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Fuel filter replacement and priming fuel lines: mk4 VW Jetta, Golf, New beetle TDI
How to drain or change the diesel fuel filter and prime the fuel lines - for mk4 VW: Golf, New beetle, and Jetta TDI 1998-2006difficulty: 1/5
Your owner's handbook and service manual calls for draining the fuel filter of water every 10,000 miles and changing it every 20,000 miles. This means install a new filter every 20,000 miles and drain it halfway through it's life. This article gives tips on changing or draining the fuel filter and then priming the fuel system.
If you are using high percentages of biodiesel such as 85% or 100% biodiesel, you should expect to change it early since biodiesel will clean out the old buildup and clog the fuel filter if there was significant build up in the fuel system. As a safe rough estimate, 1000 miles and then again at 5000 miles, your mileage may vary with biodiesel. A more economical idea is to install a small clear inline fuel filter before the main fuel filter to filter out larger particles and let you see how clogged the fuel filter may be.
Early symptoms of a clogged fuel filter are stumbling at high rpms or lack of power. A general lack of power could also be limp mode, see 1000q: TDI limp mode for possible causes and the solution. Make sure that these symptoms are not caused by an air leak in the fuel line or a clogged pickup at the fuel tank. Bacterial or algal growth in the fuel tank can clog the pickup. If your car is a 2004-2006 pumpe duse TDI, it's also possible that the low pressure in tank electric fuel pump, the lift pump, is failing, bad, or clogged. This can cause hard starting or a stalled engine due to fuel starvation.
Fuel filter basics
There is a temperature sensitive recirculation valve (called the "filter tee") on top of the filter in the fuel return line. When the fuel temperature is below 59 F, the valve recirculates return line fuel to the filter which helps raise the temperature of the fuel in the filter. This reduces the chances of gelling. When the fuel temperature is above 88 F, it should switch and send fuel back to the fuel tank. The return line fuel is warmer than outside temperature because it's heated by the pressurization and compression from the injection pump and ambient heat from the engine and fuel lines. If this valve or any of the fuel lines are not sealed well, it will let air bubbles into the fuel lines and this could result in engine stumbling or a hard/no start condition. Make sure the o-rings and fuel line clamps are seated properly. The arrow on the recirculation control valve should point toward the fuel tank to the rear.
If you have a recirculation T and want to get rid of it (due to leaking or you live in a warm area), you can use a block off T, available from kermatdi. A block off T should increase power slightly by making sure you get cool fuel. I suggest leaving it alone if you live in a cold area or use biodiesel since some warmth is a good when cold. There are also better and more productive power increases available from other modifications, see 1000q: basic power mods for more details. Note - some fuel filters on the early new beetle TDI may not have a thermostatic T. If your fuel filter is this type you can retrofit a T.
You may hear that the fuel filters are heated, they are slightly warmed by the return line fuel and that's it. There is no active or electric heater unless someone installed an aftermarket heater for a veggie or grease system and you wouldn't want diesel or biodiesel to be too hot anyways.
Although you are supposed to drain water every 10,000 miles, I have never seen any water in my fuel. Water from condensation could be present in fuel that has sat for a long time or that has contamination from the gas station. When you unscrew the water drain at the bottom it drains the whole filter.
Parts (click links to compare current prices)
vinyl/latex/nitrile (fuel resistant) gloves
chemical resistant cup for holding diesel fuel
paper towels to catch any spilled fuel
1 fuel filter (see below for part number)
For some 1998 mk4 Jetta/Golf/New Beetle TDI without thermostatic T: VW# 1c0 127 401
For 1998-2006 mk4 Jetta/Golf/New Beetle/Passat TDI with thermostatic T: VW# 1j0 127 401a
Mann and Bosch are OEM makers of filters and are just as good as the VW part. Mahle is also a good part.
Spare mickey mouse spring clip VW# 1j0 127 250
Extra o-rings (there should be replacements with the filter) VW# 1j0 198 247
Fuel filter change procedure
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Safety disclaimer - you are working with open fuel lines and fuel vapors when you change the fuel filter! Make sure the car is off. Make sure that there are no sources of ignition, spark, or open flames near the car or where fuel vapors could reach. Work only in a well ventilated area where any fuel vapors can be immediately evacuated and if fuel is spilled, clean it up before you continue working. Although diesel vapors are not as flammable as gasoline vapors at room temperature and pressure (as seen in the video below at the 1:00 minute mark - it's a demonstration only do not try that yourself!), you still want to comply with all cautions in your factory service manual. Wear eye protection at all times when working on your car. See the TOS Agreement for the full legal disclaimer. Diesel fuel will melt asphalt and rubber lines on your car so clean up any spills immediately. Make sure to use gloves because diesel fuel has a strong odor and you don't want it soaked into your hands.
