Hot start

[Pink2300]

I acquired my 1974 911 Targa from my brother-in-law because he used it very little. After an oil change at the shop, I was on the road. Soon after my coil passed away. Replaced, I was on the rode again. It was running rough so back to the shop. I have new plugs, wires and they overhauled the fuel distributor. It ran great. Then the trouble started. On occasion, it wouldn't crank on a hot start after being parked for up to an hour. After cooling down, it would crank and run great. Several times of putting me down, I took it back to the shop and they again rebuilt the fuel distributor. Left the shop and it ran great. After a couple of weeks of no trouble, it died while driving. I coasted off the road and played with the flap in the air box. It cranked and I started home. It died again and would not crank. Caught a ride home, got my truck and car dolly and went to pick up my car. I was gone about 20 minutes, tried the car BEFORE loading and it cranked so I drove it home. MY WIFE WILL NOT RIDE WITH ME until I correct the problem! (My 69 VW vapor locks all the time.) Is this 911 vapor locking or WHAT THE HECK IS GOING ON?

Thanks!

[boyt911sc]

Residual fuel pressure or ignition, it could be temp. related problem. Why was the FD rebuilt the second time?

Tony

[Pink2300]

I think the shop is baffled by the problem, so they rebuilt it again, at no cost to me. They checked the fuel pressure on both sides of the pump and were satisfied with the readings.

I did notice the temp was around 250, climbing a mountain, with about a 2000 foot elevation gain. However, we stopped for supper at a restaurant, came out and it cranked fine on that trip.

[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.

[Pink2300]

Thanks a million! I will pass this along to the shop. I know they tried a couple of the ideas you suggested, but not all.

Thanks again!

[boyt911sc]

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.

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

[Pink2300]

I have had the car for just over a year so I don't think the gas tank is contaminated. Also, I failed to mention that my brother-in-law had the engine overhauled about 6 thousand miles ago so I feel all the fuel lines were replaced at that time.

I appreciate you folks discussing the possible source of my problem. Keep up the good work! ;>)

Pink

[boyt911sc]

Does the car starts now? Ask the mechanic to check the following fuel pressures:
system
control (cold and warm)
residual

And finally ignition timing and spark. If your temp. gauge is accurate, the engine is running too hot @ 250°F. Keep us posted.

Tony

[Pink2300]

The car starts every time on a cold start, runs great. The problem arises sporadically and time traveled doesn't seem matter. It really is hard to pin point.

Pink

[Nux]

By "crank" do you mean the starter won't turn? If so, it's a common problem with the solenoid. Do a search. And then replace with a high torque unit

[Pink2300]

Not starter problem because the starter spins like a top, as does the engine. I should have said that it doesn't "fire'. Let her cool down (I assume) and I am off and running. I am pretty sure it is a fuel issue rather than an electrical (firing) issue.