Di­ag­nos­tics Doc­tor

Car Mechanics (UK) - - Contents -

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.

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