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Making a Sad Benz Happy Again

Posted May 08, 2013 10:00 AM by VickiS
Pathfinder Tags: benz cars diesel filters Mercedes

Recently, a beautiful 1983 Mercedes Benz 300SD (diesel) rolled in spitting and sputtering and not wanting to start. Here's the poor patient:

The tech had a hard time getting the car started in the parking lot to get it into the garage bay, but with some coaxing and a little sweet talk, he finally managed to fire her up and move her inside.

Sad car - old dirty fuel filters not allowing it to start

Getting to Know the Patient

Car doctors need to find out what ails their patients. When a diesel engine starts sputtering, a solid first question to ask the car's owner is, "When was the last time the fuel filters were changed?"

The answer in the case of the old Benz was, "Not since I've had it", which turned out to be about 3 years. That's actually two years longer than anyone should allow fuel filters to stay in a diesel engine without changing them.

Fuel Filters 411

Most diesel engines have two fuel filters. The first one is the primary filter that's situated between the engine and the fuel tank. This filter cleans the diesel while it's on its way to the fuel transfer pump. The secondary filter is near the engine. The diesel passes through this filter before it enters the fuel injector nozzles.

Both filters should be changed at least once a year since, A) they do a lot of work cleaning the fuel, and B) diesel gels in the winter. This particular car resides in the Northeastern U.S. where winters can be brutal. If the filter is moisture bound, the fuel could freeze in the filter -- not a good situation. Mister, you are just looking to have an unhappy car if you don't maintain that diesel engine and your fuel filters! Without proper filter maintenance in a diesel engine, fuel contamination can clog the fuel injector nozzles. Now you have another problem that needs to be fixed.


Our fine technician checked out the fuel filters first. The logical thought at the point of sputtering and learning the filters haven't been changed in at least three years is to look at the primary ignition parts. In this car, one fuel filter is a screw-on external filter which looked pretty old and beaten up. The second filter is covered in clear plastic which was super dirty, giving limited visibility to the interior of the filter. When new and clean you can see inside the filter, but the old one was so dirty we couldn't even show it to you* . Bottom line, these fuel filters were toast. They need to be replaced STAT!

*mechanic guy forgot to get a picture for his reporter.

In-line fuel filter - Clear plastic was very dirty. This is the new one.

Stock Image Fuel Filter

Fuel Pre-Pump also a Hot Mess

The technician saw moisture under the engine compartment area and noticed that the fuel pre-pump had fresh liquid around it. The tech pushed the primer and gas spewed everywhere. The seal between the moving parts of the primer had evidently worn out and was causing the leakage.

Pre-pump primer on fuel pump - Leaking diesel fuel from the top

Next Week:

Parts replacement and another problem. What did the car doctor do wrong? Can he get the Benz running again?


Midas Hudson, NY

For Dummies

O'Reilly Auto Parts


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Join Date: Oct 2008
Location: Troy, NY
Posts: 82
Good Answers: 4

Re: Making a Sad Benz Happy Again

05/09/2013 9:54 AM

My mother is still driving her 1982 Mercedes 240D. I took it out for a drive the other day and it seemed a little sluggish. I'll have to check to see if the fuel filters have been changed recently.

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