The Volkswagen group Tdi engine is a turbo charged high speed direct injection diesel engine. Until 2000 all Tdi engines were fitted with with the VP 37 injection pump.

These engines have been used in vehicles throughout the Volkswagen group which includes Audi, SEAT and Skoda. The Volkswagen Tdi engine was also used in the Ford Galaxy and some Volvo cars.

Fuel Delivery[edit | edit source]

File:VP37 SkodaTdi.jpg
Bosch VP37 fitted to a Skoda Tdi1.9 90 AGR engine

The Bosch VP 37 injection pump delivers fuel through valve covered orifice injectors which use a two stage injection to give a small pre injection.

ECU[edit | edit source]

The ECU uses signals from a number of sensors to optimise the timing of fuel delivery, the quantity of fuel and control a number of other engine functions.

VAG-COM is a PC program designed to replicate the operation of the VW dealer computers. This allows engine functions to be monitored, execution of some engine tests and some simple modifications to some engine functions

sensors[edit | edit source]

The ECU uses this signal to identify the time of injection. The ECU uses the signal to report check that injection timing is as specified and to alter the timing.

The ECU uses this signal to determine the position of the crankshaft and to identify the speed of the engine.

The ECU uses this signal to determine the temperature of the engine. Fuel injection timing is advanced in a cool engine to improve combustion. At low coolant temperatures the glow plug system is activated, with an increasing pre glow period as temperature decreases. The injected fuel quantity is also altered at low engine temperatures to aid starting. Tick over / idle is increased after starting at low coolant temperatures.

The ECU uses this signal to determine the mass of the fuel. Fuel quantity and to a lesser degree start of injection are altered to suit the mass of the fuel, cooler fuel being more dense.

This sensor is used by the ECU to determine the fuel quantity being delivered.

The accelerator pedal has a variable resistor that provides a smoothly changing resistance through its movement. The pedal also has a switch that identifies when the pedal is not depressed. The ECU uses this signal to determine power demand.

The brake pedal has two switches that are switched when the pedal is depressed. The ECU reduces fuelling when the brakes are applied.

Control[edit | edit source]

exhaust gas recirculation[edit | edit source]

exhaust gas recirculation

EGR rate[edit | edit source]

The ECU controls the exhaust gas recirculation function. It is possible to increase or decrease the EGR rate using VAG-COM to alter adaptation channel 03. Increasing the value decreases the EGR rate.

glow plugs[edit | edit source]

When the keys are turned on the ECU reads the coolant temperature to determines if it should activate the glow plugs and if so the amount of time to operate the pre glow and pre glow indication light. Also at low coolant temperature the glow plugs are activated after start up to provide a post glow. The amount of post glow is dependant on the coolant temperature. At engine speeds above 2500 rpm the post glow is deactivated. VAG-COM can be used to adapt the pre glow period TdiClub post - How To extend glow time with Vag-com

Bosch VP 37 Injection Pump[edit | edit source]

Bosch VP 37

Commencement of Injection[edit | edit source]

The start of injection timing is controlled by the ECU. The ECU uses signals from a number of sensors to determine start of injection by cross referencing sensor signal values on a commencement of injection map. To reduce the amount of NOx in the exhaust gases timing is retarded slightly from maximum power setting. Injection timing can be advanced or retarded, by a selected figure of between 5 degree advance or retard globally across the whole map using VAG-COM to alter the adaption channel 04.

  • Coolant temperature sensor

The ECU advances the injection timing to compensate for the ignition delay due to the lower engine temperature

  • Fuel mass

The ECU calculates the fuel mass by referencing the fuel temperature sensor in the injection pump. Injection duration is increased to compensate for a lower fuel mass. To allow for a larger injection quantity injection timing is advanced.

  • TDC and engine speed

The ECU uses the engine speed sensor to determine both engine speed and the crank position of TDC. Timing is advanced at higher engine speeds.

  • Injection timing

The ECU compares TDC to the injector lift sensor signal to determine start of injection.

