please help, engine keeps cutting out

[JuvUK]

Hi all,

I need some help[]

my 944 lux keeps cutting out.

it felt the same as when my DME relay failed, car will start, run for a few seconds when cut out. holding down the throttle doesn't help, nor does keeping the ingnition cranked, it starts everytime and then cuts out.
so i bought a new relay from Porsche but it is still the same.

if i open the dme relay and hold the top contact closed (with no keys in the ignition) i can hear the fuel pump running and the injectors squirting, i have also removed the fuel hose and cranked her over, plenty of fuel getting there so i am now lost.

as it starts i must have spark, i can hear the pump so i have fuel, i think this is now beyond my very limited abilities and tools, anyone have any idea what it wrong with my car?

thanks, Justin

 

[peanut]

well as you have fuel and spark to start the engine it would suggest that something is causing the ECU to disconnect the earth to your DME relay thus cutting the fuel and ignition system moments later.

I would start by replacing the DME relay with a 3x wire bypass .(See Clarkes Garage online worshop manual)

This will force a constant 12v+ supply to your ECU and the DME relay fuel pump, injectors etc .If the engine stays running then that will significantly narrow down the search.

If the engine still cuts out with the 3x wire bypass fitted my next check would be the earthing points for the DME relay and ECU which are both under the dash beside the fuse/relay box on late cars.

I assume that you have eliminated your immobiliser as being the culpret yes ?

The other possibility is your CPS . Try unplugging and replugging your crank sensor/s connector/s and wiggling the wiring down to the sensors . if there is a bad connection from your CPS then your ECU will disconnect the internal earth to your DME relay and shut down your fuel pump and injectors.

Lastly if your fuel system is overpressured and the injectors draw too much current that will also shut down the ECU .
Worth checking your fuel pressure with a guage in case your FPR valve is fubar

 

[JuvUK]

thanks peanut, if i hold the upper part of the relay closed the car will keep running, i don't have an immobiliser

 

[peanut]

ok well if the car keeps running with the second relay switch forced on then the ECU is failing to earth the second relay coil.that is terminals 87b and 87 on the DME relay.

The reason for this is probably going to be either
A bad earth connection Mp2 &MP3 both under the dash next to the fuse relay box
A faulty CPS ( crank position sensor ) Does your tach needle jump slightly when you crank the engine over ?or
A bad connection on the wires from the CPS to the ECU or
The ECU connector terminals dirty /wet or
Break in the wiring to terminal 87 or from terminal 85b to the ECU
Also check the spade connectors to terminals 85b & 87 on the base of the DME relay socket

It could be the wire from the starter solenoid to the ECU .

 

[JuvUK]

thanks again for the reply!!

now for the total newbie, can you please explain where these things are in/on the car, so i can track them down?


thanks, Justin

 

[peanut]

well a good start would be to tell me the year of your car otherwise i'm not going to know where they are and what setup you have am I !?[] Is it an oval dash model or pre-oval?

Ok its an oval dash 86.5 model ......i checked your past posts.[]

You posted about exactly this same problem back in April this year has the car not run properly since then at all ?

 

[JuvUK]

no that was when the relay failed, the new relay had it running fine everyday since then, that is why i said it felt the same as the last time the relay failed.

another new relay hasn't fixed it this time (I thought it may have been because i bought a cheap relay from ebay, so i bought a genuine one from Porsche this time, but to no avail )

yes it is a D reg 2.5 Lux with an oval dash so 86/87 ish

 

[peanut]

ok justin thanks.
Intermittant problems are the worst type because as fast as you think you've found the source of the problem it reoccurs[]

If I am going to help you I need you to do a few tests which will be diagnostic . i'll pm you
Nick

 

[JuvUK]

cool thanks.

i am at work right now but will try to carry out the suggestions this evening

 

[JuvUK]

a little update,

i tried the 3 wire bypass, no change in fault.

also the rev counter needle does indeed bounce

 

[peanut]

ok Justin thats very good because that tells me a lot actually. It certainly narrows down the problem area.
I would prefe rto do this by pm ( personal message) if that is ok with you or it will get messy and confusing with lots of different ideas and suggestions.
I'll pm you when I've finished my dinner yum yum []

 

[JuvUK]

i shall look forward to it[]


thanks

 

[JuvUK]

PMs are in the user profile right?

