73.5 Constant Fuel Pump Running.

[Artfrombama]

On my 73.5 with CIS the fuel pump runs constantly whenever the ignition is on. I've hydrolocked the engine before and had to remove the spark plugs to purge the gasoline from the cylinders (no damage).
I would like to install a timed relay like on later cars that only allows the pump to run a few seconds until the engine fires. An inertia switch wouldn't be a bad idea either.
What have you done?

[HarryD]

Later cars had this added. Early cars did not. You can look into the added complexity of a later car or just not leave the car in the run position when you are not going to start it.

FWIW, I have had my 1973.5 for 24 years and never hydrolocked it.

[HarryD]

Circuit traces 105 and 106 of the attached current flow diagram show how it was done in 1977.

G19 is a contact in the air box to tell the pump to run.

Your air box does not have this.

[thetorch]

Before I went to EFI and an ECU to control fuel, I installed a fuel pump cutoff switch. It acted as a relay on the fuel pump, tied to the tach. It would prime for 3 or 5 seconds when the key was turned on, then cutout if the RPM was below something like 500 RPM. It was easy to install in the front. Clearly a good idea to have. It's not an inertial cutoff, so unless the engine dies, it will not work in an accident scenario.

I think this was the unit: https://www.classicindustries.com/product/RE12003.html

[nickelplated5s]

I'm pretty sure CIS = continuous injection system. In other words a loop where the DME pulses injectors per it's mapping. I'm an MFI guy but my advice is to get ahold of Steve Wong from this site.

I blew a high pressure, 30k psi, return line on a 6.7l diesel once. Blew about 10 gallons in the 1/4 mile to safe parking.

[HarryD]

Originally Posted by nickelplated5s

I'm pretty sure CIS = continuous injection system. In other words a loop where the DME pulses injectors per it's mapping. I'm an MFI guy but my advice is to get ahold of Steve Wong from this site.

I blew a high pressure, 30k psi, return line on a 6.7l diesel once. Blew about 10 gallons in the 1/4 mile to safe parking.

CIS is a form of mechanical injection. There is no DME. The entire system runs on the fuel pressure. The injectors open when the differential pressure between the injector and the cylinder is high enough.

[Artfrombama]

Originally Posted by thetorch

Before I went to EFI and an ECU to control fuel, I installed a fuel pump cutoff switch. It acted as a relay on the fuel pump, tied to the tach. It would prime for 3 or 5 seconds when the key was turned on, then cutout if the RPM was below something like 500 RPM. It was easy to install in the front. Clearly a good idea to have. It's not an inertial cutoff, so unless the engine dies, it will not work in an accident scenario.

I think this was the unit: https://www.classicindustries.com/product/RE12003.html

Going to give it a try. Thanks!

[proporsche]

as mentioned -why do you want to have the key on the runing position if you are not going to start the engine?? you did not say ??


Ivan

[E Sully]

If it is hydro locking I would be more concerned with other factors than delaying the fuel pump. The injectors may be leaking or the air flow plate might need adjustment.
It seems to me the injectors should be pulled and placed in containers to check for leaks, flow, and pattern along with a pressure test of the system. The cold start valve should be checked also, making sure the micro switch is set correctly.
It is difficult to find good injectors, and quite expensive if you do. Another option would be using a similar lower priced Mercedes injector. These operate at a slightly higher pressure, so all 6 would need to be changed.

[boyt911sc]

Originally Posted by E Sully

If it is hydro locking I would be more concerned with other factors than delaying the fuel pump. The injectors may be leaking or the air flow plate might need adjustment.
It seems to me the injectors should be pulled and placed in containers to check for leaks, flow, and pattern along with a pressure test of the system. The cold start valve should be checked also, making sure the micro switch is set correctly.
It is difficult to find good injectors, and quite expensive if you do. Another option would be using a similar lower priced Mercedes injector. These operate at a slightly higher pressure, so all 6 would need to be changed.

The FP could run all day in a CIS without delivering fuel to the injectors. Even with all the injectors removed from the fuel lines a good fuel distributor will NOT allow fuel to flow through unless the plunger is elevated. Check the FD setting.

Tony

[PeteKz]

^^ Ditto this.

I installed the later fuel pump relay in my 1973.5 car to take advantage of the FP safety switch in the 1980 CIS system on my 1980 engine. But I did not have leaking injectors before I did that. Find out why they are leaking.

[Zuba]

I have a 1973.5 that had some hot start issues, I backflushed the injectors and was still losing some pressure. I had read this thread and have been running these injectors all year. Hot and cold starts are massively better.

http://forums.pelicanparts.com/porsche-911-technical-forum/1025109-1973-5-911-fuel-injector-4.html

The other thing I did that has helped is wired a momentary switch to the cold start injector in the airbox as on the 1973.5 it sprays when the starter is cranking no matter what temperature the engine is and would cause an extremely rich issue when having issues hot starting.

The new injectors and manual option to spray fuel at cold starting have made the car even more fun to drive!

I do like the fuel pump cutoff and will be adding that this winter