I have a 1999 Mercedesbenz C250 turbodiesel, with an automatic gearbox and 169,000 miles. Recently, I had to remove the inlet manifold and, once it was on the bench, I could see a considerable amount of oil inside. Having degreased it, I decided to decoke the EGR valve at the same time.
The reason for carrying out this work is a lack of power under normal driving conditions. On a steep gradient, the vehicle speed drops dramatically. There is no excessive smoke under these conditions, but there is a delay with the turbo picking up when I use the kickdown, then it labours to increase speed. Does this point to a turbo or crankcase breather restriction problem? Or is it something else?
The vehicle is regularly serviced and I always change the oil and oil filter every 5000 miles, while the fuel filter and pollen filter are renewed at every second oil change. RD Oates As you have not mentioned the engine management light, I am assuming this has not been illuminated at any stage.
Having cleaned out the EGR valve, it can hopefully be assumed that this is not sticking, although without checking it this cannot be ruled out entirely.
One of the most likely causes of power loss in your Mercedes is the mass air flow (MAF) sensor, which often does not leave a fault code logged in the system. Try cleaning the MAF sensor’s hotwire element using isopropyl alcohol or a dedicated cleaner spray. Do not use any other type of cleaner, as this could leave a deposit on the hotwire. Also, ensure that the element is fully dry before starting the vehicle.
Another point to check is that the vacuum hoses on the manifold and systems are correctly fitted and are not leaking air into the system. It’s surprising how much one dislodged hose can affect performance.
Finally, it’s worth checking that the turbo wastegate regulating valve
The turbo wastegate actuator on the Mercedes-benz C250.