- Category: Tech Blog
- Created on 05 August 2014
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Replacing the Fuel Filter is part of Part of VW TDI 20K maintenance. Part of what makes TDIs so efficient is its high pressure injection system. But this system requires excellent quality fuel for both economy and longevity. The fuel filter is the critical element in protecting your fuel system and getting the economy you want from your TDI.
Filters can get clogged by old or dirty fuel. And even if you don’t have an incident of fuel filter clogging, replacing the fuel filter is part of your TDI’s 20K maintenance.
Where is it?
The fuel filter is on the passenger side of the engine compartment, just in front of the coolant overflow bottle. The one shown below has been in the car for 20K, but also for over 3 years because the car was stored for one year. It clearly looks like it’s ready for replacement.
The fuel filter has lines from the tank to the pump on the left, and a return on the right that goes through a thermostatic filter tee. The injection pump pulls fuel from the tank—there’s no in-tank pump on rotary pump TDIs—and also returns fuel back to the tank. Why? Because diesel fuel both cools and lubricates the injection pump, so it pulls more fuel from the tank than it needs to run the engine. The fuel tank becomes a big cooling reservoir to keep the pump cool.
The thermostatic tee helps the TDI work more effectively in cold weather. When the fuel is cold, it circulates fuel between the pump and the filter, only returning it to the tank once the fuel has warmed up. For some reason 1998 New Beetles did not have a fuel filter tee as original equipment, although they’ve been retrofitted to most cars.
You’ll only need the tools shown below, plus a 17mm box wrench (maybe) to bleed the fuel lines. Oh, and a new fuel filter.
Remove the fuel lines and filter
First, remove the fuel lines that come from the tank to the filter and the pump (the ones that don’t have the tee).
The filter shown here as OE clips. Often owners have retrofitted hose clamps. Once the clips are moved off the hoses, you can remove the fuel filter (or Mickey Mouse) clip, and lift the fuel filter tee out of the filter
Next, push the fuel lines off the filter.
The hoses frequently get stuck to the filter, and a little pressure with a screwdriver to break the hoses loose from the filter. Be careful not to distort or tear the hoses.
The filter is held in the car by a sleeve clamp with a v-shaped fitting that tightens down on the filter body with a Phillips head screw. Loosen that screw (shown at the bottom of the picture below) and the filter holder loosens up.
Sometimes the filter—like this one—gets rusted into its holder and has to be rotated back an forth a bit to free up and remove. The filter holds about a pint of fuel, so have a pan ready to empty the filter contents.
Preparing and installing the new filter
Remember, the injection pump pulls fuel from the tank to the pump and then to the engine. The pump cannot pull fuel from the tank if there is air in the system. Opening up the fuel lines to replace the fuel filter introduces air into the system. To limit the air in the system, you need to pre-fill the new filter before installing it. If you don’t do this the car will not run. The exception to this is ’04 and later cars with an in-tank lift pump, or cars where the pump has been retrofitted.
You can pre-fill the filter with diesel fuel if you have it available, or any fuel additive, like Stanadyne or PowerService. Remember the fuel is circulated through the filter and back to the tank, so the additive will quickly be diluted with the rest of the fuel in the tank. Although it’s not essential, it can be helpful to have the fuel tank ½ full or more when replacing the filter, reducing the work the pump has to do when re-starting the car.
The filter media will absorb the fuel or additive. It’s worthwhile to fill it, let it absorb the fuel for a minute, and top it up.
Slide the filter back into its base, and tighten it down. Note the height of the filter in the holder.
Too high and the hood will hit it. Too low and the fuel lines to the pump will rub against the intercooler piping.
Next, replace the o-rings on the tee. These come with fuel filters, and are important to replace to prevent air leaks.
The pick is a good tool to remove the old o-rings, but if possible put the new ones on by hand to avoid damaging them.
Once the o-rings are in place replace the tee in the filter and insert the clip. Make sure the clip seats properly in its holder, shown below.
Air leaks can make your TDI hard to start, and limit the power it makes.
Restarting your TDI
If you’ve pre-filled the filter, and worked pretty quickly and therefore limiting the time for fuel to drain back to the tank, you may be able to re-start the car without difficulty.
The best way to restart your TDI, however, is to Prime a TDI's Fuel Filter with VCDS. This method runs the fuel pump and clears air out of the system without any potential damage to your fuel injection system.
Start the car and rev it to 2000-2500 RPM and keep it there for a minute or so. The engine will probably stumble as it clears air out of the system. But with luck it will smooth out and idle normally. It’s important to rev the engine on its initial start, because at idle it’s much more likely to stall and not re-start.
The best way to fully prime the system is using a vacuum pump. We really like the ones by “Mighty Vac“.
If the car does stall and won’t re-start, you may have to bleed the air out of the lines to the injectors.
With a 17mm box wrench, loosen the nut on the tops of #1 and #2 injectors.
Have someone crank the engine over until fuel sprays out of the fitting. Stop cranking the engine over, tighten the nuts, and start the car, once again revving it to 2000 RPM or so until it runs smoothly. Sometimes you’ll have to repeat this step, or also clear injectors 3 and 4.
Once you have the engine running smoothly check the clear fuel lines for bubbles. You’ll have some at first but as the car idles they should disappear. If they seem to persist re-check the filter tee to make sure it’s seated properly.
A clean fuel filter is an essential element to protecting your fuel system and allowing your TDI to deliver optimum power and economy. Regular changes will make sure your TDI runs smoothly and starts in the coldest weather.