Fuel filter location circled in yellow. If you have a mk4 VW Passat TDI the fuel filter is to the right (looking at the engine) of the engine.
Draining the fuel filter on VW TDI
The diesel filter has a water drain at the bottom. Drain any collected water every 10,000 miles. Just place a catch cup underneath the drain at the bottom and turn the white knob to open it. See below for a close up of the drain. Because of the quality of diesel fuel sold in North America, it's rare to have any water present and you will probably not see any water in a daily driven car.
Replacing the fuel filter
If you have a 2004-2006, make sure the car is off since there is an electric in tank fuel pump that pressurizes the fuel lines.
To replace, loosen the phillps screw that holds it and remove the fuel lines. Pull off the wire mickey mouse pin in the direction of the arrow. Then release and slide the spring clamps back. The fuel lines may be stuck to the filter so try twisting the line to break the seal before pulling it off. You can leave the return lines on the T attached because you can just pull the T off.
Pull out the filter and replace the o-rings on the thermostatic T. Make sure to replace them or else excess air bubbles can cause fuel supply problems. New ones should come with the new filter. Wipe up any fuel spills and make sure that while you are doing this procedure, that there are no possible sources of ignition or open flames anywhere near the car and adequate ventilation to clear any fuel vapors. Make sure to wad a paper towel over the fuel line when you remove it. The fuel lines should not be under pressure if you have a <2004 but some fuel may spill out. If you have a 2004 or newer or any model with a lift pump in the fuel tank, the fuel lines will have some leftover pressure if you just had the engine running.
When you replace the fuel filter, fill it up with fuel to prevent a dry start (see more on priming below).
Here's a video summarizing the procedure on a similar engine:
Here is what's inside the fuel filter housing, click to enlarge the thumbnails.
Priming the fuel system or purging air (if your TDI engine can't start if you changed the filter)
To prime the pump, first fill the fuel filter and fuel lines with fuel. If you didn't change the injectors or do something that emptied the fuel lines, just filling the filter should be enough. If you did empty the fuel lines and rail, prime the system fully. The method depends if you have an earlier or later style fuel system. Early styles (up to 2003 in North America) use a Bosch VE type TDI injection pump and no electric fuel pump. Later styles (2004+ in North America) are pumpe duse and use an electric fuel pump in the fuel tank.
No electric fuel pump
The injection pump is self priming but it'll take forever to start if the fuel lines are dry. This is hard on the battery and can overheat the starter. By priming, most of the air is removed and the engine should start with minimal cranking. If the fuel lines weren't run dry, just fill the filter and leave it at that.
Apply suction on the return line at the fuel filter or fuel return line until fuel has circulated through the pump. You can use the small 4 mm braided hose connecting the top of the injection pump to the #4 fuel injector or the return fuel line T shown below. The problem with loosening the filter line at the filter is that the plastic T can become brittle and snap off. Remember, twist the hoses to break the seal before pulling it off. Don't bother using your mouth, you must use a vacuum pump to generate enough force. A vacuum brake bleeder or a "mity vac" hand pump will work.
Some people loosen the fuel hard lines at the injectors to purge air. This also works fine but if you just removed the fuel injectors, counterhold the injector bodies to avoid twisting their hold-downs.If you didn't, they'll be frozen in place and won't need to be counterheld. Wrap a rag and around the line to prevent fuel being sprayed everywhere. Do not use your hand to hold the fuel since the diesel is under pressure. Although you only see "instant death" fuel pressure at the injector nozzle (in the cylinder and not exposed by loosening the fuel line), I would still avoid exposing bare hands to any pressurized fuel.
Pumpe duse engine with electric fuel pump
Turn the key to on and leave it there for a second. Repeat. This runs the electric fuel pump in the fuel tank to prime the fuel system. Repeat a few times and the car should start without too much bucking. This assumes that you filled the filter.
An advanced way to prime the fuel lines is to attach a clean hose to the fuel filter outlet exiting the filter and going to the engine (the one in the middle). The hose should empty into a clean jar. Turn the key to ON but not start. This will run the pump for about a second as long as the engine isn't running and is very effective at purging the air out. Repeat until you see fuel in the jar and then empty the fuel into the fuel tank. If the fuel rails in the cylinder head were run dry, you can also do this method on the return line coming back from the engine.
All models continued
Replace all the clamps and check for any loose tools, paper towels, etc. Take a test drive and check for any leaks. There may be many bubbles initially because of air pockets still trapped somewhere in the system. If there are still lots of bubbles after a day, check for leaks at the lines, the o-rings, the T, or at the fuel pickup in the fuel tank.