 

[peanut]

sorry justin I assumed you knew.
click on my ID 'peanut' then select 'email peanut'

I sent you my email address to make it easier . I'm not getting a copy of any of the pm's I'm sending via PCGB so normal email messaging would be more reliable.
Nick

 

[Frenchy]

Yep, if not the DME Check the throttle position switch TPS there should be a "click" when opening the throttle, checked when engine is off and you manually operate the throttle from under the bonnet, could possibly be the flywheel sensors.

Keep us posted, it will fix !

 

[JuvUK]

fingers crossed!!

i am missing using my car for work!! (i'm having to borrow the wifes car)

 

[JuvUK]

Hi all,

i wanted to give you another update, and to thank you all for your help and suggestions.

after a lot of different tests and some cleaning up and double checking, it turns out that my fuel pressure regulator had failed and stuck closed giving me a very high fuel pressure which in turn made the injectors draw too much current and then to shut off.

one new regulator and she is back up and running

another side effect of the failed fuel pressure regulator is to cause the DME relay to burn out as well, so if you seem to be getting through relays quickly it could be worth doing a fuel pressure check


thanks again all, Justin

 

[peanut]

justin it would be nice if you gave me the credit for diagnosing the fuel over-pressure fault by suggesting disconnecting one of the injectors to confirm the fault.

I also suggested the reason for the fault being the FPR valve , and explained how the fuel over-pressure causes the ECU to shut down the injectors due to them drawing too much current.

I work very hard on this and other forums spending many hours to help owners with their car's no start and non running issues with a good success rate and a little bit of credit now and again is a welcome recognition for that effort []

 

[peanut]

Justin i have posted our email communications relating to this thread to enable the thread to have a resolution which might help other members resolve a similar problem with their cars in the future.

Hi Justin
Ok I'll try and give you the gist of my previous email in brief.
The 3x wire DME relay bypass I had you test has given me some useful information. It has narrowed down the components we need to test .
If the engine still dies after starting with the bypass in place that should mean that the DME relay and the ECU (engine management Unit) are not the direct cause of the problem.
A couple of questions.
Q1 how many seconds after starting the engine before it dies ?
Q2 How much fuel in the tank ?

It looks like there is either a fuel pressure issue or one or more of the ECU sensors are not supplying the necessary data to the ECU for it to keep the fuel pump running.

One more simple test .
Disconnect any one of the electrical connectors to any one of the fuel injectors . You'll need to pull off the wire locking circlip which holds the connector onto the injector.
Try starting the car with the DME relay removed and the 3x wire bypass in place. Tell me what happens.

If the FPR (fuel pressure relief) valve isn't functioning correctly it could cause either
too little fuel pressure for the injectors to work properly or
too much fuel pressure which would force the ECU to shut down the injector supply

Disconnecting one injector reduces the fuel pressure and in an over-pressure situation allows the ECU to earth the DME relay, fuel pump and injectors.
Regards
Nick

Hi Nick,
Ok so today's test completed, with one injector unplugged the car started and ran for longer (30 seconds ish) it was very lumpy as to be expected, then stalled, re-starting was very difficult.Any use?
Regards, Justin

Hi Justin
yes that is good news because it means we are definitely on the right track.

It looks like your fuel pressure is too high and the result of that is that the injectors draw more current which is registered by the ECU so the ECU shuts down the injectors by disconnecting their earth internally.

There are several reasons why your fuel pressure could be too high. The obvious and most common one is that the FPR fuel pressure relief valve is faulty but there could be a blockage to the pressure relief tube from the FPR back to the fuel tank .

Justin
If you attach a fuel pressure guage to the fuel rail test point and start the car you should be able to see what is happening to the fuel pressure .

use the 3x wire bypass to test the fuel pressure initially . you could also test the flow rate of the pump at the same time by running a hose off the fuel rail test point into a clear coke bottle or similar.
Run the pump for 15 seconds by connecting the 3x way bypass and multiply x 4 to get the flow rate per minute.I used an old plastic kitchen measuring jug marked with mls

If those tests check out ok then check the overflow return from the FPR to the tank is not blocked

Don't forget using the 3x way bypass for all your starting ,running and testing is giving you a guaranteed continuous power supply to the ECU , fuel pump, injectors etc which eliminates any doubt over the DME relay being part of the problem.

If you make the 3x injector test again try measuring the voltage from the terminals of the unused electrical connector to earth both before starting the engine and after the engine cuts out. It should be 12v+ . If its less then the ECU is likely to be faulty .

 

[JuvUK]

Hi Nick,

yep indeed your help was invaluable, and i did send you an email to thank you personally.

thank you again for helping me with this, i am very grateful.