There is some question about Volkswagen and Audi Tdi fuel filter replacement intervals. A few variable exist. Whether using Diesel or Biodiesel is the biggest factor. The factory replacement interval for the Tdi fuel filter for cars using regular petroleum diesel is to have the fuel filter changed every 20,000 miles to be safe. The factory service information gives little clue about Biodiesel Tdi fuel filter change intervals, as they do not sanction the use of Biodiesel in their engines. We recommend replacing the fuel filter on a biodiesel system every 10,000 miles.
Diesel fuel injection systems up through 2003 have a distinct disadvantage. The fuel pump must suck the fuel from the tank through the filter. Those models are the most prone to power loss due to filter clogging. Starting with the Pump Duse fuel injection in 2004, a small electric fuel pump, called a transfer pump, is located inside the fuel tank. This pump feeds fuel at 4 to 11 psi to the fuel system, so the tandem pump on the head does not have to suck the fuel. The CR Common Rail fuel systems have two electric fuel pumps. One pump in the tank, like the PD system, and another to bring the pressure up higher to feed the main pump driven by the timing belt. The high pressure fuel pump raises the pressure to 8000 to 27,000 psi for the injectors, depending on injection needs.
If a Tdi fuel filter clogs, the pumps after the filter get starved. And starvation is always a progressive chain of events. Since the fuel pumps are cooled and lubricated by the diesel fuel, starvation will eventually damage them. Cavitation can also occur. Cavitation is when any pump is sucking vacuum pockets in a fuel or other liquid by pulling on the liquid too hard. Those vacuum pocket fill with the vapor of the liquid they were drawn from. When cavitation occurs in the fuel filter, the pump receives those bubbles, lowering lubrication and cooling from the diesel fuel. And the bubbles can also affect injection quantity and timing, as well as damage the pump. This may cause the Check Engine Light (MIL) to illuminate on the instrument cluster, and Diagnostic Trouble Codes (DTC) for that problem to be stored in the Engine Control Unit (ECU). Usually by this point, some pump damage is starting to occur in all 3 types of fuel system. But the pump usually fails slowly, and no one might catch the fact. When cavitation occurs inside the pump, the expanding bubbles rip metal off, and the pump fails rather quickly.
If we suspect a problem because you lost power, were towed in, or we see debris or goo inside the old filter, we will want to do some appropriate testing. Your Service Advisor will let you know before we proceed if you are just in for a service or a simple fuel filter change. On all models, the Technician needs to evaluate your fuel system with his computer. He will look through the appropriate measuring value blocks (MVB) to check some important specifications, then perform some tests. Different systems require different readings and tests.
On models up through 2003, those with an external fuel injection pump, we will want to do a fuel system restriction test to check for the amount of vacuum present as the pump sucks the fuel from the tank. We will also check the maximum vacuum the pump can pull, a clear sign of the condition of the primary pump section of the injection pump. Normal fuel system restriction vacuum for regular diesel is 3 to 5 inches of Mercury, expressed as 3″ Hg to 5″ Hg. For biodiesel systems, restriction spec is 4″ Hg to 6″ Hg, sometimes as high as 7″ Hg in the winter. The fuel pump maximum vacuum should be between 24″ Hg to 27″ Hg for a good pump. A regular diesel fuel pump can wear to as low as 20″ Hg and still be able to pull fuel from the tank in most cases. A biodiesel fuel pump can only wear to as low as 22″ Hg. Maximum injection pump values below those specs can lead to the engine failing to restart after sitting over night, or in cold weather; especially with a low fuel level and/or parked aiming uphill.
If we find restriction vacuum readings in excess of these specifications after we replace the fuel filter, then your Service Advisor will inform you that we need to look inside the fuel tank. There are 2 check valves, a screen and a reservoir level valve inside the fuel sending unit inside your fuel tank. Any of these could be causing the high restriction. If not checked and cleaned or replaced, those components will likely continue to damage the fuel injection pump until failure occurs. The labor for tank inspection, cleaning and/or replacing the fuel sending unit and valves varies from 1 to 2 1/2 hours. Sometimes the fuel itself simply must be changed.
In Models made 2004 and later with the PD and CR systems, we may find metal flake in the filter, and later in the tank. A very tiny amount of tiny metal flake is normal. Many times with the Pump Duse models from 2004 – 2007, we find a failed or failing pump, but the engine is still running. In the 2009 and later CR models, if there is more than just a gram of metal flake is found, we definitely need to check out the pumps and review the system. If the engine is running fine, we still may find a pump weak or inoperative, or starting to shed metal flake. If the engine is not running well, or will not run, then we may be too late, and the entire fuel system may need to be replaced. Just a note, it would seem that the Jetta and Golf series for 2009 and 2010 are the cars that we have seen with significant metal flake in the fuel filter. We have not seen any such issues with the Touareg V6 or V10 Tdi, nor with the Audi A3, A6, Q5 and Q7 Tdi.
Improper procedures when changing a fuel filter on a CR model may result in fuel system failure. There is always some metal flake around the outside of the filter element. When the filter cartridge is removed, it can drift towards the center of the housing, so the housing must be completely evacuated and cleaned. That metal flake will now be in the center of the new filter and will go straight to the main high pressure fuel pump. There is a tiny screen just before the main pump, but the openings aren’t small enough to stop an ant. Any metal flake will damaged the HPFP. Any air left in the filter housing will damage the main pump. The bleeding procedure should be performed with a computer and then repeated. And after initial startup, the engine should be left for at least 5 minutes at an idle.
Check with your Service Advisor for the proper intervals, and the date and mileage of the last replacement of your Tdi fuel filter.
Save your fuel pumps! Change your Tdi fuel filter often!