This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
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.
The first step is to make sure that the fuel in the system is unpressurized. As mentioned previously, the fuel accumulator is spring-loaded to keep the fuel in the system pressurized when the engine is turned off. I recommend that you only disconnect the fuel filter and the accumulator after the car has been sitting a while, preferably overnight. If the replacement must be made recently after the car has been run, then open the connection to the top of the fuel accumulator slowly and carefully. You want to make sure that you depressurize the system slowly, so that fuel doesn't fly everywhere. Make sure that you wear safety glasses, as gasoline can sting your eyes.
To remove both the fuel filter and the accumulator, simply disconnect the fuel lines from both the top and bottom of both units. Depending upon when the last time these were replaced, it may be difficult to unscrew these lines from the units. If necessary, disconnect the line from the top of the filter, and the bottom of the accumulator, and remove the two as an assembly. This will allow you to take the two over to your workbench to disconnect the hard metal line that joins them together. These two units are simply held on with a hose clamp that can be unscrewed.
Make sure that you have a few paper towels handy, as there will be some fuel spillage from this process. Also make sure that you perform the replacement in a well-ventilated area, as the gasoline fumes can build up. I also recommend wearing gloves: gasoline is not the healthiest chemical to get on your skin.
Installation of the new filter and accumulator is a snap. Simply install the new units using the original hose clamps, and attach the fuel lines. Before starting the engine, make sure that all the fuel you may have spilled has evaporated, and that your fuel lines are tight. Take the key and turn it towards ignition position without starting the engine. I also like to disconnect the CD box as well just to make sure that there are no sources of ignition around. When you are confident that the fuel system is pumping fuel through the new filter and accumulator without leaking, start the engine.
Shown here off to the left side of the engine compartment is the fuel filter (left) and accumulator (right). The working space is tight: you may want to remove the large black plastic blower hose that connects the blower motor to the fan. Make sure that you let the car sit for a couple of hours before you release the connections. If the fuel accumulator is working, the system will still be pressurized, and fuel may spray out. The accumulator typically holds the pressure for about a half an hour, but this time may vary among different cars. Be careful when you are removing the steel lines from the tops of the filter and the accumulator. These lines can become corroded and be very difficult to remove. If you can fit them in there, use two wrenches to disconnect the lines. Place one wrench on the filter, and the other on the line. Using this method will reduce the probability that you might damage one of the lines, or the accumulator. Sometimes a faulty fuel accumulator will cause various warm-start problems on the CIS-equipped 911. Try replacing your accumulator if you have problems with vapor lock or difficult starting when the car has been running for a while. Also keep in mind that the fuel pump check valve works in conjunction with the accumulator. A faulty check valve will generate the same symptoms as a faulty accumulator. A new fuel pump is roughly five times the cost of the accumulator, so most people replace the accumulator first.