What can stop working when it gets hot?

[tref]

Gentlefolk,

For a while now I have had an intermittent cut-out on my 944 S2. Driving along, engine suddenly dies. Coast to a halt, and after a few seconds it wil start again. This has become more frequent, but still it hasn't "proper broke" so it is some-what difficult to diagnose. It does appear to be heat related though.

It is a sudden cut, so don't think it is fuel related, though I have changed the fuel filter as a precaution (and feeling guilty because I haven't changed it in a while). Have swapped the DME relay several times, each time the previous one works, but it still cuts out now and again, so I don't think that is the problem.

When it cuts and you try and restart it, the tacho needle bounces, so I am guesing the crankshaft sensor is ok.

So... what components do you think there might be that could decide to suddenly stop working for no apparent reason, and then start again shortly afterwards? the only ones I can think of is the coil, or immobiliser... which I don't know where it is yet. Immobiliser is my first guess, because it is a non Porsche item, and I don't know how well it was fitted, but... any other ideas? Anyone had this before and found a solution?

Best regads,

Tref.

 

[tref]

Hmmm... immobiliser cuts a green wire with a black tracer somewhere under the fusebox - any idea what that is?

Thanks,

Tref.

 

[VanhireBoys]

Hi Tref
An intermittant problem is a nightmare to diagnose.... It may be the ignition switch itself perhaps.... Ive seen one a while back that had an issue just like that and the give away was the key was warm.. The contacts in the ignition switch were bad/pitted and after a while of running the resistance got so big that it cut the DME relay.. It took us a bit of running about to diagnose this .. When the car cut out I was straight to the DME and found the ignition side on terminal 86 was lost... After a while it was back and the car started.. It was an 86 Lux but its probably the same principle... ! This car would be cut for about 30secs so it may be unrelated...!

The fuse box layers all form a sandwich and it could even be a bad connection between these.. Not unheard off !

On another note my Turbo started to do this in '07 and I changed the DME.. No change .. It happened to be the Airbag Control Unit which feeds the DME.. It had started to go faulty.. I ran another feed to the DME until I got a control unit off Mr.eBay US !

Take note of when exactly it happens - are you going left for example, Is it after a bump..? Is there a way of bypassing the immobiliser to rule this out.... ! gather the evidence, observe and think outside the box... Keep asking yourself why and Good Luck !

 

[VanhireBoys]

Green with a black tracer is to the feed side of the fuel pump on a turbo anyways ...A cut in the pump supply would not be sudden..... I will have to get my pdfs out again ! Work has become very interesting all of a sudden...[]

EDIT.. There seems to be an ignition module which fires off the coil .. Both feed back to the ignition switch via the fusebox .... Start and check all these..

I am open for correction now as its the first time I ever looked at an S2 diagram its for a 90... Its sheet 8 Volume 5 page 158-160... !

 

[tref]

Thanks very much. Good idea about the ignition switch, hadn't thought of that! Anyone know if they suffer the same on later (1991 specifically) cars? The early dash cars I think used the same ignition switch as a Mk 1 Golf, and similar problems.

I am going through the remnants of two alarms and an immobiliser I have found so far:-(

The immobiliser does indeed cut the green and black wire... or did;-), and also it looks like the starter circuit at the ignition - via a couple of fuses... there are connections to black, red, the ignition switch connector itself, and a Red and black wire that looks like it should go to the ignition switch... Haven't got to the bottom of that yet!

I have been looking out for other similarities of when it cuts... today, I got to the bottom of the drive, (20yards?) waiting to pull out, and it cut. Previously it has been on a straight section of motorway, the North Circular (fun!), and yes, twice just after going round a tight-ish bend in a 40mph... I keep finding patterns that are then confounded!

 

[944 man]

First Id renew the earth straps - not dear and always due. Then Id look at the ignition switch, as they wear out also, but theyre far dearer... After that its a nightmare checking third party additions (alarm & immobiliser) and sensors etc for continuity.

If it wasnt earths, switch or immobiliser though, youve been very unlucky.


