Tuesday, May 21, 2013

Cranking issues with the W124s

Having cranking issues?

If you are having difficulty starting the car after the engine has been off for some time, there are a few key components to check.

Do be advised this article is more applicable on the KE-Jetronic Engine Management System
ie Pre-1991 W124s and W201 and some other models [ eg 300SELs ]

1) Fuel pump check valve
This is a one way valve that comes mounted on the fuel pump when newly purchased.
If the check valve is faulty, it will allow fuel to flow back into the tank after the ignition has been turned off, this causes pressure in the fuel line to drop. It is also discovered that impurities in the tank may cause the check valve to malfunction and it may also be such particles that is causing the loud buzzing of fuel pumps as they get trapped by the pump mechanism.

Bosch 044 fuel pump's check valve

2) Leaky injectors
After the ignition is off, if the injectors are leaky, it will also cause the fuel pressure to drop and results in difficulty of cold cranking. If there are also issues with warm cranking, the leak may be severe. It is noteworthy that old injectors start spraying at 2.5 bar of pressure whereas new injectors will only start spraying at 3 bar of pressure.
W124 injectors

3) Vacuum advance system
When engine is cold, the thermo vacuum valve is activated and will allow the vacuum advance unit to initiate more advance at the spark plug distributor in order to properly start the engine when cold. 2 components at play here. The vacuum advance unit and the thermo vacuum valve. It is also possible that the vacuum hoses and connectors that joins the 2 components may be leaky.

Thermo Vacuum Valve/Switch

Vacuum Advance Module