My 944 keeps stopping

[sam116glan]

Hello anyone who can help
I am quite knowledgeable about cars in general but have no experience of the porsche 944.
This is the situation. I bought the car which had been standing for a few months and was not firing on no 3 cylinder. This was sorted by taking out no 3 injector it was bunged up with crap and after cleaning out the car started and ran perfectly for about 8 minutes. This is what I have done since following forum and manual advice etc.
The car is early 1985 2.5 ltr.
Fuel pump relay changed.
Fuel pump connections jumped All fine.
Fuel Pump pressure checked at the rail ok
Fuel filter changed.
Fuel tank drained flushed and new petrol.
Battery voltage checked at all the recomend positions from the manual. 12 volts at coil etc.
Coil changed
Distributer cap changed
Rotor arm changed.
This is the interesting part.
When left overnight the car starts fine & runs for about 5-8 minutes then cuts out due to lack of H.T. spark (checked with strobe light and can be seen stuttering then stopping)
Will not start again for about 15mins then will run for about 30 seconds
Leave for an hour or 2 will run for about 2-3mins then cuts out.
Next day starts fine and runs for the usual 5-8 mins.
One thing I haven't checked yet is the oxygen sensor in the manifold do you think this could cause these symptoms?
I await your suggestions
Thanks
Sam

 

[Keith D]

Ok, so I may not have the answer but a very similar fault was on a front line Ambulance (very awkward to detect) but it turned out to be a faulty coil, when cold all was well but as it got warm one of the internal windings would expand and break connection, left to cool and the connection would re make. Worth a look at and best of luck. Keith

 

[robwright]

That was one of the things that I would suggest. I have known of coils breaking down when hot before now. The other thing I would check is the speed and reference sensors on the flywheel bell housing. They also could be failing with heat.

 

[924nutter]

I endorse the seggestions made by the last two posters. I have also had a similar symptom and fix. Of course I come from the days of points, condensors etc.

 

[chrisg]

You've certainly elliminated a very comprehensive list of potential issues. Id go with robs suggestion, either or both of the sensors at ate back of the engine just above the flywheel. I had a similar issue last year with one of them being very intermittent !

 

[eddieedmo]

could be the ignition module failing. Nick []

 

[zcacogp]

Does it cut cleanly, or stumble and die over a period of seconds? Do you know whether it is failing for lack of fuel or spark? Oli.

 

[sam116glan]

In answer to replies about the coil it has been changed both coils perform in the same way ie no spark after 5+ mins when cold. cant believe 2 coils would fail after the same amount of time and when I swap them back over stillno spark for 15mins so its not the coil. Yes it does stumble about 3 times over 30 seconds before stopping. Where is the ignition module? How do i check the sensors at the back of the flywheel?

 

[sulzeruk]

One of the sensors will be knackered, you just need to find out which one. One of the wires will have gone brittle and will be breaking down with the heat. Have fund getting them out! Is there 1 or 2 sensors on your car? If it is one, easy change. If 2, crank the engine and see if the tacho bounces. Disconnect the front sensor (B I think) and then check again for tacho bounce. If not that one, change the other one (D), make sure you get the wires back in the right place. There used to be B and D idfents on the wires but most have long gone. Make sure the plugs are round the right way as well as they are identical. May be worth checking that they are but that means belling out the wires to the DME plug in the passenger footwell. Would need the wiring diagram for that as well. Alasdair

 

[geoffbateman]

You can check the sensors either with a straightforward resistance check at the plug (located at the rear of the engine, both plugs are on a single bracket. The resistance values are in the workshop manual. The function of the sensors can be checked at the Engine management unit plug. The engine management unit is located in the passenger side footwell under the plywood removable panel. The manual gives you the terminals to check. You'll need a good old analogue meter as a digital won't work. The sensors are hall effect and should flick the meter needle as the engine is turned over. The meter needle should flick up to about 2.5 volts if the sensor is in good order and correctly fitted. I would suggest that you start the engine up and wait for it to stop and then check the function of the sensors. By the way, have you checked the DME relay isn't dropping out. Does the car have an immobiiser fitted?

 

[alwigley]

Another possibility is that the HT lead between the coil and distributor has an internal fracture that is exacerbated by heat........ have seen this several times before on other cars. Worth borrowing a known good set of HT leads and swapping them (if you have not tried already). Hope you find your problem !

 

[sam116glan]

Thanks for the tips I have changed the main coil dist H.T. cable.I will check the sensors at the bell housing next. What do you mean by relay dropping out? Yes it does have an imobilisor fitted but i cant see how it can interfere with the h.t. a sit should only affect the fuel supply and l.t. which is ok.

 

[Hilux]

Have you checked the relays?

 

[peanut]

just to add my 2p worth [ If you fit a 3x wire bypass in place of your DME relay and itstill cuts out you can eliminate the DME relay being at fault.The bypass will provide constant power to the ECU fuel pump injectors and indirectly the ignition system via the ECU If the engine doesn't cut out with the bypass fitted you should check the ECU and DME earthing points which are under the dash and next to the fuse/relay box. It might be worth cleaning them anyway With all this damp and rain about at the moment I'd check inside the ECU connector and make sure no water ingress . If you have no spark but have fuel when it cuts out then it would point toward the ignition amplifier if one is fitted to your car and of course the coil. It might be worth fitting a fuel pressure guage and watching what happens to the fuel pressure when the engine cuts out.

 

[sam116glan]

Thanks for all the replies. I have now changed sensors but still no improvement. I will try the bypass again for the relay and do a fuel pressure test.the computer box I have looked inside & all looks good no dampness etc. I will definately try the earth situation as the other side of the car fuse box has been wet in the past but ok now although I have a problem with left hand headlight, right hand front indicator and horn not working so I need to do a continuity test on these items as there is power at the fuses. Can anyone tell me if the H.T. spark must work if there is L.T. at the coil as I have low voltage when the H.T. stops. Could a fuel problem stop the H.T.Thanks for any tips already received.

 

[peanut]

not sure about your model but if it works the same as the 86.5 oval dash cars then the HT system is controlled by the ECU which pulses an ignition amplifier which amplifies the very low signal so the the coil can operate. If the ECU isn't pulsing the ignition amplifier for whatever reason then you'll never get a spark. The ignition amplifier is mounted close to the coil usually on the wing and has its own heat sink which must have a good clean contact with the wing to dissipate the heat. The ECU uses the information from the engine temperature sensor , throttle position switch ,engine speed and air flow meter in order to determine the best ignition advance and fuel delivery for a given load and condition and pulse the injectors and the ignition amplifier so all of these sensors need to be functioning correctly to get a spark and fuel delivery. Thats a very basic explanation of course. The ECU controls the fuel and ignition delivery by earthing the injectors and amplifier internally. It is essential that the two earthing points under the dash next to the fuse/relay box are good and clean or neither the ECU or the DME relay will operate properly.

 

[sam116glan]

Thanks for your reply peanut. I will look for the ignition amplifier. I havnt noticed it on the wing my car is an 85.3. with regards to the earth at the fuse box area it was very corroded and I have moved it to the same place as the battery earth under the bonnet using a thick cable so that should be ok.

 

[eddieedmo]

on my S2 the ignition module is on the n/s inner wing just behind the headlamp below the expansion tank , don't know if yours is the same. Nick []

 

[sam116glan]

Thanks Nick I will look for it there as well. how big is it andwhat shape?

 

[eddieedmo]

just search on google for porsche 944 ignition module and you will see photos. Nick []