1983 Porsche 944: Starts, won't stay running

[DeadPorsche944]

Ok, I have read the forum on every thing to do with a car not starting. Here is what I know:

First the symptoms:
The car starts for a second or two then stops. I have squirted carb cleaner into the intake and once or twice I was able to accelerate it up to 6K RPMs before quickly dying.
So: obviously there is spark and fuel. Is there enough fuel?

How I got here:
Bought a car that didn't run. Put in 3 DMEs and the third one was the charm. A switch was put in to force the fuel pump on (new fuel pump) and a switch to force the DME on. That was good enough to get it home. Clutch died on the way home (Tulsa to Atlanta).

Just replaced the clutch and took it for a ride. Felt like the brakes were on, and the rears were. Rebuilt the rear brakes and it now rolls nice, but the car won't start. I took out the switches and fixed the wiring to the DME/FP relay. The fuel pump turns on when the computer is trying to start the car. There is spark, so relay is working nicely.

I tweaked the position of the speed sensor so that I
have 4.5 V pk-pk.

Still no difference in the car's ability to stay running.

Wit's end!!!

[DeadPorsche944]

Note:
I took the car for a 15 minute drive (with the rear brakes not releasing) prior to bringing it back to the garage and working on the brakes. My foot snagged the wiring that was hooked to the switches on the way out of the car. That is why I re-did the wiring.

Is there some other wiring in the driver's foot well that could cause the car (1983 Porsche 944) not to stay running?

[Bradical]

There is a port on the front of the fuel rail to take fuel pressure. If I were in your situation I would remove the fuel rail and test the injectors, I would guess that the injectors themselves or the fuel filter is clogged.

[DeadPorsche944]

Why would you think that? The car was running fine a few days before. Same gas. Nothing changed except the wiring that forced 12v to the fuel pump and DME. Something else under the dash could have gotten unplugged or disconnected.

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[DannoXYZ]

Measure AFM output voltage at DME.
Measure manifold vacuum.
Check for leaks between AFM and intake-valves.

[DeadPorsche944]

I'm stuck on a biz trip this week. I will try the suggestions when I get back. Thanks for your responses.

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[shadetree944]

I'll second the afm hypothesis. More than I'd care to admit I've forgotten to plug the afm back in after doing work in that area, and the symptoms are what you describe.

[txhokie4life]

vacuum leak.

[DeadPorsche944]

Originally Posted by shadetree944

I'll second the afm hypothesis. More than I'd care to admit I've forgotten to plug the afm back in after doing work in that area, and the symptoms are what you describe.

I don't remember unplugging the AFM. Isn't that plug up near the front of the engine compartment? I basically messed around near the crank and speed position sensors.

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[DeadPorsche944]

I had a few moments (10 minutes) to work on it today. I removed the DCM relay and basically shorted the computer and fuel pump on. I wanted to eliminate it as a possible source why the car won't start. No change. I did monkey around with the distributer cap a bit when I was trying to get it to restart, but I am getting spark. I ordered a new cap & rotor just to eliminate that. They show up on the 6th. Anyone know where to get an adapter that fits the fuel rail? It looks like an M12x1.5 and my Bosch fuel pressure guage is an M12x1.25....

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[DeadPorsche944]

Cap & rotor & spark plug wires are in. Will be checking tomorrow, hopefully

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[Bradical]

FUEL RAIL PRESSURE GAUGE KIT 924S / 944 / 944T at LINDSEY RACING - Your Porsche Performance Parts Center

[mikepellegrini]

TPS is unplugged?

[DeadPorsche944]

Originally Posted by Bradical

FUEL RAIL PRESSURE GAUGE KIT 924S / 944 / 944T at LINDSEY RACING - Your Porsche Performance Parts Center

Ouch! $85. I guess that wouldn't be so bad if I didn't already have a guage set.

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[DeadPorsche944]

Originally Posted by mikepellegrini

TPS is unplugged?

From what I can tell by looking at it, it's not unplugged. I will go to the computer and check levels at the connector

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[DeadPorsche944]

Ok, quick update. I snuck out to the barn and installed the new sparkplug wires, distributor cap, and rotor. No change. The car will not stay running very much after I release the key.

I did notice that if I stepped on the accelerator, the car died faster.

I spent some time looking for any loose connections, unplugged and replugged the AFM. The throttle position sensor connector is a bit harder to get to, so I just wiggled that one a bunch. I found a loose wire up by the driver side fire wall (center of attached picture) and removed the vacuum hose for a better shot of it. The vacuum hose was reconnected.

Anyone know where this should plug in? What is it for?

This is getting really frustrating...

[DannoXYZ]

That does not look like a factory wire. Trace to the other end and see where it goes . Do this to solve your problem once and forever:

1. Measure AFM output (green/red wire) @ DME connector with key ON, car not running

2. Measure AFM output with engine cranking and just starting and dying

With a recording 'scope, this is really easy. You see, the DME is programmed to blindly squirt fuel and fire spark-plugs to start the car. Then it inspects all the sensors to arrive at proper fuel-injector duty-cycle to match actual air-flow.

Without a proper AFM signal within a second of starting the car, the DME will shut off the injectors. That's it, black & white, all-or-nothing.

ALL the other stuff you've done is on the output side of DME and causes shades-of-grey variations. Plugs/wires/rotor, etc. will cause less-than-optimal running with stumbles at worse. But doesn't cause out-right non-running.

Input issues are more severe. The only other cause of all-or-nothing start-then-stop symptoms is the engine-coolant temp-sensor at front of head.


You can also make troubleshooting more time-efficient by using a single-procedure to test an entire system of individual components. For example, pull the fuel-rail and place little cups under each injector. Crank for about 10-15 seconds and you should have 8-10cc of fuel. This tests:

- ignition-switch
- speed & ref sensors
- DME relay
- DME
- fuel-pump
- fuel-lines
- fuel-rail
- fuel-injectors
- fuel-injector wiring
- FPR

You can the immediately rule out all those components as being a culprit and move on to other tests to find the problem, such as measuring AFM output.

[DeadPorsche944]

Thanks! Because of your post, I found an article (pdf) on how to check the AFM. I've got the 12V unregulated version.

It has to be something with it not getting power. Originally I had no power to the DCM, so I bet they are related. I hope to have some time to tonight to check this out . I will update after I see what is going on with the AFM power and signals.

[DannoXYZ]

No power to DME? Most likely DME relay and less likely ignition-switch. You've already made 3-way jumper (or switches) to power the fuel-pump and DME directly, so that shouldn't be a problem. Although permanent solution is to get DME relay working correctly.

I suspect a problem with input-sensor wiring in the bundle that got yanked out. The wiring is old and crack-prone leading to shorts or breaks. It's a pain, but measuring voltages & impedances at each terminal of DME connector may pinpoint a problem.

Search for "DME and KLR test plan" for a step-by-step procedure to trouble sensors and wiring. Ignore KLR tests as they're for Turbo models.

[DeadPorsche944]

Thanks. I will do so. The DME/Fuel pump relay is working. I removed the switches and fixed the wiring to it (ground wire was disconnected). I later jumpered out the relay to make sure it wasn't the problem. I'm out of town until Saturday, but I will find that article and be ready to test when I get back.

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