difference between early and late cars: Cable driven - this has a pull cable from the pedal to the throttle valve, press the pedal, it pulls the cable which operates a butterfly (small pivoted disc) to let air into the engine. Fly by wire - This has a pedal TPS (throttle potentiometer) when you press the pedal it increases resistance on the TPS, the ECU uses this information to determine the power going to the TB and opens the butterfly by means of a motor. This issue you have is that the pedal TPS can fall out of sync with the TB and requires regular resets (once every 2-3k miles or when ever you switch fuel rating, 95 to 99ron etc)

I guess if they're marginal they might be OK to drive with, but if the light comes on I tend to stop and limp home or summon the flatbed. Reason being (Lloyd tell me if this is wrong) that unburnt fuel will be passing through the engine and into the exhaust. Cats don't like unburnt fuel, and new cats are well expensive. I was told £2k on an Audi I used to have, don't know the number for a 996.

100% spot on - miss-fire = unburnt fuel. This takes off the special coating on the monolith and has a bearing on the reaction rate. Once damaged thats it, it only gets worse and eventually pull a P0420 cat below threshold or just emmision failure at MOT time. Don't panic too much as from what you are now saying it could be MAFS, get it scanned asap If you need any parts give me a PM