Will Not stay running

hamiltrf · 06-08-2003, 03:26 PM

I have a 85.5 944 NA. It will start but not continue to run. What I know is that the fuel pump comes on when ignition is turned on but goes off as soon as the car starts. I know this by the sound of the fuel pump and watching the voltage to the fuel pump with a multimeter. As soon as the car stops running the fuel pump starts again. I have replaced the DME relay with no change. I also replaced the Air Flow meter also no change. From other forums it seems as though the speed sensor could be causing the problem. I have disconnected both the TDC sensor and the speed sensor one at a time, that causes the engine to not start at all. Is there a ways to check these sensors besides an oscilloscope?

todwic · todwic's Garage

funny you should ask...
yes there is.

take your multimeter and set it at the rpm position. Pos lead to secondary coil wire, neg to ground. If you get an rpm reading sensor is good. switch the sensors around (plug and all) and do again. No rpm reading? wildly fluctuating rpm reading? sensor bad or maladjusted.
Take a portable shop flourescent shop light and take sensor out of bracket. hold rpm sensor end on the switch of the light and move sensor around it untill you feel a pulse. your fuel pump should kick on and off.
same sensor out of bracket, wave a piece of metal or another magnet right under the sensor. You should hear your fuel pump kick in.

todwic · todwic's Garage

you can also use the ohm setting and figure it out, and you can also use the DME connection and see if you are getting a pulse, tho I forget which con. point it is. Clark's garage has it all spelled out for you.

hamiltrf · 06-08-2003, 03:56 PM

My Multimeter has no RPM setting it is not specfically for auto. I do not understand your other two approaches as the fuel pump will run if the engine is not turning over but the ignition is on.

todwic · todwic's Garage

How about this: switch your sensors and your plugs. they are the same part #. If you don't want to do that, read the disclaimer.

hamiltrf · 06-08-2003, 04:27 PM

I will try switching the sensors. If one is bad what will be the affect of switching the sensors, ie what new problem should I expect to encounter because I now have a bad TDC for example.

todwic · todwic's Garage

well if your rpm sensor is just "kinda" bad, but too bad to read the teeth, and you swap them, then all it has to do is read the tdc pin. Don't ask me how it works, I just know switching the sensors worked on my car and another person's car. it's worth a try.

carsontc · 06-09-2003, 05:06 AM

Todwic's right...sometimes the resistance test is good but they still don't work...weird...swapping them and seeing the problem 'move' with the sensor proves it...a scope test may show otherwise but we all don't have a scope

the sensor connectors have 3 wires – 1 of the end wires is a shield and should read ‘open’ to either of the other 2

between the other 2 you should read 900 ohms to a little over a 1000 ohms

at http://www.clarks-garage.com

these sections have a lot of good info:

http://www.clarks-garage.com/shop-manual/eng-15.htm

http://www.clarks-garage.com/shop-manual/ign-02.htm

got a Haynes manual?

hamiltrf · 06-09-2003, 08:21 AM

I have a Haynes manual and the set of shop manuals for the 944. I have not seen a good description of a cause for the fuel pump power to come and go except on this forum. Tonight I will try some of the suggested checks and let everyone know the outcome.

Jason White · 06-09-2003, 10:31 AM

Does your car have a porsche factory alarm with the key hole on the fender? I had the same feul pump problem replaced the DME fuse and everything until some one told me the alarm cuts off the fuel when its armed. So I just disconected the alarm in the passenger foot well.

JRW

hamiltrf · 06-09-2003, 05:10 PM

Ok, I interchanged the TDC and speed sensors, while I was at it I checked to make sure they were connected as shown in Clarks Garage, they were. So I started the car and it did the same thing. Starts but will not continue to run. The fuel pump shuts off when the engine is running but starts again as soon as the engine stops. The car does not have a factory alarm system, in fact it has no alarm system. I am wondering is there a way that the ignition switch could cause this problem?

todwic · todwic's Garage

Entirely possible that it's your ign. switch.

Moneyguy1 · 06-09-2003, 07:53 PM

What is your air flow meter is inoperative? Just a thought, but if there is no reading, the computer would think the engine was stationary and the fuel pump would not be operating.

Is this possible?

carsontc · 06-10-2003, 04:47 AM

go back to clark's...sounds like the FPR

http://www.clarks-garage.com/shop-manual/fuel-07.htm

hamiltrf · 06-10-2003, 08:21 AM

I have checked the AFM it showed fine. But I also replaced it with one from a running 944 no change in how the car performed, Starts but then quits. Have not checked the FPR yet but will try tonight should be easy check. Thanks all

carsontc · 06-10-2003, 08:59 AM

when I tested for a no-start problem I ran a wire from the + battery terminal directly to the small black wire on the ignition coil to by-pass the ignition switch’s ON position...this will supply +12 to the coil and the DME relay...if it runs...buy Todwic a beer and buy yourself a new switch

(btw...this is how Todwic was able to amass 17 'parts' cars over the last several months)

hamiltrf · 06-10-2003, 10:31 AM

I had my son check the FPR as detailed in clarks-garage, the disconnect a fuel injector, unfornately it did not correct the problem. I will try carsontc idea for checking the ignition switch. The car will run if you force the DME relay on. I have already confirmed that.

hamiltrf · 06-10-2003, 04:32 PM

I just got finished checking the car again. While I thought that the problem was the same it has changed since interchanging the TDC and speed sensor. While the car does stop a little after it start each time. Now the fuel pump runs the entire time. If I were to bet I say that the old speed sensor now TDC is bad. What do you believe?

todwic · todwic's Garage

I beleive if you have the scratch, replace em both. I like Heinies

carsontc · 06-10-2003, 06:06 PM

so then the problem 'moved' with the sensor...I agree...I would suspect the original speed sensor

as the car is pushing 20 replacing both is a judgement call (as is a Heineken's for Todwic) but the other one will probably fail some day and if you have the 'scratch'...