Intermittant misfire

[zcacogp]

Chaps,

Posted on here for alternative opinions ...

My S2 seems to have developed an occasional misfire. It happened once over the weekend, and two or three times on the way in this morning. It seems to happen in the mid-range (rather than at idling), and under load. When driven gently it is as smooth as usual. First time it happened (Saturday evening) was when the engine was cold and it was fine for the rest of the journey. This morning it happened a couple of times while the engine was hot.

My immediate suspicions are leads and rotor arm. The dizzy cap was new moderately recently (within the last year or so), but the leads and arm are at least 5 years old. They don't seem in bad shape visually, but I think I know better than to trust visual appearance for these things.

So, new leads and arm are to be fitted, which I am hoping will cure the problem - they were on my list of 'things to do' anyway, and this has precipitated doing them. However, what else could it be? What else should I be looking at if the new leads and plugs don't solve it.

(For reference, the 'cut' is a bit of a stutter, is very intermittant, and clears after less than a second. The plugs are less than 5k miles old and the fuel filter about 20k miles old. The fuel tank is over 3/4 full. I have done no diagnosis on the problem whatsoever, and the car runs fine otherwise.)


Oli.

 

[scam75]

Hi Oli

Sorry to hear of this. I had a similar issue when I got mine and it turned out to be the connections to the AFM. I did however change cap, arm and leads before I worked it out! Mine eventually went from mis-fire to dying on throttle (it would idle but not rev) then randomly start working again for no apparent reason.

I have since replaced the AFM with a MAF kit just in case it had a relapse![]

Stuart

 

[Dockrell]

Oli,
With the cold damp weather we are having now don't forget the light grey gasket underneath the distributor cap - it could be original and in poor condition. Also there should be a dust cover under the rotor arm - missing on my car. Both are available cheaply from your local Porsche Centre.

Mike
white 2.7 automatic

 

[zcacogp]

Stuart, thanks. I'll check the AFM connections carefully.

Mike - great suggestions! Can you tell me more about the two things you mention; I don't believe there is any gasket under the distributor cap at the moment (I presume it sits between the cap and the distributor housing), and I'm not sure there is any dust cover under the rotor arm either - from memory, that simply bolts onto the three legs of the cam shaft pulley behind it.

Hmmm. Call to OPC necessary. Thanks.


Oli.

 

[Dockrell]

Oli,
If you have access to the 1991 944 PET take a look at page 334 - the parts for the distributor.

Part 28 is the gasket under the distributor cap. It blends very well with the alloy housing and I only identified the need for it by looking at PET. As it blends in so well I would expect that you have a gasket but did not notice it. Think mine was the original because it was hard and brittle when I took it off.

The dust cover is black plasic and just sits under the rotor arm - part number 14 in the PET diagram. To help hold mine in place whilst I struggled to fit the distributor cap I used a minute amount of super glue in the locating slot.

Mike
White 2.7 automatic

 

[simkin911]

Firstly, I'd point out by saying I can't add much but to say.....

...I read an article (readers car 911 & PW) within the last year - the chap bought a lovely looking blue '90 s2 with 160k on the clock. All seemed fine apart from an occ misfire. Eventually got to the bottom of it and it turned out to be a broken valve spring.

Of course, I suspect this would appear to be well down the list of possible causes but just repeating here as it may be something you wish or need to exclude later on.

Hopefully someone will come along and say it can't be this and I'm gurgling nonsense.

 

[zcacogp]

Mike,

Thanks. I have to say, I am a smidge confused - I don't have the '91 PET but I do have the 88 one, and I fear you are looking at the wrong distributor diagram. On page 331, there is a diagram of what I suspect is a 2.5 or 2.7 distributor and it has a part 28, part number 944 602 257 00, "Sealing Sleeve", which does look like a sort of gasket under the distributor cap. There is also a cover, much as you describe, part 14, part number 944 602 803 00, "Dust Cover". But there are no such parts in the diagram on page 333, which is for the S2 layout (clearly different as the distributor cap is very different.) This is bourne out by the conversation I had with the chap at Stratford OPC (after reading your original post!) who said that such covers and gaskets were only fitted to the 2.5's and 2.7's.

So, without wanting to be rude, are you looking at the right diagram?

Simkin, thanks. I recall the article, and am fervently hoping that that is not the problem with mine. Reasons to think (hope) that it is electrical: 1) I have had the front of the engine stripped down a couple of times recently, the last time was just before this problem manifested itself. At the same time I removed the plugs and the plug leads. I therefore suspect it is something I have disturbed. 2) The problem only manifests itself within a certain rev range, under a certain amount of load. It then 'clears' itself (and can be felt 'clearing' itself.) True, it does feel very like a mis-fire on one cylinder but it also 'feels' very electrical to me. 3) The rotor arm is cracked (i know I cracked it through baffoonery when I last had the front of the engine stripped down) and at least 5 years old. The plug leads are (I assume) of the same age - neither have been changed during my custodianship of the car, and I fear they could be heading for their 10th birthday pretty soon. I therefore suspect electrickery trickery is to blame, and will change these parts (leads, rotor arm and plugs) before anything else. If the problem persists then I'll think again.

Thanks also to ChrisG on here, who called me up this afternoon to offer wise words of advice and a good sounding board for problem solving. Thanks Chris. It was appreciated.


Oli.

 

[Black Beauty]

Check your speed and reference sensor.

You may or may not have seen my previous posts about my misfire, it turned out that the resistance of the reference sensor was dropping off when the engine got hot. I checked the resistance readings using the guide from Clarks website and and the resistance was ok when cold, within spec, but dropped right off the scale when hot. £100 later and problem sorted - she has been running for a month now with no issues []

Maybe this should be in a top tip section or useful things to check section

Good luck
S

 

[xenon]

Oli I've just looked at my '91 PET and it does quite clearly show item 28 (sealing sleeve) and item 14 (dust cover) on the distributor of a 1991 S2. Illustration 901-00, update 203, revised 28.08.2006, however I believe this rather "conventional" looking dizzy is for the turbo which is not made clear.

