The core of the D-Jetronic system is the relationship between the engine control unit (ECU) and the manifold pressure sensor (MPS). The ECU stimulates the MPS with signals and in return the MPS helps to form the basic pulse that is sent to the injectors.
Each pulse sent to an injector causes it to open and spray atomized fuel. The longer the pulse the more fuel is emitted.
The MPS operates by reading the pressure level at the intake manifold, which typically ranges from a vacuum of zero inHg to 17 inHg. An incorrect reading of the pressure will therefore cause the wrong amount of fuel to be injected.
If the vacuum is too low then too much fuel will be injected and the engine will run rich.
This is a key point. When troubleshooting a D-Jetronic engine one of the first steps is to eliminate all vacuum leaks. For example on the Mercedes 450SL a range of accessories and systems are operated with the same intake manifold vacuum. For example:
- Central locking
- Locking seat backs
- Fresh air flaps
- Hot water faucet
- Cruise control
- Automatic transmission
- Engine timing advance changeover switch
The 450SL has two vacuum storage tanks and as they age they can start to leak. Most of the accessories use vacuum “pods” which contain rubber diaphragms, which can tear as they age.
Any leak here could cause the MPS to incorrectly read the intake vacuum. There is no point trying to tune a rich-running D-Jetronic system until the vacuum leaks have been fixed or disconnected/isolated from the engine first.