The fuel temperature sensor is one factor that tells the ECU how to adjust the amount of fuel injected. When the fuel is warm, it's less dense and burns more readily so the ecu injects more fuel to get the same mass of fuel and retards the timing. When the fuel is cold, it's more dense so the ecu injects less fuel and advances the timing. Hot fuel has the same effect as higher cetane fuel at a lower temperature because it burns more readily. Timing and injection quantity is also based on many other factors, fuel temperature is just one of them.
If this sensor fails, it will set a check engine light (aka. DTC/MIL/CEL) and the ECU will replace the faulty value with a nonsense temperature of -5oC. This lets it use a "fall back" setting so that fuel economy and engine operation is only slightly affected. A dealership may tell you that the entire injection pump is bad and needs to be replaced although this one sensor is actually user serviceable. Why not just ignore the light? The check engine light will result in emissions test failure in many states that have emissions testing, and the engine will not inject the correct amount of fuel. The car is still safely drivable, but your fuel will be slightly off peak economy.
Here are some values you may see if you put the sensor in water for testing. The sensor pins on the harness are 4 and 7.
0°C - 15200-17300 Ω
10°C - 9250-11500 Ω
20°C - 5500-6500 Ω
30°C - 3790-4270 Ω
40°C - 2650-3100 Ω
50°C - 1800-2200 Ω
60°C - 1200-1600 Ω
70°C - 850-920 Ω
80°C - 600-660 Ω
90°C - 425-480 Ω
100°C - 325-370 Ω
The top cover for the injection pump can also leak, this article shows how to replace it.
Parts (click links to see current prices)
triangle security socket (triangular injection pump socket) Bosch PN# "0-986-612-605" from KermaTDI, dieselgeek.com , Metalnerd (these are cheaper and just as good as the Bosch part)
torx bit -T20 and T10
vinyl or nitrile diesel resistant gloves and paper towels
fuel temp sensor - bosch PN# "2 464 509 015-001" OR VW #"028 906 040
c" (note - the fuel temp sensor is the same for all 1996-2003 VW
diesels) from dieselgeek.com
injection pump top cover seal - bosch PN# "2 461 015 008-000" OR VW #"028 130 717 a"
full TDI injection pump seal kit
First clean the area around the injection pump cover. You don't want debris entering the sensitive area of the injection pump. Make sure the engine bay is cold and have a fire extinguisher in your workspace. Make sure all combustible fumes are exhausted from the area and do not introduce any ignition sources or sparks while working on the fuel system. I also suggest wearing gloves to keep diesel fuel off your hands.
Wad some paper towels behind the injection pump. The mk3 pump is normally tilted backwards so you only have to stuff paper towels behind and to the side of the pump. The mk4 pump is tilted forward so quite a bit of fuel will spill out. At least 5-6 paper towels are required to catch the fuel. Remove the injector fuel return line and wrap with a paper towel to catch any spilled fuel - the fuel system should not be under pressure in these cars that use a Bosch VE injection pump since there is no electric fuel pump anywhere on the car.
3x T20 bolts and 1x triangle security
bolt with the triangle socket circled below. The injection pump cover should
come off easily, be prepared for some fuel to spill out. You don't
want the fuel on the coolant lines or any other rubber hoses so make sure the
paper towels are stuffed behind the pump!
Unscrew the fuel temp sensor (T10 x 2 screws) and replace. It's the black thermistor, the thing that looks like it's holding a pill. I don't know the exact torque for the T10 screws, but 3-4 lbs should be more than enough, the T10 screws are very small and LIGHT hand tight is more than enough force. Any more and you will strip the torx head. The heads are pretty shallow. The sensor will only fit well one way, the numbers should face up.
You can also check the resistance of the fuel temp sensor and compare it to the old sensor. It should be about 1300-1800 ohms at room temperature. With the ignition on, one side gets about 5V and goes to the ecu, the other side is ground. Also inspect for unusual wear or dirt in the injection pump area. It should be very clean with no visible wear. Inspect for bubbles or specks in the electronics and plastic - there should be none.
In my case, the bad sensor was related to an electrical short on the wiring harness. Carefully inspect the wiring harness for shorts, cracks, or rubbing. A bad sensor could also be related to water in the fuel.
Installation is the reverse of removal. If you decided to damage the triangle head bolt by hammering a socket onto it to remove it, here are some specs on the bolts.
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