Porsche Enthusiasts Club Forum

its not likely to be the fuel pump but an easy test is to connect a piece of garden hose onto the test nipple on the fuel rail and put into a 2lt coke bottle or similar.
Replace the DME relay with a 3x way bypass (see Clarkes Garage Workshop manual)

With the ignition off connect terminal 30 of the DME relay socket (always live 12v+) to terminal 87b which feeds the fuel pump. Let the pump flow for 15 seconds then pull the bypass out. NOTE ( connect terminals 87 & 87b first to avoid potential shorts or other excitment)
Measure the fuel in a measuring jug (initiative required ie Wife's fluid measure jug) multiply by 4x and you'll have the pump's flow rate for 1x minute . From memory its likely to be about 2lts + but you'll need to check( edit 1350mls thanks JDS )

Next you should measure the fuel pressure which will eliminate the FPR (fuel pressure relief valve) Probably about 3 Bar but check .
Check to see how much pressure drops over next 30 minutes (should be about 0.5 Bar or less) this will eliminate the FPR and non-return valve in the pump both of which maintain fuel pressure.

From the symptoms you describe it sounds like it is temperature related so i would check the engine temperature sensor which controls the ECU also check to see that the fuel tank breather is not blocked and building up a vacumn. (test with the fuel cap off next time )

I'd locate the special ground points for the ECU and DME relay and the Fuel pump and clean those. Good earths are always a good place to start on 20+year old cars

The crank sensor is again unlikely to be at fault. Its only a magnet with a coil of wire wrapped round it. Easy to check the coil's resistance with a multimeter (refer to Clarkes garage guide) its more likely to be the the sensor's electrical connectors corroded or oxidised contacts or brittle wiring due to heat .If your tach needle jumps a little when cranking the chances are its working ok.

The ECU gets its power from the DME relay terminal 30 when ignition switched on. If the ECU doesn't see about 225rpm when cranking it won't energise the DME relay's second coil to supply 12v+ to the fuel pump. It also won't provide a pulsed earth to your ignition system.
Remember the pump only runs for about 1/2 second when you switch ignition on. Just long enough to pressurize the fuel delivery system. Its not until the ECU sees the correct data input from the AFM and Engine temp sensor and 225rpm from the crank sensor etc that it will actually provide earths for your fuel pump and ignition system so you get a spark and fuel.

Fuel flow test is 1350cc in 30 secs.

thanks JDS so thats mmm lemmeesee 1350 cc's on the Wife's measuring jug I'd forgotten its 1xcc = to 1xml doh

Thanks for the advice still going with the CPS as there's no movement on the tacho when you crank the engine and it's clearly signal related, my original question still stands in how best to get to the connector? I can see it but it's a real PIA to get to, I think it will be harder reconnecting the new one than disconnecting the old one, it's right under a cable guide and one of the fuel regulators.

why replace something that you haven't tested or proven faulty? The head of the sensor has to be set at a very precise gap to the flywheel which will be very difficult to achieve in such a confined space. I would test it with a multimeter first before making such a tough job for myself.

Follow the cables until you find the first electrical connector. Split the connector and test the crank speed sensor . If the resistence is correct you'll not achieve anything by changing the sensor. Clean the connector terminals and see if things improve.

Have you checked to see if you have 12v+ at terminal 30 of the DME relay ? Its a direct feed from the battery via the ignition switch but it is live with the ignition switch off ,without that nothing is going to work because it supplies both the DME and the ECU

heres a testing and refitting guide on Clarks that might help. http://www.clarks-garage.com/pdf-manual/ign-02.pdf

Peanut thanks for the advice, if the problem hasn't gone I'll go down the route you suggested but I have replaced the CPS after some considerable pain, snapped a tendon in right middle finger when I fell forward with my finger between the sensor and the adjacent bracket, and effort. This was both on my part and the invaluable help of a friend who had the advantage of actually being able to grip things properly which is more than I can do at the moment. With the cunning that years of experience of working on 928's brings Jason did what I couldn't and got the CPS out.
Once reassembled I got the car up to a temperature whereby the fans cut in and the engine kept running, previously when the temperature got up it cut out. I shall have to take a chance and drive it a bit further tomorrow and see what happens, I'm hoping nothing and that my problem is solved, if not I shall come back and re-read Peanuts post.

I always get Mrs peanut to do jobs like that she loves getting greasy and oily up to her armpits and her fingernails broken ...not
I take it that previously the car didn't reach sufficient temperature for the fans to cut in before the engine cut out yes ?
Personally I can't see any relationship whatsoever between the CPS (magnet) and temperature related cut-out problems but will watch this space

While taking the engine out a Manta GTE I cut my right middle finger OFF at the knuckle, only the skin was holding it on. Once they found out I needed it, due to working with tools ( insert your own gag ), they re-attached it. I aways said that I got it put back on just so I could say goodbye to my mother in law.

I dropped an Allegro engine on my LH index finger about 25+ years ago and it burst open, 9 stitches and unable to get the engine back in for 2 weeks, so had to go to work on the bus

That's one way of getting out of driving an Allegro

Credit where credits due. I'd never cough to having an Allegro, fortunately my good taste means that I don't have to, I had the Allegro's equally crappy big brother, an Austin Princess 2200!

Personally I can't see any relationship whatsoever between the CPS (magnet) and temperature related cut-out problems but will watch this space I can only assume that contact was being lost somewhere inside the sensor due to expansion when hot? I'm no electrician, or mechanic either judging by my finger, perhaps John Speake may be able to throw some light on this? I took the car out again tonight a bit further than last night and it didn't miss a beat so i'm hoping this was the problem. I also changed the ABS relay whilst working on the car as the warning lights for the ABS and diff control kept coming on, even when starting from cold and before moving. Again John Speake suggested that this might be the problem and he may well have been right again as there were no warning lights after I fitted the relay. Just as well as the relay set me back £86.00 including p&p!

the senser is essentially nothing more than a magnet with some wire wrapped around it so the only thing that could happen to it is a open circuit hence the suggestion to check it for resistence with a multimeter.

Well all I can say is whatever the physics behind it the change of CPS hopefully appears to have worked as I went out tonight and dorve around in varying conditions and it didn't cut out, it was also quite pleasant not to have the bright red warning graphic for the ABS dazzling me in the dark!

To be honest the Allegro only cost £50 with a spare engine and loads of MOT, It was only a tempory measure ..... honest