Porsche Enthusiasts Club Forum

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Before it goes in, check the fuel pressure. You can do this if you have a measuring jug etc. How much fuel's in the tank atm? Generally, more fuel in the tank will help an duff pump but it depends if the fuel's congealed inside.

Its got quate of a tank but this has never been a problem. Pro 9 reckon it has 2 different issues due to the starter and the running but I`m not convinced as the problem started together. I`m off work this week so I`m going to have a look but I tried when I bought it and I never found anuyhing it just decided to go one day and has been fine up untill now.

OK off this week so time for a look had a good day today with the car and as so far here-
I cleaned up another two earths I found and she now starts everytime so thats the starter sorted but still wont run but I have done the following-
Coil checked cleaned and ok
afm checked seems ok
ignition circuit checked and seems ok, the new dme I bought died but I think that was me checking things.
She is getting a spark and when the engine died I kept the ignition turning the engine over and it was still sparking like a goodun.
I took the fuel feed to the fuel rail off and turned the ignition so the battery light came on and it didnt give a pre-pressure spirt or any sign of pressure so I turned the engine over and it has fuel flowing out but I cant remember from my college days if it should really fly out been an electric pump as when you turn the key for it to start the fuel acts more like a mechanical pump (only pumps on the key no pressure first like I would have expected with an electric pump). Also if i touch the dme relay (I have the cover off so its exposed) I can make the pump come too life on the 1 circuit in it, the other is the ignition I think and seems to power everything fine. If I put a live battery feed to the pump part of the dme so I can hear the pump running and try and start it it dosnt make any difference just wants to cut out straight away as usuall. I think tomorrow I shall remove the pipe between the afm and the throttle body and rig the dme to see if I can get more air to the constant flow of fuel, something else I forgot to check is whether the fuel will really come out the fuel rail feed fast if I touch the pump part of the dme I`ll check this aswell, anything that anyone else can think of would be appreciated as I`m missing driving her now.

Hi there,

Did you ever get to the bottom of this? My 944's started to do this again after 2 years. Previously, it was in with Strasse in Leeds for a week before they found out the cause. Swapping the ECU with a second hand one fixed it. Guess it was a dry joint in the control panel. I'm gonna try swapping out the DME this morning, fingers crossed!

Rich

No worries, I think I found your outcome on your other posts. AA man came out yesterday and diagnosed it to be a faulty fuel pump. I'll start a new thread rather than hijacking this one

Rich

Frog..

From what you have said it sounds like the car will start and run but fails under heavy load.


I would suggest that you start with testing the fuel system and eliminate that if you can.

Remove the large test nut on the fuel rail and attach a length of garden hose into a 2lt coke bottle.
Remove the DME relay and replace it with a 3x way bypass cable made up with spade connectors as per Clarkes garage guide.
With ignition off...Put the first connector into terminal 30 then the other into 87b .

The third spade connector can be fitted to start the car but only 2x are required for the fuel pumpto run for testing).

The fuel pump will now run continuously . Run the pump for 15 seconds then remove the bypass. Pour fuel into a measuring jug and multiply the quantity by 4x (ie 1 minute) you now have the fuel flow of your pump in lts per minute. Check flow rate against your workshop manual .

Next you should attach a fuel pressure guage to the same test point ( on ebay for a tenner),refit the bypass and check the fuel pressure (which should be about 2.5 bar or 3 bar depending on your model.

Then remove the bypass and allow the pressure to drop. It shouldn't lose more than about .5 Bar over 30 minutes I believe but check the proceedure on Clarkes garage.

If all that is ok you should have eliminated anything in the fuel delivery system being at fault , save possibly blocked injectors ,poor ECU/ DME earthing or an engine management fault with the ECU

To eliminate the DME relay replace it with the 3x way bypass terminals 30 , 87, & 87b if the car doesn't start and you have no spark then the DME relay is not at fault

Rich I've only just seen your post .

It is extremly unlikely to be your fuel pump at fault.

You need to be aware that a lot of so called mechanics know nothing about the Bosch fuel injection and engine management systems.

On the Porsche 944 (and any other car using the Motronic ) the fuel pump does not run when the ignition is turned on.

The pump runs for about 1/2 second then switches off again.This is sufficient only to bring the fuel pressure up to 2.5Bar or higher to allow the car to start
(This is a safety feature designed to prevent fire in a crashed car)

Once the ECU receives correct data from the various sensors (like minium 225rpm from the crank speed sensor ) it will turn on the DME relay which energises the fuel pump and the ignition system to produce a spark.

To test the fuel pump
If you use a 3x way bypass cable (see Clarkes Garage workshop manual) to replace the DME relay the fuel pump should run continuously .

Cheers for the info Peanut,

Yeah, its a good job i didn't replace the fuel pump because it turned out to be a faulty Fuel Pressure Regulator. It was a pretty simple job to replace and as an added bonus, start time is a lot quicker than it was.

Rich