Quote:
Originally Posted by dap930
Pink,
The '74 911's fuel pump has a check valve built into the banjo fitting coming off side of the pump. If that leaks through, the CIS residual pressure bleeds off too quickly causing vapor lock. Then there is the fuel accumulator that may have a bad diaphragm causing fuel pressure to bleed off instantly, causing vapor lock. Finally there is the pressure regulator piston in the fuel distributor that may be sticking due swelled o-ring seal that is not compatible with ethanol in the fuel (not likely since the fuel distributor has recently been rebuilt).
As for it dying while driving, since the car has been sitting for some time, the screen filter in the bottom of the fuel tank may be clogged with crud causing the fuel pump to cavitate due to low fuel flow, particularly if the pump gets too hot due to the reduced flow. The filter is removed from the bottom of the tank (after draining the tank) with a 22mm (I think) allen wrench, or a suitable sized double nutted bolt head will work. The filter screen can be cleaned or replaced if necessary.
Finally, if you plan keeping the car, I would recommend changing out all the fuel lines including the tunnel lines, with ethanol compatible hoses. The original hoses are very old and can spring a leak easily when exposed to ethanol in the fuel today. Len Cummings ("BoxsterGT" on this forum) can fabricate new ethanol compatible hoses with correct metric fittings to replace all your lines. The original tunnel lines were made of a nylon material that get brittle and cracks easily. Len's new tunnel lines are made of polyamide which Porsche started using in 1976 and is used in all new vehicles use today.
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Dennis,
The loss of residual fuel pressure in a
CIS will not cause vapor lock as you claimed. You can not produce a vapor lock in a K-Jetronic. Why? Gasoline is an incompressible fluid and system pressure of 70 psi. plus will displaced any bubbles, gas, or vapor present in the fuel lines. This is a myth.
A vapor lock could occur in a carb system because the system pressure is very low at 5 psi. or less. In a 930 system (CIS) where the system pressure is more than 100 psi., the fuel lines will readily be purged as soon as as the FP’s start to run.
For sake of discussion, produce, create, or improvised a condition you called “vapor lock” and I will prove you wrong. I enjoy doing experimental tests and let’s do it.
Tony