(Customer Share) MB Diagnose SD C4 What to Do When CDI Lack of Power

Diesels in the workshop often has common problem that lack of power or some people describe it as the kick down not working.

Firstly do some basic checks like oil levels, fuses etc.

Step 1: listen for any escaping air under pressure when you are driving at full load. If this is the case you will probably have a boost pressure leak from somewhere. The common areas are: Intercooler, rubber boost pipes and the inlet manifold itself. If you suspect a boost leak then listen at the front of the engine and see if you can hear where it is coming from while a friend gives the engine some RPM.

Step 2: get it plugged into a Star diagnosis I.E SD Connect 4 and get the codes read. This is the most important thing to do before buying ANY parts. Assume there are many people have the same problem as what they have read and replace the same parts and be confused why it hasn’t fixed it. Get it plugged in. It will save you much money in the long run I promise you.

The most common item to fail and nearly always brings up a code is the infamous Air Mass Sensor or MAF (mass air flow). It is always worth doing a manual check with the live data just to be sure that is the cause.

A common fault code that comes up is a fault with the boost pressure sensor. It will say either under boosting (lack of turbo pressure) or over boosting (too much turbo pressure). So many people will replace this sensor for this fault when the sensor will be working perfectly.

Checking the sensor values with a Star diagnosis helps determine this. Under boost: Check boost pipes for leakage. Over boost: has it been mapped badly? Is the turbo actuator or vanes sticking?

We see quite a few inlet ports shut off motors failing too. The V6 diesel, 220CDI and the 270CDI all suffer. This will give you lack of power when they fail and an engine light will come on in the case of a V6 CDI. A check on a Star diagnosis will bring up codes for this.
I hope this helps.

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