Pelican Technical Article:
Fuel Filter and Accumulator Replacement Wayne R. Dempsey
911 (1965-89) 930 Turbo (1975-89)
New filter, new accumulator
Replace the accumulator if you are having a CIS warm start problem
Cleaner running fuel system
Replace fuel pump, or fuel lines.
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.
Check out some other sample projects
from the book:
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 doesnt 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 its 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 doesnt 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 doesnt 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.
Followup from the Pelican Staff: ok, got it. - Nick at Pelican Parts
Comments: I recently replaced a working but leaking fuel pump and fuel filter on my 71 911 and am having problems with the restart. I c an hear the fuel pump coming on but am not getting any fuel it seems. What Is the problem? What is the proper way to bleed the system after disconnecting the fuel system? Thanks.
September 2, 2014
Followup from the Pelican Staff: You do not have to bleed the fuel system, it will bleed when you crank the engine. - Nick at Pelican Parts
Comments: I don't have a starting problem, car starts fine. while driving car the engine stops running traveling at 30-60 mph, when I turn the key to the off position then to the on position the engine starts running again, it seems something has reset in order for the engine to run again. I changed the electronic part of ignition switch but I still have problem
August 30, 2014
Followup from the Pelican Staff: You could have a relay failing, like the main engine relay. - Nick at Pelican Parts
Comments: My 1978 911SC Targa, I can be driving 60mph and the engine shuts down, turn key off then back to the on position and the car compression starts and runs fine, then it might not do it again for 2 months or in 5 minutes. I changed the electronic part of the ignition switch, nothing changed. Any suggestions?
August 23, 2014
Followup from the Pelican Staff: I can't help without knowing what is missing from the system when you are trying to start it. When your engine doesnt start youll want to check the basics. Check spark, fuel injector pulse and pressure fuel, volume and quality. Once you figure out what is missing, it will be easier to diagnose.
- Nick at Pelican Parts
Comments: I have a 1977 Porsche 911 and just recently have had trouble starting it. Only way i can keep in running is by cycling fuel pump relay, otherwise engine will flood and raw fuel comes out exhaust manifold. Any ideas??
August 17, 2014
Followup from the Pelican Staff: The fuel pressure could be too high. I would perform a fuel delivery system test. Check fuel pressure, volume and quality.
- Nick at Pelican Parts
Comments: Just ordered the one listed for a 1977 Porsche 911S, but there was no bottom line connector on it, it only had two on the top and a small screw on the bottom some sort of attenuator?
Which one do I need to order if the one listed is not correct?
June 18, 2014
Followup from the Pelican Staff: You may have been sent the wrong part by accident. Give our parts specialists a call at 1-888-280-7799 and they can help figure out which part or repair kit you need.
- Nick at Pelican Parts
Comments: How to remove end of arm.?
June 9, 2014
Followup from the Pelican Staff: Use a shop press. - Nick at Pelican Parts
Comments: my friend bought a 1981 targa that has not started in about 4 yrs . I cleaned out the tank and added new gas. the car turnes over and has new oil and filter and i am not getting any gas to the injectors yet. This is a factory wide body slant nose, black needs wk built in a program called Weissach and imported over to the states in 1985. we want to sale this car.
April 23, 2014
Followup from the Pelican Staff: Sounds like the fuel pump is faulty. - Nick at Pelican Parts
Comments: I have a porsche sc 911 1983 and need to no how to wire a new ac switch
March 29, 2014
Followup from the Pelican Staff: Is it a factory A/C system? - Nick at Pelican Parts
Comments: Hi, I'm trying to start a 1975 911 that has been sitting for about 12 years. Drained the fuel tank strainer was clean and refilled with fresh gas. Pumped fresh gas through the accumulator and filter to a container till clean. Air flap was stuck, but working it a bit loosened the piston right up. No start and plugs are dry, but it tries once in a while. I don't have a fuel pressure gauge, but what could I try next? Thanks
December 24, 2013
Followup from the Pelican Staff: I would grab a fuel pressure gauge so you can properly test fuel delivery. If the unit is corroded, fuel may be restricted. - Nick at Pelican Parts
Comments: Nick, I appreciate your suggestions. However, it turned out to be something I had not considered. It turned out the nearly 40 year old connector on the fuel pressure regulator had developed some corrosion I live in Santa Barbara. A quick clean up of the connector and I can now hear the regulator bimetal lever actuate. It starts instantly - good for another 40 years :
November 2, 2013
Followup from the Pelican Staff: Thanks for the follow up. We appreciate it. Glad you got it fixed!
