Tech Q & A

Testing turbo boost pressure

Q: My 300CD is low on power; it seems starved for fuel or boost. My car only has about 55,000 miles on it and a few thousand miles on the fuel filters. It starts almost instantly, which to me indicates a sound diesel engine. I cannot find a reference on measuring turbo boost in the books recommended by George Murphy. I would like to eliminate the turbo as a suspect by measuring boost and comparing to nominal. I installed a temporary VDO boost gauge using a T-fitting on the pressure line to the ALDA automatic pressure compensator from the switch on the firewall. I see a maximum boost of 7 psi under full load above 2,500 rpm. When I release the throttle pedal and drive at a steady-state speed of 50 mph, the boost drops to a few psi. Based on that measurement, I’m wondering if I guessed the wrong place to install the T for the gauge. What is nominal boost value and profile versus rpm and where do I install a T to make the proper measurement?

A: First, check that all fuel lines are clear (diesel thickens if it sits unused) and that the fuel pump is supplying enough fuel at the right pressure for the injection pump to do its job. As your 300CD has only 55,000 miles, many factors might allow it to start with low fuel demand, but lose power when running with higher fuel demand. Pages from Section 09-425 titled “617 Turbo boost test” from the Shop Manual that you received from George Murphy shows what a dealership technician will do using a dynamometer. It is unlikely that you have a dynamometer in your garage at home. Here is a method that may help you solve the problem if you are familiar with your car’s engine compartment and if you have ruled out other causes of power loss at speed. If not, visit a shop that knows these cars.

Start by disconnecting both plastic lines to and from the switchover valve between the aneroid compensator on the injection pump and the banjo fitting on the back of the intake manifold. Air should flow freely through this valve when de-energized. If the aneroid compensator is not getting the same boost pressure as the intake manifold, fuel enrichment will not occur and will cause your turbodiesel to perform more like a 240D (no one wants that). Once the turbo spools up, the boost should be about 8-9 psi. This will vary with engine age and mileage. Another way to check its operation is to disconnect the plastic boost pressure line from the injection pump at the switchover valve. Set the parking brake, ensuring it is in park. Bring the rpm up to around 4,000, then connect and disconnect the rubber hose to the switchover valve. There should be a noticeable jump in rpm when connected. (Be warned. This test is not for the faint of heart.) This paragraph is from Ken Adams, Mercedes-Benz dealer shop foreman.

Stumped by a Smart Key

Q I was hoping you could help me with a Smart Key problem on my 2010 E550. I have owned the car for five years; it needs a new Smart Key battery every two to three months. Batteries in both my Smart Key and my wife’s Smart Key last about the same length of time. It’s annoying. I’ve noticed no other electrical problems. Could the problem be a weak auxiliary battery? If so, could you please send the procedures to test and replace, if necessary? I don't even know where it is in the car. Thanks.

A The auxiliary battery has absolutely nothing to do with the Smart Key battery life. Usually when key battery life is short, it is because the Smart Key is being left in close proximity to the vehicle; a distance of 10 feet is recommended as the safe minimum, 20 feet is better. If you keep a Smart Key near the car, it uses “near field communication,” to signal the vehicle as if you were approaching to enter it and drive away. The previous “Keyless Go” keys did not have near field communication. Also, watch-type batteries (typically CR2025) have a low capacity; a high-quality battery such as Duracell or Energizer will help. As for replacing the car’s auxiliary battery, the information sent to you by George Murphy should help, although I advise having the dealer – or MBUSA Roadside Assistance – check your vehicle and determine if either the main battery or the auxiliary battery needs attention. MBUSA Roadside Assistance is notably less expensive than having your dealer do this; they do not have a separate labor charge to diagnose and change the battery. You pay only for the battery. Your car’s glove box contains information about how to contact MBUSA Roadside Assistance, if you have that service available.