79 SC Losing residual pressure.

[limble]

The car 79 911 with std CIS system

I’ve been having intermittent starting problems. They occur when the car is hot and/or cold. The car turns over but takes too long to finally start.

Things that I have tested and the results:
Fuel accumulator- holds pressure at the top fittings and a vacuum at the bottom fitting. I replaced this last year when I was having a similar problem but it turned out to a broken spring in the check valve.
All injectors spray a nice pattern and do not drip.
Control pressure – good
System pressure – good
Residual pressure – loses pressure very quickly (0 bar @ 7 min), hence the title of the post.
Check valve – replaced a year ago and still has pressure at fuel accumulator
Fuel pump – more than adequate flow it was replaced with the check valve.

It will always start easily right after it is shut down (less than 5 min).

The cold start valve is hard to see let alone test. Does it require a partial engine drop top test.

What else could cause a loss of residual pressure?

[ossiblue]

There is a pressure valve in the FD as well that may be causing a pressure loss if it is leaking. I'll try to find a diagram for you but in the meantime, look in the rear of the FD (firewall side) for where the fuel line and a return line connect. That is where the pressure valve is located, inside the body of the FD. Hopefully someone will post the diagram soon, if I can't find it.

[ossiblue]

This is the best I could find. It's component #15 in the diagram. Leaks are usually due to grit or bad 0-rings.

[limble]

If this leaks should I see gas dripping? Or does it flow back into the system some how?
I read on a different post that it could be a small hole in the membrane in the fuel distributer. I ordered one to try a rebuild but I'm reluctant to go that deep into it until I replace parts that aren't so finicky. Yeah, yeah I know there are no parts on a Porsche that aren't finicky.

[Bob Kontak]

I did the cold start valve test. It was over ten years ago. I am almost certain I had to drop the engine several inches. It was a PITA.

I was able to pull it back a couple of inches to see if it sprayed. You need an allen wrench to get the two bolts out.

[ossiblue]

You wouldn't see fuel leaking if the pressure relief valve is sticking open. This is just one possibility that would be on my list to check as cleaning and 0-rings are cheap. The diaphragm could also be suspect, as you mentioned, and that would be next on the list if R/R of the relief valve doesn't cure your problem.

[limble]

Ossi,
Thanks I'll order the kit and give it a shot.
Bob,
The partial to check the cold start valve is next

I found the diaphragm, mentioned in my second post, here and have it in hand.

BOSCH FUEL DISTRIBUTOR DIAPHRAGM FOR 6 CYLNDER ENGINES | eBay

[boyt911sc]

I would advise that you isolate the system and identify where the pressure loss occurs rather than just buying parts and replacing them. You maybe lucky sometimes but you cannot depend on luck all the time. Install a fuel pressure gauge and find out whether it is the FP check valve, fuel accumulator, or the FD return line. Fixing the problem means finding the root cause not the symptom/s.

Tony

[limble]

Boyt911sc,
I agree with you. I don't like throwing money at this and I haven't. I have checked the FP (plenty of pressure and flow rate within spec) fuel accumulator checks out good, the FD return line is where I found the low residual pressure. Now I'm trying to determine, when the engine is shut down, where the pressure loss is occuring. And as I understand it the CIS system is pressurized until the next start. There are no drips and physics tells me it has to go some where.
The Bentley manual, I think, only has you check pressure at the FD line. Where else can I hook a pressure gauge that will help me make a proper diagnosis?