Fuel Quantity[edit | edit source]

The quantity injected is controlled by the ECU. The ECU uses signals from a number of sensors to alter the end of injection by cross referencing sensor signal values on a fuel quantity map. Fuel quantity can be adjusted, globally within a pre set range, across the whole map using VAG-COM to alter the adaptation channel 01. Fuel quantity on start up can be adjusted using adaption channel 05.

  • Accelerator pedal position sensor

The ECU determines power demand based and fuel quantity based upon the position of the accelerator pedal.

  • Fuel temperature sensor

The ECU adjusts injection duration in relation to the mass of the fuel to allow an appropriate volume of fuel to be injected

  • Coolant temperature sensor

The ECU uses the coolant temperature sensor signal to determine starting injection quantity

Idle Speed[edit | edit source]

The idle speed or tick over rpm is controlled by the ECU. The ECU determines rpm via a signal from the engine speed sensor. Idle speed can be adjusted within a pre set range using VAG-COM to alter adaptation channel 02. The ECU increases idle speed when the engine is cold.

Chip tuning[edit | edit source]

Using specialist equipment and software it is possible to alter every engine parameter. Injection timing, glow plug function, injection duration and turbo control can all be altered with great scope for control under different conditions.

Reading and writing the hexdump by chip removal and replacement[edit | edit source]

The engine control files can be read by removing the chip or chips that contain the control hex dump and reading them and with a EPROM reader. Care has to be taken to remove the chips without damaging them or other ECU components. Typically a hot air rework / desoldering station is used. The chips can then be replaced with a suitable socket that will take a suitable rewritable EPROM. Commonly used EPROM writers

  • Willem

Reading and writing the hexdump by OBD socket or direct connection to the ECU[edit | edit source]

On EDC15 and EDC16 ECUs it is possible to read and write the files through the OBD socket using suitable hardware tool. For some ECUs it is necessary to unplug the connectors from the ECU and read/write with suitable hardware connecting to the ECU socket or individual pins. Commonly used tools include-

  • Byteshooter
  • Galletto
  • Optican

Modifying the hexdump file[edit | edit source]

File:Skoda Timing.gif
Tdi timing map text view with WinOLS
File:Skoda Timing2.gif
Tdi timing map 3D view with WinOLS

The Hexdump can be modified using software designed for ECU modifications or a HEX editor. Any modification must include correction of checksums within the file.

It is advised to have a thorough understanding of how diesel engines work, how the Tdi engine works and effects that any modifications will have before modifying the ECU files. It is possible to cause damage to the engine by making modifications without knowing the function and limits of the engine and components.

WinOLS manual and demo download- Demo has no file export or checksum correction.

Biofuel conversions[edit | edit source]

Biodiesel[edit | edit source]

Many unmodified engines have been successfully fuelled with biodiesel. Starting in 1994 Volkswagen offered biodiesel compatible Tdis. In order to achieve this Volkswagen changed the fuel tank, fuel sensor, all fuel lines and hoses and the fuel filter. Bosch supplied injector pumps for compatible Tdis that had biodiesel resistant viton seals and a different ring magnet in the inlet to the inductive pickup injection quantity adjuster

Pure plant oil[edit | edit source]

Single tank conversions[edit | edit source]

A number of German companies have developed single tank conversions for this engine to run on fuel grade rapeseed oil.

  • Elsbett perform conversions and sell kits for DIY conversion. These websites have details of a Elsbett Tdi conversion
  1. American Elsbett conversion workshop write-up
  2. Audi Elsbett conversion write up

Technical discussion about running Tdi engines on PPO

Twin tank conversion[edit | edit source]

Numerous companies have developed twin tank conversion kits for this engine. Due to the engine being direct injection care should be taken both to ensure that the engine has warmed up sufficiently before changing to PPO and to ensure that the PPO has been flushed from the injection equipment before extended shut down. Website detailing a DIY twin tank installation

Links[edit | edit source]

VW 1.9 TDI industrial engine .pdf file contains details of the ECU control and sensors

Website and forum dedicated to VW tdi engine