Another Warm Start Problem Post!

unzio · 06-07-2010, 04:35 AM

Hi everyone, I'm looking for a few answers to questions that I have. First, the details: 1976 911S, 2.7, with the classic no start after it warms up (heat soak/vapor lock) problem. I have a fuel pressure gauge tee'd in post fuel accumulator, and the pressure drops almost instantly to 0psi after shut down, probably within 20 seconds. If I close the valve on my gauge set tee, then the pressure holds, which means that everything post-accumulator (injectors, etc) is fine. So, the questions are;

1- After searching on this forum for 2 days, it looks as if the general consesus is either accumulator or fuel pump/check valve. My issue is this; if both the accumulator and the check valve should maintain fuel pressure in the system, then if only one is bad the other should still do the job, correct? Am I missing something? My fuel pump does not have a check valve that is visible from the outside, so I am assuming that it is internal, if it is there at all.

2- If the cause of the problem is vapor lock, then why doesn't the fuel boil when the accumulator and check valve are functioning properly? Is it because the pressure raises the boiling point?

3- The car runs fine otherwise, but my fuel pressure gauge reads 85 psi running. I don't have a manual yet (should be here tomorrow!), but I've read many times that 65-75 psi is the norm. Is this an issue I should be looking into? My fuel pump looks original, although it doesn't look like any of the photos on our host's parts pages. The outlet is not on the end of the pump, it is a hollow bolt/banjo that is threaded into the side/housing of the pump.

Thanking all the experts in advance...

ossiblue · 06-07-2010, 05:49 AM

Welcome Unzio, you've found the best spot for all your Porsche problems/needs. I'll address each of your questions in order.

1. If the accumulator is bad, it will leak fuel into the return line to the tank so the check valve on the fuel pump is of no matter. If you connect your test gauge before the accumulator, you can test the check valve. If it holds pressure, the accumulator is faulty.

2. Essentially, yes, the accumulator raises the boiling point as long as pressure is maintained.

3. Your warm pressure is way too high if, in fact, that is an accurate reading. 85 psi works out to about 5.8 bar which is system pressure for your car. The warm controlled pressure should be about 3.7 bar. I am surprised your car is running smoothly with that reading. Be sure you have your gauge hooked up properly and do a retest--connected between the fuel distributor and inlet to the WUR, with the shut off valve located on the WUR side of the T fitting.

Again, welcome. Now would be a good time to include some pictures of your car--tradition--as well as some information in your ID, such as your location, so members can know if you are close by for additional help.

unzio · 06-07-2010, 06:22 AM

Thanks ossiblue!

Duh, I feel dumb! I guess fumbling around in the dark without a manual can lead to mistakes! I have my gauge connected between the accumulator and what I now realize is the return line! I thought it was the supply/pressure line!

I guess what threw me off was that the fuel filter is connected on the other port of the accumulator, so I assumed it was supply. Regardless, I can probably make a quick diagnosis now by pinching either the return line or supply line at the tank at shutoff. This will tell me which of the accumulator or fuel pump is bad.

As far as where to hook up the fuel line, there are 2 fuel lines on the WUR, one goes to the fuel distributor I think, the other looks like it is tee'd into the fuel supply line. I guess I should hook my gauge into the first line, correct?

ossiblue · 06-07-2010, 06:37 AM

Yes, hook your gauge up to the line coming from the fuel distributor and has a flare nut fitting at the WUR. The other line on the WUR has a banjo fitting and is the return line which T's into the return to the tank.

Joe Bob · Joe Bob's Garage

One or the other not working will cause a pressure drop. Mine was the check valve....lot's cheaper than the other.

unzio · 06-07-2010, 07:14 AM

ossiblue, so the fuel supply line does go into the accumulator first? Then through the filter and out to the distributor?

ossiblue · 06-07-2010, 07:15 AM

Quote:

Originally Posted by unzio

ossiblue, so the fuel supply line does go into the accumulator first? Then through the filter and out to the distributor?

Yes.

unzio · 06-07-2010, 07:43 AM

OK, so my warm pressure is 55 psi, bang on 3.7 bar. And of course, now my pressure doesn't drop when I shut it down! So much for my earlier diagnosis!!!!

I'll keep trying to start it and shut it down while watching the pressures, as this problem was intermittent. I'm pretty sure it was pressure drop and vapor lock, as the only way to get it started was with a shot of ether. I'll keep you posted, but if anyone has any ideas, let me know.

unzio · 06-07-2010, 08:11 AM

Update:

I let the engine get hot, and my fuel pressure is about 70 psi/ 4.8 bar. I think that may be a little high, but still within acceptable limits, correct?

As for the pressure bleeding off, it did drop to zero on this occasion, and the car wouldn't start. I got it running again and tested the pressure drop by pinching the fuel return line at the same time as a helper shut the car off. The pressure dropped to zero.

I redid that test but this time pinching the fuel supply line (fuel pump outlet) at the moment of shut down, and the pressure held at about 45psi. Funny thing, it kept holding even after I let go of the "pinch". Perhaps the internal check valve in the pump is getting weak and can only hold back the flow if the pressure isn't too high?