Warm start problem

[TX911SC]

I’ve read numerous threads about warm start problems but would appreciate it if you could help me narrow down my problem / component and point me in the right direction (i.e. fuel filter, fuel accumulator, WUR, or other component). I replaced fuel filter…..problem continues.

Indications:
1. Cold start after sitting 24 hours or longer: starts immediately / runs perfect.
2. Re-start within 10 minutes of shutting down: starts immediately / runs perfect.
3. Re-start after sitting 30 minutes: Only starts after continuous cranking; 5-7 seconds; must repeat 4 to 6 times before it starts. I try not to give it any throttle (to protect my air box), but I usually have to give it a little or it won’t re-start at all.

Thanks for the help.

[ossiblue]

Originally Posted by TX911SC

I’ve read numerous threads about warm start problems but would appreciate it if you could help me narrow down my problem / component and point me in the right direction (i.e. fuel filter, fuel accumulator, WUR, or other component). I replaced fuel filter…..problem continues.

Indications:
1. Cold start after sitting 24 hours or longer: starts immediately / runs perfect.
2. Re-start within 10 minutes of shutting down: starts immediately / runs perfect.
3. Re-start after sitting 30 minutes: Only starts after continuous cranking; 5-7 seconds; must repeat 4 to 6 times before it starts. I try not to give it any throttle (to protect my air box), but I usually have to give it a little or it won’t re-start at all.

Thanks for the help.

If this was my car, the first thing I'd check is the residual fuel pressure. You've probably already read about it, so checking the pressure and the duration it holds would be my first step.

[TX911SC]

Sorry.....car is '83 SC 3.0 stock.

[TX911SC]

Originally Posted by TX911SC

Sorry.....car is '83 SC 3.0 stock.

Thanks.....reading up on "residual fuel pressure" now.

[TX911SC]

Read a few more posts. Looks like I'm wasting everyone's time until I buy a fuel pressure test kit.

[Walt Fricke]

I wouldn't say waste, but you've got it.

[Paulporsche]

You definitely need the gauge.

If you need help with using it or interpreting the results, let us know.

Quite often the fuel accumulator or the fuel pump check valve are to blame.

[stlrj]

Originally Posted by TX911SC

3. Re-start after sitting 30 minutes: Only starts after continuous cranking; 5-7 seconds; must repeat 4 to 6 times before it starts. .

The assumption here is that it is not getting enough fuel. But then again it might be flooding.

How can you tell which it is? Try disconnecting the thermo-time switch and see if it starts. If it starts right up with the TTS disconnected, then you know it's flooding since you just eliminated the cold start valve from spraying extra fuel while cranking.

Also, keep in mind that it is possible to diagnose most CIS problems without the need of gauges.


Cheers,

Joe

[boyt911sc]

Originally Posted by ossiblue

If this was my car, the first thing I'd check is the residual fuel pressure. You've probably already read about it, so checking the pressure and the duration it holds would be my first step.

TX,

Listen to the above advise. And pay close attention to whom you get advise. If your system was flooded, it would not start immediately after sitting over-night. You could re-start the engine after 10 mins. simply because you have not totally lost the residual fuel pressure. You need a pressure gauge to solve this problem and pressure tests would direct you to the culprit/s. A fuel pressure gauge kit is mandatory for fuel injection system troubleshooting regardless you have a Ketronic (CIS) or not.

Tony

[targa80]

I had this problem at one time in my 1980 SC and yes it is good to have a fuel pressure test set to check the residual pressure that is built up by the fuel pump. once the car is shut off the system should hold that pressure as long as there are no leaks and the internal check valve in the fuel pump is operating correctly. Using the fuel pressure test set and a few made up hoses I was able to isolate the fault. In my case the internal check valve in the fuel pump was bad. I purchased an external check valve and added it to the fuel line at the pump. If the fuel accumulator is at fault it is a torn diaphragm inside and the fuel leaks back to the tank.

Here is a pic of the parts I had ordered. you need to get the end cap and crush washers along with the check valve to mate to the existing system.

[Walt Fricke]

The old hands are guessing lack of residual fuel pressure. When the engine is cold (and everything else is working fairly well), it will start. It doesn't need residual pressure when it is cold. The CSV squirts, the warmup regulator richens things, and it starts. The pump builds up system pressure almost instantly.

When it is hot, and you restart it pretty much right away, as described, chances are there is still enough residual fuel pressure to help get it going, even though the things which richen it for cold starts aren't richening it (because they aren't supposed to).

You seem to be describing the intermediate condition, where the residual pressure has fallen enough that it isn't helping, but the engine is still warm enough that the cold helpers are turned off.

I've never really understood just why, even when hot but little or no residual pressure, the speed with which the pump pressurizes the system wouldn't cause it to start. But it is simply a fact that in this sort of lukewarm condition, without residual pressure it is hard to start, and we don't need to understand more than that, if we measure the fall off of residual pressure, and it is too fast after shutdown, there will be problems.

My guess as to why posits fuel and pumps or fuel system parts hot enough to vaporize the fuel at atmospheric pressure. The higher residual pressure raises the boiling point enough that this doesn't happen, so no vapor lock. The pressure is permitted to drop off, just not fast enough to overtake the cooling of the hot parts.

With the valve on the pump and the injectors in tip top shape, and anything else which might cause a leak back into the return line (WUR, system pressure regulator) you can expect residual pressure to be higher quite a bit longer than the values Porsche/Bosch give.

But the gauges and a watch will tell right off if this is an issue. Hook them up, pull the plug on the backside of the air measuring plate assembly (bit of a reach around the left side of the top of the motor, but you can feel and pull it), and you can just turn the ignition to run and the fuel pump will start. Much easier than jumpering the fuel pump relay. Let it run a little bit. You'll probably see 5 bar in the system pressure setting. Then turn the key off and take a look periodically at the gauge. If you don't have references, go to Jim Williams' CIS Primer site.

Don't forget to plug that plug back in when done. It is there to insure that the fuel pump won't run unless there is air flowing through the intake (as in the engine is at least idling). The start position of the ignition bypasses it. You can get along with it unplugged, but at the expense of the pump running when you don't want the engine running but want the key in the run position for this or that reason. And it is a safety feature - hit something, kill the engine, the pump won't just keep running. Just don't turn the car over and expect it to do its job.

[malmos911]

I have the hot start with my 74 911.Could it ever be an electrical issue? My mechanic states the pressures are good and he thinks it's electrical,maybe the coil gets hot and malfunctions? Sorry, didn't mean to steal your thread.

[stlrj]

Originally Posted by Walt Fricke

You seem to be describing the intermediate condition, where the residual pressure has fallen enough that it isn't helping, but the engine is still warm enough that the cold helpers are turned off.

Even 30 mins. after shut down I will guarantee the engine is cool enough ( below 45 degrees C ) and well within the operating range of the cold start valve. Disconnecting the thermo-time switch during that time and cranking the engine will tell you in an instant if the cold start valve is operating, leaving all guesswork out of the equation.

Been down that road many a time...


Cheers,

Joe

[911SauCy]

My '78 SC had the exact same issue.

Replaced fuel accumulator, no more problem.

My thought, FA was original, probably could use to be replaced anyway. My fuel pump and check valve are new as well.