911 SC Fuel Pressure Diagnosis

Mar 29 2012 porschedoc 911 Variants 2 Comments

We are having a very brief cold start problem in this 911SC. After the car sits for about 8 hours, it struggles to start for a few seconds. Once it has started once, it will start immediately for the next 6-7 hours like everything is normal. Once it sits overnight, it struggles again. This makes diagnosing the issue very difficult, as you can’t tweak something, check it, tweak something else, check it. Instead you get one shot, then have to wait 8 hours for it to act up again. And repeat the process.

There are a couple things that can cause this, including the cold start valve, thermo-time switch, and the warm up regulator (WUR). On any CIS system, knowing what your system pressure, control pressures (hot and cold), and residual pressure, are critical to diagnosis. These are all tested by attaching your fuel pressure tester between the WUR and the fuel head, and using a jumper switch on the fuel pump to check the individual pressures.

Our pressures all checked out within spec, which is good, this means our fuel system should be good. On to the thermo time switch and the cold start valve testing.

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Michel

April 3, 2012 at 7:40 AM

I am having exactly the same problems.

My WUR pressures cold and hot are now within spec but after 10 hours of standing still I can hardly get the car to start. After that initital first start you can shut it off and the car will start perfectly 2hours later or 6hours later but when you leave about 10 hours between starts no joy.

I am suspecting a leaky cold start valve. When you start up and leave the car running for about 5 minutes pressure goes to 3.8 bar. This pressure stays high for several hours after shut off and will go slowly down to 1 bar. I believe this high residual pressure makes the cold start valve drip. After 10 hours a high amount of fuel is in the cilinder intake , to this is then added extra fuel by the TTS that fires the CSV and the WUR also delivers extra fuel as the cold control pressure is about 1.4 bar at 10 degrees.

I think this causes an overfuelling at start-up.

In addition the leaky cold start valve will probably also increase fuel consumption at normal driving.

Let me know what you have found to be the reason for this behaviour.

Michel

    porschedoc

    April 5, 2012 at 12:44 PM

    Hi Michel,

    Yes, it was indeed the cold start valve. I will write a blog post on the replacement soon, we have been swamped, so behind on the blogging. One other thing you can try, is that right away in the morning after it has sat, push and hold the accelerator to the floor and try and start the car. It should fire right up as you are adding a large amount of airflow to match the fuel that has leaked out of the cold start injector. It is a little bugger to replace, but can be done by just removing the heater blower and going around the driver side of the motor. It is 100% done by feel though as you can’t see anything.

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