Checking the additional fuel supply valve (Mercedes-Benz E-Class W123)

1. The valve should open fully when the coolant temperature is 20°C and close when it rises to 70°C.

2. To check the valve on a cold engine, remove the valve box and blow into it: air should pass. If not, the valve is faulty.

3. To check the valve on a warm engine, start the engine and let it run, while idling, pinch the hose between the valve and the manifold. The engine speed should not change. A change in the crankshaft speed indicates that the control plunger is jammed and the valve needs to be replaced.

4. To replace the valve, drain the coolant, disconnect the cooling system hoses and remove the valve.

5. When installing a new valve, replace the sealing gasket.