Random Stalling

[ScoobySteve]

Okay, so I wash the 944 and pull it back into the garage for waxing. Midway into the garage it stalls. Not a sputter, just suddenly off. I try cranking....it won't catch. I pop the hood and look in the belt inspection hole fearing the worst. It looks okay. I get back into the car and it starts up fine.

I finished waxing and moved it back outside for inspection...no problem. I'm ready to go grab some lunch and it starts, idles for about 30 seconds, and stalls. I crank (tach bounces) but no start. I wait 60 seconds and it starts. This time it runs for 2 minutes then, abruptly and without fanfare, stalls. I decide to take the other car to lunch so I need to futz with the garage to pull it back in. That involved it idling for 2 minutes and me driving it in....no problems.

As some background, the car had been sitting for a while and I just completed the following. (Note, I drove around for a couple of hours and 150 miles last weekend -- no problems)

1. Brand-new DME relay
2. Used fuel pump (newer "blue" style)
3. Plugs
4. Fuel Filter
5. Fluids

I'd almost rather have a car that didn't start at all then one which stops at random times.

[Born2bwild1191]

I have never had this happen to me in a Porsche, but i know in my VW bug i had the exact same problem. Some days it would run fine, then it would just stall for absolutely no reason. I would wait a minute or so and it would start right up. This went on for about a month and it kept worsening. In my case, my fuel pump had actually slowly fallen apart, until one day it wouldn't start at all and i noticed the pin in the fuel pump was suddenly gone. I dont know if that will help you or not, but seeing as you just replaced the fuel pump it may be something to look for. Not necessarily falling apart, but just a bad pump somehow.

[ScoobySteve]

UPDATE:

It's started right up. And is holding idle without drama.

BUT, I've noticed it's spitting condensation out the tailpipe. I've noticed a trend where when first started after sitting a few days it does this and the idle is slightly rough. I had some less-than-stellar gas in at while it was sitting but I've since put many miles and more gas in it. I'm wondering, could this be the residual effect of bad gas? Obviously, gas with water in it will tend to settle in the bottom of the tank. Should I pick up some fuel dryer as a precaution?

After driving around and sloshing the fuel idle is perfectly smooth with no vibration.

[Jeff Laurence]

I would check 1. ignition wires,
2. DME relay
3. remove battery and see if there is a hole where water can seep down the wire bundle that goes to the brain- it can get into the brain and cause intermitent failures.
Good luck

[Jeff Laurence]

sorry ,skip step 2, and yeah try alcohol in the tank

[Dtchy]

Cracked DME solder joints??

IIRC this has the same syptoms, like the engine suddenly stalling...

[ScoobySteve]

UPDATE:

Thanks for the ideas, everybody.

I went out this morning and started up the car. Everything ran fine for 15 minutes, getting up to temp. I also revved a bit to make sure I was giving the injector drivers and ignition a workout. After 15 minutes I shut things down.

I'm thinking this is somehow water-related. I analyzed the events of yesterday and realized that the average length of time the motor would run without stalling was proportionate to the time since washing. (Hehe, I guess I can't drive it in the rain)

I don't like things like this hanging over my head so here's my punch list of things to check:

1. Under-battery area for holes
2. Passenger DME footwell for any signs of water
3. Tightness of DME connector

BTW, the stalling behavior itself is odd. The tach doesn't drop to zero, it continues displaying accurate (falling) engine speed. Doesn't the tach get its signal from the DME? I'm trying to think of what sensor shorting (if that's it) would cause the DME to continue to output a tach signal but completely stop normal engine running. I have to think if this were just an ignition problem in the engine compartment that it would sputter as it died -- and not immediately restart. It's like for some reason the DME suddenly thinks the engine is off. I thought the only way that happens is if it doesn't think it's rotating -- but I have a tach signal from the DME so it knows it's rotating.

[Jeff Laurence]

After trying to seal my battery tray with no sucess, I mounted the brain on a piece of 1/8" aluminum and mounted that on the side of the tunnel beside the pass. seat. Now the water has to flow uphill to get into the brain. I have driven through monsoons with not even a hiccup since then.

[AKCJ]

I would install a fuel pressure gauge on the end of the rail. Then you can check for fuel pressure the next time it stalls. Are you sure the (newly installed used) fuel pump is running when the car is stalled?

I would also double and triple check that (new) DME relay. I have heard of new ones being bad.

Good luck and let us know.

[mallard ducks]

One possibility is that the batterycables and some of the grounds are relatively unprotected from the sort of spraying that you do when washing.

A good wash with lots of rinsing could soak things enough to even prevent you from starting unless you dry things up.

After I made a cover for my battery out of a paint roller tray liner for when I'm washing it and I started towel drying the grounds on top of the fenders after washes (they always get soaked) I stopped having any after car wash starting/weak starting issues.