Draining or changing the diesel fuel filter and priming the fuel
system - for mk3 and mk4 Volkswagen TDI
difficulty: 1/5
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list
Introduction
Your 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 one every 20,000 miles and drain it halfway through it's life. This article discusses changing and draining the fuel filter, and priming the fuel system.
If you are using high percentages of biodiesel such as 85% or 100% biodiesel, you should expect to change it in the first 1000 miles and then again at 5000 miles since the biodiesel will clean out the old buildup. This can clog the fuel filter if there was significant build up in the fuel system. These are only rough guidelines, your mileage may vary with biodiesel use. If using a high percentage of biodiesel, I also suggest installing a clear fuel filter before the main fuel filter to filter out most of the stuff 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. 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 or bad.
Fuel filter basics
There is a temperature sensitive recirculation control valve on the return line. When below 59oF, the valve recirculates the warm fuel to the filter and helps to regulate the fuel temperature. When the fuel is above 88oF, it should switch and recirculate fuel back to the fuel tank. Your 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. Note that 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 no start condition. Make sure the o-rings 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 will also increase power slightly by making sure you get cool fuel at all times. I suggest leaving it alone if you live in a very cold area or use biodiesel since some warmth is a good when very cold. 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 one.
Although you are supposed to drain water every 10,000 miles, I have never seen any water in my fuel. Condensation water would be present in fuel that has sat for a long time or contamination from the fuel station. The water drain also drains the whole filter.
Parts (click links to compare current prices)
diesel 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 1996-1999 mk3 Jetta/Passat TDI: VW# 1h0 127 401e
From tdiparts
, Meyle
filter
From ecstuning
(halfway down page)
For some 1998 mk4 Jetta/Golf/New Beetle TDI without thermostatic T:
VW# 1c0 127 401 -
Genuine VW/Bosch , Meyle
filter
For 1998-2006 mk4 Jetta/Golf/New Beetle/Passat TDI with
thermostatic T: VW# 1j0 127 401a
From
tdiparts, Meyle
filter
From
kerma
From worldimpex: genuine
VW, Mann
filter, Meyle
filter, Mahle
filter, Bosch
filter
Fron ecstuning: Bosch
filter , generic
filter , Mann
filter
From mjmautohaus: Mahle
Filter , Mann
filter, Bosch
filter
Procedure
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Caution - you are working near open fuel lines in this procedure! Make sure that there are no sources of ignition 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. Diesel fuel will melt asphalt and rubber lines on your car so clean up any spills immediately. Comply with all cautions in your factory service manual and with common sense safely procedures. Make sure to use gloves because diesel fuel has a strong odor and you don't want it soaked into your hands.
| MK3 car fuel filter circled in yellow
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MK4 car fuel filter circled in yellow
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Draining the fuel filter
The diesel filter has a water drain at the bottom. It's purpose is to let you drain out any collected water. Drain it 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 collected in a daily driven car.
Replacing the fuel filter
To replace, just loosen the screws that hold it and remove the fuel lines. Note the wire clamp or any o-rings on the thermostatic T. Make sure to replace them or else excess air bubbles can cause fuel supply problems. 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 but will spill out. If you have a 2004-2006, make sure the car is off since there is an electric in tank fuel pump that does pressurize the fuel lines.
When you replace the fuel filter, fill it up with diesel fuel to minimize priming time. You can also use diesel purge or diesel power service.
Below left is a picture of the fuel filter. Below right is another picture showing the mounting bolts (you don't need to loosen those to remove the filter) and the fuel lines. Note the white water drain screw at the bottom of the filter.
Priming the fuel system
All Bosch VE type TDI injection pumps are self priming but you can save wear on the starter and battery by priming the fuel system. It's better to not use the self priming injection pump to draw fuel all through the system because this forces you to crank the engine for a long time which can overheat the starter and put excess drain on the battery.
To prime the pump, first fill the fuel filter and fuel lines with fuel. Then apply suction on the return line (at the white plastic T fitting) until fuel has circulated through the pump. Do not attempt to use your mouth, use a vacuum pump as a suction source. You can also remove the return fuel line at the injection pump. This should let the engine start with minimal cranking.
Although some people loosen the fuel hard lines at the injectors to purge air, I do not suggest this since it's best to leave the fuel hard lines alone. Although this will work, putting vacuum at the return line is faster and cleaner. If you do loosen the fuel hard line at the injector, 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 and is not good for your hands.
Take a test drive and check for any leaks.