Porsche 911 (1965-89) Fuel Filter and Accumulator Replacement

One of the simplest tasks to perform on the 911 is the replacement of the fuel filter and fuel accumulator (on CIS equipped cars). The fuel filter filters out the fuel that is pumped into the fuel injection system from the tank. With older cars that might have rusty tanks, keeping the filter fresh and clean is a very important task to keeping the fuel system running clean. Even microscopic particles can build up and clog the tiny holes that are a key part of the carburetors or fuel injection system. This project details the replacement of both the fuel filter and the fuel accumulator. For the early cars (1965-early 1973) and the late-style Carrera (1984-89), you only need to be concerned about replacing the fuel filter.

The fuel accumulator is part of the Bosch K-Jetronic system, or CIS, that was used on the 911 from mid 1973 thru 1983 and 911 Turbos up until 1989. The accumulator contains an internal spring-loaded diaphragm that prevents vapor lock by keeping the system pressurized after the engine has been shut off. Vapor lock occurs when the fuel pump and lines become heated up from the engine. The gasoline in the lines and the pump actually evaporates and transforms into a gaseous state. If the fuel pump is turned on when the fuel is in this state, it tries to compress and pump the vaporized gas, which it cannot do. The only remedy is to wait for the gas to cool down to the point where it liquefies once again and can be compressed and pumped by the fuel pump.

Vapor lock rarely appears when the car is running. This is because the fuel in the system is constantly recirculated back into the tank. Fuel is pumped from the tank to the fuel injection system, and then the excess is returned back to the tank through a return line. This process of recirculation keeps the fuel pressurized in the lines so that it doesn't have a chance to evaporate. The process of circulating the fuel back into the tank also helps to remove heat from the fuel system. The added pressure in the system from the action of the pump also reduces the chance of the fuel evaporating.

If you are having warm start problems on your CIS engine, there are a couple of things that you can do. First and foremost, replace your filter and your fuel accumulator. As mentioned previously, the purpose of the accumulator is to keep the system pressurized, so if you are experiencing warm starting problems, then chances are this unit is not performing adequately. If you happen to get stuck somewhere, and need to leave quickly, carry a can of compressed air with you. This is the air in a can that you use to blow out dusty computer equipment. Since it's compressed as a liquid, it has a tendency to absorb heat as it expands. Blow this compressed air over the lines in the engine compartment and the fuel pump in the front of the car, if you can reach it. This should help cool down the fuel system a bit quicker.

Another important component of the system that often fails is the one-way check valve inside the fuel pump. A faulty check-valve allows the system to become depressurized when the ignition is turned off, thus defeating the purpose of the fuel accumulator. If a replacement accumulator doesn't solve your fuel related CIS starting problem, then you might have to replace your fuel pump.

Another smart idea is to keep the fuel tank full most of the time, and to not let it run down low. The CIS system recirculates the fuel back to the tank, therefore a full tank will act like a large cooling center for the fuel. When you shut the engine off, you might want to try leaving the fuel pump running (turn the key one notch towards starting position) for a minute or two. This should help cool the system and reduce the chance that vapor lock will form in your lines. When the engine is turned off, temperatures have a tendency to spike, as the cooling effects from the fan are stopped.

The replacement of the fuel filter is quite easy on the 911, although access to the area where the filter is located may be a bit difficult. On cars equipped with the large black blower hose, I recommend that you remove this in order to gain some more access to the fuel filter area.

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Time: 1 hr
Tab: $10 to $80
Tools: Wrenches:https://www.pelicanparts.com/Porsche/...
Applicable Models:
Porsche 911 (1965-89)
Porsche 930 Turbo (1976-89)
Parts Required: New filter:https://www.pelicanparts.com/cgi-bin/..., new accumulator
Hot Tip: Replace the accumulator if you are having a CIS warm start problem
Performance Gain: Cleaner running fuel system
Complementary Modification: Replace fuel pump, or fuel lines.