- Nick at Pelican Parts
Comments: On 74 Carrera any suggestions on modification to accommodate the new Bosch fuel filter since old filter NLA and the modification kit for the 74 fuel filter is also listed as NLA. Any diagrams/pictures of the old modification unit your numbers -911-356-901-00-OEM
Fuel Filter Update Kit, 911 1974?
October 24, 2013
Followup from the Pelican Staff: I don't have any photos to share.
I opened a post in our forums. A Pelican community member may be able to answer your question. - Nick at Pelican Parts
Comments: I have a '76 turbo that is difficult to start warm or cold if it has sat for a while. I have replaced both fuel pumps. To listen to the pumps I have disconnected the 2 pole connector behind the mixture control unit so that the pumps run when the key is turned. I must let the pumps run for about 25 seconds then hear a change in tone deeper and then the car will start right up. If I do not wait for the sound change the car fires for a second and then dies. Car runs great once started. Is this something the accumulator could cause, it is the original, 57K miles. I also replaced the filter.
October 23, 2013
Followup from the Pelican Staff: If pressure drops off the accumulator could be the issue. First I would check fuel pressure, volume and quality. You want to be sure the pumps are delivering enough fuel before condemning the accumulator. - Nick at Pelican Parts
There is some sort of device attached to the gas tank and it's the one making the whinning sound. Looks to me like an electronic pump of some sort.
September 18, 2013
Followup from the Pelican Staff: Is the device attached to the small tank on the top of the fuel tank? Can you share a photo? - Nick at Pelican Parts
Comments: I have a 1992 Carrera C2 and I recently replaced the DME relay because the car was almost stalling when applying the gas and I began to experience the smell of gas in the car. That all stopped. I've also noticed that in the trunk, right above the gas tank, something's making a whinning noise and it's been this was for about a month. Does anyone know where the noise is coming from?
September 17, 2013
Followup from the Pelican Staff: There isn't much above the fuel tank on your model, except for the fuse box. Could there be a relay that is noisy? I would remove the fuse box lid and see if the noise is coming from inside it. - Nick at Pelican Parts
Old Enough To Know Better
Comments: On a 85-89 Carrara, does the fuel pump run before the starter motor is engaged?
July 27, 2013
Followup from the Pelican Staff: No the fuel pump relay should only be engaged when cranking or running. A relay may be stuck or someone bypassed the wiring. - Kerry at Pelican Parts
Comments: I replaced my fuel pump, fuel distributor, fuel injectors, air box check valve, fuel filter and fuel accumulator and I still have a serious issue with restarting a hotnormal Temp. engine. Now thinking about replacing the car. Can anyone please advise. Frustrated 1979, 911SC, 3.0L, owner.
May 25, 2013
Followup from the Pelican Staff: In between the fuel distributor and the air intake below is a 3mm adjustment to fuel mixture that may need adjustment since you replace the fuel distributor. Also there should be a warm up regulator, cold start injector and frequency valve that all have an effect on how much fuel is injected on a hot restart. Are you lean on the restart (will it start easier with starter fluid) or are you flooding out (do you need to open the throttle all the way to get it started. Pull a spark plug and see if it is clean or black and full of soot. - Nick at Pelican Parts
Comments: my 82 911sc wont start i was driving and it stop while i was at a light hasnt started since unless i spray starter fluid i already replace the filter and pump but i dont hear the pump going as the key is turned on please help.it did not have any cold or warm start issues
May 1, 2013
Followup from the Pelican Staff: Check the engine for fuel pressure. If there is none, check for power and gorund to the fuel pump when you try to start the engine. - Nick at Pelican Parts
Check out some other sample projects
from the book: