[Grady Clay]

Here is the common problem and why.

While earlier cars suffer this indignation, the ’69 was the first to really suffer. The reason is the circulating fuel system and fine rust particles in the gas tank.

The outlet fitting in the gas tank is actually a very fine mesh brass screen filter. It is located horizontally a few cm above the lowest point in the tank. The lowest point is where the drain plug is located.

With the circulating fuel system, Porsche cleverly routed the return to the tank to a banjo fitting at the drain plug. This allowed the continued use of the prior gas tank and not to engineer a new part. All is good.

Over time, fine rust particles wind up in the tank. Some comes in with the fuel. Some is from corrosion of the inside of the tank.

Very fine particles pass the screen filter and are retained by the fuel filter. These are discarded with the used filter. Even finer particles pass through the engine with no damage. The largest particles remain in the tank. Some stay stuck to the screen but most fall off when the fuel flow stops and fall to the bottom of the tank. As they should, they gravitate to the lowest point – at the drain.

Here is the rub.
With the circulating system, the return at the drain plug stirs up the sediment and suspends it in all the fuel. Over time and many miles of continuous driving at first, the suspended sediment covers the screen filter. Eventually this reduces the fuel flow to the engine and effects performance, i.e. it stops.

Frustrated, you pull off and when it won’t restart, you turn off the fuel pump. With no flow, the particles on the screen fall off and drop back down as sediment. Towing the 911 home or to the shop sloshes the fuel causing more cleaning of the screen. Checking fuel pressure shows normal pressure. Replacing fuel filters does not help.

As the screen becomes increasingly restricted, the time/distance until no-run becomes shorter. I suspect sitting there with the ignition off and swearing while flailing arms jostles the fuel enough to get the car to run for a bit more.


Diagnosis:
The first thing to do is measure fuel flow. This is best done with a long fuel line from after the filter in the rear. Return the fuel to the gas filler. You can let this run for an hour or more, taking flow measurements regularly. If the flow decreases over time, you have the problem. Do this waaay outdoors. Be careful.

Another useful test is to Tee-in a pressure gauge at the engine and use a long fuel hoes to a gauge in the cockpit. You can then monitor the pressure while the flow is restricted. It will drop to zero or close. This has the danger of a fuel leak. Be careful.


Maintenance:
You can siphon almost all of the fuel out of the tank via the level sender opening. Remember to replace the sender gasket. Many have use-once crush washers under the five scews.

Cleaning the fuel tank at the drain is part of regular maintenance. Since you need to remove the front pan (4 bolts) to get to the drain, it is not much more effort to remove the screen filter fitting and clean it.

The brass mesh is so fine that the rust particles can almost totally clog the flow yet the wire screen pattern is still visible. Many an expert mechanic hasn’t recognized that the screen was clogged because he could see the screen.

These screen filter fittings are still available new if the screen is damaged. There is a plastic 914 version of the screen filter fitting but I don’t recommend it in a 911.


Improvement:
I replaced my screen filter fitting with new and installed the good-used original in the gas heater supply port. This way the return fuel is dispersed and doesn’t disturb any sediment. It just sits at the bottom until I drain and clean the tank.


This problem is not as much an issue with CIS. I think this has to do with the ‘swirl pot’ design, the vertical position of the screen, the screen size and the significantly larger size of the filter. It still needs regular cleaning.

Best,
Grady