Simon

 

[tref]

OK, before I soldered the green and black wire back together I tried starting the car with it disconnected... If this is as suggested (and I think the following bears that out). It ran for a short while (few seconds) on residual fuel pressure, and then cut out, surprisingly suddenly. So I'm not discounting the immobiliser as being the problem again... The trouble is of course, I can't prove I have fixed it, only find out sometime that I haven't!

Regards,

Tref.

 

[tref]

Earth straps - another good one - thanks. Hmmmm... think I would have murdered that one if it were a problem moving the car back onto the drive on the starter motor though!

 

[bertelli_1]

When it cuts out you need to check if the plugs are sparking & if there is pressure in the fuel rail. This will point you in the right direction. Personally my money is on a faulty fuel pressure regulator or alarm issue.

 

[944 man]

Is the FPR failing likely to cause a sudden stop? I thought that they struggled for a bit first.

With regards to the earth strap: I know it sounds unlikely on the face of it, but Ive had a failing earth strap cause random and sudden power failures and it goes and comes back as though on a switch (causing my glow plugs to operate, which is what gave it away).


Simon

 

[bertelli_1]

You'd be surprised how suddenly it'll stop. The earth can be checked by measuring resistance between the engine block & chassis.

 

[tref]

Unfortunately it fixes itself before I get anywhere near pulling a plug, or even pulling an HT lead and putting a spare plug on the end of it - by the time I have done that it will start again.

I have now disconnected the immobiliser, and am hoping that may be it... The wires on the fuel pump connection were crimped. On the starter circuit it was a 4mm2 wire that had been cut, and a scrawney little wire out and back in it's place... thinking about it... that probably wouldn't cause it to cut out, more likely an under-dash fire whilst I am happily winding the car back up the drive on the starter motor!

I have just been out for a 5-10 mile drive, and all was ok. The trouble is... and I should have said this before, when I say hot, I mean hot ambient temperature, not when the car is hot - the car can be nigh on cold and it will cut out, so long as it is hot outside... which of course, now it is not.

Thanks everyone for your help, any further suggestions or debate most welcome!

 

[VanhireBoys]

Just looked at the wiring diagram again and looked at the ignition module AKA an ignition final stage ! I had a Nova many moons ago and it used to cut out like this as well. There was an ignition unit on the left of the engine bay.. It had male and female plugs so I bypassed it and hey presto - problem solved !

I would not however start blaming this until all the earths and connections have been discounted ! BTW Simons suggestion of the earths is a good one .. []

Keep us posted

 

[944 man]

Im going to offer, in a similar vein, the connector behind the rear of the cam cover. Regularly stops my 2.7 from starting...

 

[tref]

To tempt fate, after 500ish miles so far it has been ok, no repeats of the previous cutting out and restarting problems... I did buy an ignition final stage module as an insurance policy, and carrying it around along with a coil and DME relay just in case, but it does look as if it were the immobiliser.

Thanks to all who took the trouble to give it some thought and comment,

Tref.

 

[peanut]

Tref if you fit a 3xwire bypass in place of the DME relay and try that for a while it might cure the intermittant problem which would narrow down the list of possible culprets to just a few. It would be a useful test.

The 3x way bypass should bypass the immobiliser but it depends how it has been fitted. Most immobilisers are very crude and probably just cut the 12v live feed wire to the fuel pump which is fed by terminal 87b on the DME relay via a fuse. The immobiliser cable connections could be simply crimped to the fuel pump supply feed wire and taped over with electrical tape and are likely to be connected just after the fuel pump fuse. Worth checking the connections in case you have an oxidised connection.

If you want to bypass the immobiliser temporarily you probably just need to connect the 12v+ on the fuel pump fuse directly to the 12v+ terminal on the pump itself. (The fuel pump is earthed in the boot behind the carpet under the hatch lock)
For a permanent solution simply find the 12v+ feed from the fuel pump fuse and follow it until you find where the wire has been broken for the immobiliser . Remove the 2x wires from the immobiliser and re-join the 2x cut ends of the pump feed supply wire.

On my 89 S2 the 12v+ fuel pump feed wire is red /black from the DME relay to one side of fuse 2 (16A) and black/green from the other side of fuse 2 to the fuel pump

Sorry Tref I didn't read the whole thread and missed the part where you removed the immobiliser. doh![]

If you have a re-occurance it might still be worth trying the 3x way bypass.