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Poor running

ian harvey

PCGB Member
Similar to Talat on the Spa thread, my car has developed a problem that may be due to a temporary lack of use.
The car continues to start instantly but will not tick over without throttle assistance and does not run cleanly; cutting out when lifting off the throttle and generally needing to be coaxed along.
I'm not happy driving it like this, it may leave me stranded so I'm trying to eliminate those things that are easily swapped before having it trailored to a garage.
I've swapped the DMA relay and the MAF unit functions correctly, without improvement.
The distributors and leads are all new so should be good (all Porsche), Boxsey has helpfully suggested, among others, the Idle Stabilization valve (is this the MAF unit?).
The fuel filter and presumably fuel pump are potential culprets I guess. Can anyone tell me where they are sited, please? How do i restrict fuel leakage when removing - is it possible to crimp the pipe? Any replies and/or further suggestions of other home checks gratefully received. Ian.
I have a theory on this. I was telling Talat about it whilst at Spa. I am happy to be corrected if my info is wrong. I am a bit flaky on the details as i don't have the info to hand but here goes.

If the car has been left a while and the battery disconnected or allowed to go flat it might be worthwhile having an adaption done. Now i know some of you will be saying at this point, "no need just disconnect the battery and the car will re-adapt" . true to a point.

My understanding is this. Adaption does 2 things (this is where i may not be 100 % correct as i don't have the details to hand) 1, it sets the idle stabilization, via the Idle Stabilization Valve (the bit that works when u do a battery reset) 2, it centers lambda control (the bit that can only be done by a proper adaption). Doing just a battery reset will make the car run better but won't cure idle problems 100%

If you get an adaption done and your problem goes but comes back when you disconnect the battery, it might be worth changing the lambda sensor (O2 sensor).

Hope this makes some sense. When i get a chance i will find the full info. There is a specific piece stating all this somewhere, possibly in the 964 workshop manual. Adaption can be done with a K300 hammer, PST2 with the right software or one of the generic Doug Boyce style scan tools (do a rennlist search).

I should add that if adaption cures your problem it is also worthwhile checking for air leaks (intake hoses, vaccum pipes , jubilee clips need nipping up etc) As the adaption could mask these, as that is what it's designed to do.

Do a web search for cleaning 964 ISV , lots of pics of where and what it is.

Thanks Laurence. It does make sense. I have always disconnected my battery via the ultra convenient battery kill switch. On re-connecting the car always has to re-learn tickover. If I plan to use the car again within a few days I leave the battery and it usually starts (ticks over) much more easily without the hunting that lasts for a few seconds before it works things out for itself.

I frequently check the hoses/clips etc but there may be one that I haven't spotted.

Whatever has caused my issue has been an instant thing rather than a gradual creep. This is why I'm tempted to swap the fuel filter (they eventually fail, I'm told) and then maybe the fuel pump. That's why I'm curious about cutting of the fuel to minimise spills.

My ISV has been bench tested and is clean and appears to work fine.

If this procedure fails I'll have it trailored to a garage and then have the adaption test done as you suggest.

Thanks very much for the reply, Laurence. Ian

Hi Paul,

That is very useful, thanks indeed. I'm still interested to know how I can separate the filter without losing loads of petrol, if anyone knows...


To add to Laurence's suggestions. The O2 sensor signal is not used by the DME (ECU) for about the first 90 seconds from firing up from cold. So if your car is fine for the first minute or so from cold start up and then the problems start it could indeed be related to a dead O2 sensor. BTW if the car recently had a service before the problems started don't discount the garage forgetting to replug the sensor after the service which will cause the same problem. I've known this happen!

Idle adaption via a Bosch hammer is indeed worth trying. Because at the same time the garage should see any fault codes that might be causing the issue (e.g. it was the hammer that confirmed my own problem with the hall sensor). Talat's car HAS to have an idle adaption after every battery disconnect because his C2 was originally a C4 and still has the C4 DME. All C4 DMEs need idle adaption after battery disconnection which is a right PITA! Usually the C2 doesn't which is good otherwise there would have been no sense in Porsche fitting battery cut offs to the RS model.

Thanks Paul, Steve,

Half my trouble is that I'm not happy driving it to a garage as it is which is why I'm trying to eliminate some of the easier/cheaper consumables such as relays and filters. The car has not been serviced or otherwise disturbed in any way recently and the problem is immediate after start-up rather than after a while. My car has cat by-pass so not sure if that has any effect on the potential for trouble with the lambdar sensor but it has been fine for years.

Can't remember who suggested the fuel filter but it was suggested that the paper element swells and fails causing a partial blockage which could explain why my car has suddenly hit trouble.

The article you posted, Paul (thanks again), implies low pressure in the fuel system after a period and therefore minimal leakage. Does this apply equally in the vicinity of the fuel pump?

Just for info the cat bypasses are fine as long as they a lambda sensor still fitted (hence why they are supplied with a hole to take one) and should have no adverse effect on the lambda function. From what you say about the problem being there from cold leads me to think the lambda is not the problem here. FWIW I think you need to risk driving or flatbedding it to the garage for Hammer diagnosis to check the ignition systems. Presuming that they have one?

Laurence told me that it was necessary to reset the lambda sensor with the Hammer following re-connection after isolation. I always used to isolate Roobarb and I isolate Kermit. They both start up with a fast-idle after re-connection. Not worth driving an inch until they stop fast-idle as they just stall so easily. Takes a minute or 2.

RS, N-GT and Cup intended to be isolated but does re-initialise the car. Ctrl-Alt-Delete.

I remember the settings having to be adjusted when I removed the cat, may have been a new Lambda, not sure. Disconnecting the battery has never been an issue in many years.

Steve, I'm sure you are right, a Hammer session will be needed but a new filter is cheap and easy-ish to swap so I'll try that first. Will a local (non-Porsche) garage with the correct Hammer be able to diagnose most problems?

I stick by what i said until someone shows me some proof otherwise. O2 centering can only be done via proper adaption. Resetting your DME will only set ISV NOT O2. I am not suggesting that the O2 sensor dies but rather that it causes the centering to be wrong, which then gets masked by adaption curing/hiding the issue. If the sensor dies completely it will throw a code !

The fuel filter lies above tank height (just about)so when NOT under pressure fuel only dribbles out if at all. When you open the lines you will need to catch (rag etc) the initial spurt that is still pressurized. Be careful when reconnecting the lines that they do not rub on any body work.The picture showing the fuel filter is a good case in point it's running close (too close ?)to the bodywork(though of course it might be fine when the engine is put back in !!! doh) ! I would not recommend crimping/ clamping any of the lines unless they are fairly new, not sure i would want to even then. Possibility of the line crumbling !

Laurence Gibbs said:
Given what adaption does, i can't see why a C4 should be different to C2? DME coding yes, adaption no?

Laurence it's anecdotal evidence from Rennlist and independents that a C4 always needs an adaption after battery disconnect. Can't really give you much more info than that or the actual reasons why. I remember Talat's car stalling at Oulton (battery had been on and off a few times prior to the track day). Luckily Jonny of unit 11 had a hammer with him and after an adaption it ran perfect. On my C2 I disconnected the battery on many, many occasions and never needed an adaption.


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