First and foremost, we advise you to not start an engine conversion without adequate diagnostic equipment. The DDE 4 ecu is not OBD-II protocol so a lot of cheap diagnostics will not work. Higher end diagnostics will usually work. We would recommend Inpa diagnostics, which is a cheap BMW specific system and is what we use for setting up ecus. We recommend buying a cable and software package from cable-shack.

Diagnostics House of Torque

So, you have got your engine fitted and it won’t start. What should you check first?

We would check the following things:

  1. Check that the main relay is functioning properly, see our wiring guide for how this works.
  2. Check the lift pump is running and you have fuel.
  3. Plug in diagnostics and read error codes. Pay particular attention for if there is an ews error code. If there is, you may have to resync the emulator. Please check out our page on resyncing and look for reasons of why it has come out of sync in the first place.
  4. Check target rail pressure and actual rail pressure. To start you need around 300 bar of pressure. We are increasingly seeing engines not starting due to high pressure pumps that have been sat for so long dry that they are no longer functioning. However, before you go replacing any parts, check your wiring and error codes first. A good wiring check is to make sure you have 12v on one of the MAF sensor pins. This is easy to access and the 12v feed is common across multiple engine sensors.

Once you have your engine up and running there are a few common issues.

  1. Grey smoke after a cold start. This is quite common on these engines especially if you have removed the catalytic converters. You can check that your glow plugs are wired up. If the smoke is excessive then it is likely to be the injectors dribbling fuel into the cylinders over time which is then burnt off at start.
  2. Another issue which is very common on vacuum actuated turbos is over-boost or bad boost control. This is often noticed when you floor the accelerator, the engine starts running very rough and misfires. This happens because the ECU detects over-boost and cuts fuelling to protect the engine. Over-boost can easily be confirmed using live data logging through programs, for example, Test0. You should check:
    1. Vacuum system is in good condition without leaks.
    2. Turbo actuator moves relatively freely. When you first start the engine, you should see the actuator move and hear the exhaust note change.
    3. Often the vacuum solenoid which controls the turbo fails. We only recommend replacing with genuine solenoids as we have seen many ‘new’ pattern parts not working out of the box.


Turbos are also another common failure point. This is primarily when you remap an engine with an old tired turbo. Combined with over-boost due to poor turbo control, this is a recipe for a quick death.

Injectors are another part which can be often tired. If you have rough running issues, then it can be useful to do an injector leak off test. For solenoid injectors, most good diesel shops can refurbish them. For piezo injectors, only Bosch can replace the piezo crystals so we would only recommend getting refurbished injectors direct from Bosch.