Cold starting your Mercedes Diesel – facts and fiction

Each year people ask me about how to start their Mercedes diesel engines in cold weather.  I usually end up dispelling myths in addition to giving advice.  I decided to write an article to look at some of these myths and back up some good advice with facts about how the starting systems in these cars are designed to operate.

For simplicity, I will use a 1983 300D Turbo diesel (123.133 with OM617a) as a basis for this article.  The same theory and functions apply to a wide range of Mercedes diesels from the 1970s through the 1990s.

Myth:  “When the preglow indicator goes out, the engine is ready for starting”

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Starting the engine

While my wife and son are visiting family out west, I’m able to dedicate an entire week of spare time that I don’t normally have to the car.  The first major task was for Wednesday evening:  get the engine running.  This required several smaller jobs be completed:

  • Install the fuel tank
  • Install the fuel pump and hoses
  • Install throttle linkage and ignition components
  • Top off or change basic fluids
  • Installed transmission linkage

These were all pretty straight forward tasks, except for installing the transmission linkage.  As some of my followers have said, the bushings are difficult to push on, and its not immediately clear how they fit.  This helpful photo from a friend explains it quite well.

The difficult part is press it onto the shift levers.  Specifically, the one buried deep in the transmission tunnel.  After some creative use of locking pliers, and about an hour of laying in awkward positions under the car, it was installed.  A special thanks to my friend Adrian for enduring this torture and successfully installing it!  This is the type of job that would have been much easier with a lift!

After everything was installed, we cranked the engine with the starter, but without spark plugs.  Oil pressure came up after a short time and we were ready to go for it.  In went 5 gallons of fresh gas, and 6 new Bosch W7DC spark plugs.

We cranked and cranked.  No firing.  We checked the fuel supply at the engine, then at the output of the pump… they were dry.  It seems that with the arrangement of the fuel tank and pump, 5 gallons isn’t enough to naturally flow in the direction of the pump, and the pump isn’t strong enough to prime itself.  The solution was to jack up the back of the car to help the fuel move forward, then run the pump with its output  running into a small container.  Once we had fuel here, we connected everything up again and confirmed fuel delivery to the injection pump.

More cranking.  After a while, it started to fire a bit.  It reminded me of starting a diesel in the dead of winter.  There was some fuel burning, but not enough to run on its own.  We  keep at it.  And just as we were about to give up and look for other problems, it started!  Rough, but it ran on its own.  It seems that it takes a long time to get fuel down to the injectors after all those years.  Here is the engine after running for a few minutes:

The engine sounds great!  We let it run long enough for the thermostat to open.  Oil pressure and temperature looked good.  It also smoothed out somewhat and no longer has a defined miss as in the video.

Everything was smoking a bit as the old nests and dust burned off the engine and exhaust.  We also seemed to have disrupted a mouse’s home in the exhaust system:

Overall, a greatly successful evening.  It ended up taking about 6 hours to do all of the above.