On the next page is what I would call an S2 distributor (i.e. with all the plug leads coming out of the top, vertically) and that does not have the dust cover or sleeve.

I'd be changing the cap, arm and leads first - expensive though []

 

[Dockrell]

Oli, Ian,
My 944 is a 2.7 so that was the illustration that I was looking at in PET. It does initially appear that the first illustration is for a 1991 S2 but the engine codes are for the 2.5 and 2.7 8 valve versions - Sorry.

Mike
white 2.7 automatic

 

[zcacogp]

Mike - no need to apologise, thanks very much for the suggestion. It does seem a little odd that they are fitted to the 2.5's but not the 3.0 model.

Ian - I'm confused. Are you saying that there should be, or shouldn't be, a cap and seal on the S2 dizzy? (It's slightly academic as the nice man at the OPC probably wouldn't sell me them anyway - he knows I have an S2, hence believes they are un-necessary!)


Oli.

 

[xenon]

I'm saying that there should not be either on an S2 dizzy. [8D]

 

[zcacogp]

Ian,

Thanks! Fits with my understanding, Mike's understanding and the understanding of the guy at my local OPC parts counter (in increasing order of trustworthyness)! []

As above tho', they sound like a good idea, so I wonder why they were only fitted to the 2.5's and 2.7's, and not to the 3.0?

New rotor arm and leads arriving today (hopefully), so I'll be reporting back on the results.


Oli.

 

[xenon]

How much was the arm and leads? IIRC the arm is about £45!

 

[zcacogp]

Ian,

Arm was (I think) £24 inc. VAT. Leads were the more expensive end of the game at well over £100. (Bert quoted me less than £70 by eMail about 6 weeks ago but that has now nearly doubled. Parts prices do seem to fluctuate quite a lot these days.)


Oli.

 

[zcacogp]

OK, job done. I think it's cured the problem.

Old leads off (getting the king lead off the coil is a prize fiddle), plugs out, rotor arm off.

Made a schoolboy error fitting the new rotor arm and managed to break it! Bl00dy annoying - there are three spacers on the screws on the back of the rotor arm, and while the new one is supplied with new screws it is not supplied with new spacers. And if you fit it without the spacers from the old one it cracks! That's £24 down the drain.

The spark plugs looked pretty grotty (it accumulated grot in the tubes while I changed the waterpump a while back) and it turned out I didn't have any new ones on the shelf. So I cleaned them up thoroughly, and cleaned the tubes as well with some solvent and a long paintbrush, cleaned the old rotor arm, and re-assembled everything.

It seems to be as it should be, although the the old fault was very intermittent so I won't be sure for a few days yet. It also seems slightly smoother, which also bodes well. I'll pick up some new plugs later on this week (they're not expensive) and fit them when I can.

Fingers crossed it really is fixed and wasn't isn't a valve spring ... although I am still cross about the rotor arm!


Oli.

 

[zcacogp]

I'll definitely call this one "Solved" now. Just had a boys weekend away with a very good friend, thrashing the S2 about as hard as one possibly can around the south coast. No misfire at any stage whatsoever, and the car was as reliable as a swiss watch - even in temperatures of around -8degC.

I have yet to work out the MPG, but given the amount of time it spent on or near the rev limiter, I don't expect it to be that good! I also learned quite a lot more about the handling characteristics of it, having slid it around a LOT on a combination of black ice and wet leaves. No complete loss of control at any stage (quite), but I do feel as if I know it better now than I did on Friday, and if I do ever get it completely out of shape I should now be in a better position to catch it.

All in all a great weekend. Certainly the most demanding I have been of the S2 since I bought it, and it felt utterly bulletproof at all times. And it does seem to go tangibly much better as a result; Italian tune-ups really do work .... []

Oli.

 

[Tom]

Oli,

I'd be very interested in hearing whether you've successfully fixed the problem because my S2 has a very similar and extremely intermittent issue: It has happend on three occasions that I can remember - Thursday last week, a couple of months ago and perhaps a couple of years ago. As you described, the problem is only under load and at middling revs a sudden hic in power delivery. In the most recent occurrences, it persisted to some degree for the whole journey but was fine if not pushed too hard.

 

[hotblack944]

Did you make it to The Langton Arms? []

ORIGINAL: zcacogp

 

[zcacogp]

Tom,

Read the thread, and the post just above yours; the problem is definitely cured.

Most misfires are caused by problems with the spark system, and almost all of those on the HT side. When did you last change the dizzy cap, HT leads, rotor arm and (most importantly) the spark plugs?

If you have a misfire problem, closely examine all the components mentioned above. Any sign of wear, damage, cracking or tracking and they need replacing. Clean them all (apart from the plugs - these are cheap enough to replace) thoroughly with something like carb cleaner, and dry them out (10 minutes in a very very low oven is good.)

The S2 (along with many other multivalve cars) has long tubes the plugs sit in, and these can collect muck and junk. Take the plugs out and, using a small stiff paintbrush and some carb cleaner (or similar), clean the tubes very carefully, until there is no dirt left in them. (Yes, the dirt will drop down into the cylinders along with a good quantity of carb cleaner and no, it doesn't matter.) I used getting on for half a can of carb cleaner to do this to my car, which is quite normal.

Check the LT connection to the plug, although this will either work or not (it rarely shows problems.)

Fit new plugs and new (or carefully cleaned-up) leads, rotor arm and cap.

This should solve a large proportion of mis-fire problems.


Oli.