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”

Although the owners manual states this, it isn’t completely true for very cold temperatures and older engines.  The glow plugs need 30 seconds to reach their maximum temperature of 1180C.  The glow plug relay is designed to only show a preglow light of 30 seconds if the ambient temperature is -22F!  If you want to maximize the amount of heat your glow plugs are producing for a cold start, they need to glow longer than the light suggests.  But how?

Caption here
Temperature of the typical “quick-start” pencil glow plug is shown here as curve b.  Curve a shows the early style pencil glow plugs used before August 1980 (no longer available as a replacement part)

Myth:  You should cycle the key several times to power the glow plugs longer before starting in the cold.

Preflow Time
This chart shows the actual time range the glow plugs will have power (b) vs the time the light is actually on (a).

While this isn’t a terrible thing to do, its unnecessary.  The glow plug relay continues to power the glow plugs after the indicator light on the dash board goes out.  There is a “safety shutoff” which keeps the glow plugs powered up and warming 20-35 seconds after the lights goes out on the dash board.  So, there is no need to cycle the key… the glow plugs continue to heat.  You can hear the relay when it finally does switch off, it makes an audible “clunk” noise.  If you’d like to cycle the key, after you hear the relay switch off would be a logical time, but by then the glow plugs have probably heated to their maximum temperature.

Myth:  Don’t crank the engine for too long when attempting to start.

Sometimes its useful to have a look at the owners manual for even the very basics, like starting your car.  In the section “Starting and Turning off the Engine”, you will find two very important statements.  1) “Release key only when the engine is firing regularly…” and 2) Do not interrupt the starting process”.  Now, I know it may sound like abuse of your starter, but they really mean it. Keep cranking as long as it takes and don’t stop. 30-60 seconds possibly. You will find that as you crank, the engine will slowly turn faster and faster until it eventually is turning fast enough to run. It takes time to build up the heat necessary for the engine to run on its own. The starter was designed for this abuse (and its why these cars have such big batteries).  If your starter dies because of this type of usage, it was probably on its way out anyway.

So what does all this information translate into when you actually have to start you car?  A few years ago I recorded a video of a cold start at about 5F.  The car was a 1982 300D turbo with only 152k miles on it.  But the car didn’t have the best care earlier on in its life.  It had less than ideal compression in at least one cylinder.  The injectors were iffy.  It had 15w40 conventional oil it in.  I didn’t plug in the block heater.  In other words, it was not well prepared to start in cold weather.  But, I had to goto work.  So, I followed my own advice and gave it a shot.  You see me wait for the glow plug relay to click off, then I start cranking and I don’t give up until it starts.

So a car that didn’t sound like it should start did start.  And yours can too.
Now that you know how to start your car in the winter, lets talk about what should do to prepare it before you really need to test it out:
  • Adjust your valves.  Properly adjusted valves ensure you’re getting the most compressions possible in the cylinders.  I typically do this each fall before the winter starting season begins.
  • Install a properly sized battery with clean high quality cables.  The original battery in these cars are large for a reason.  Its important that battery terminals and cables are cleaned and free of corrosion.  Clean the negative battery cable where it attaches to the chassis.  If any cables have been repaired or replaced, ensure high quality parts were used.  Also ensure the ground strap from the engine to the chassis is clean and secure.
  • Use a thinner oil.  According to the owners manual, 15w40 engine oil (a commonly used weight for diesel engine oil) is only recommended above 23F.  For colder temperatures 10w30 can be used (down to -4F).
  • Consider synthetic oil.  In October 1979 Mercedes Benz North America released a bulletin to its dealers about cold weather operation.  Even nearly 40 years ago, they stated Mobil One “can also be used to good advantage”, but cautioned that it is released for “winter operation only”.  Synthetic oil has a lower pour point than conventional oil.  That is, it flows better when its cold.  Better flow means the engine will turn faster with the starter, which will result in faster or more successful starts.  Modern synthetics are now popular for use in these older vehicles year round.
  • Use a fuel additive… or should you?  I personally have never used a fuel additive in 20 years of driving these cars in the winter.  But I always used fresh winterized diesel fuel.  About the coldest we get here in New Jersey is a few degrees below zero F and I’ve never had an issue with fuel gelling.  Trying to start a diesel engine in cold weather with summer diesel fuel in it will require a fuel additive, however.  Kerosene can be mixed with summer diesel fuel to improve fluidity, as mentioned in the owners manual.
  • Use a block heater.  Mercedes originally recommended a block heater for starting in temperatures below 0F.  That is, they expected the cars to start without a block heater down to 0F.  But using a block heater in any cold weather significantly reduces the effort to start the engine.  Plug the block heater in at least an hour before starting the engine, longer if possible.  Most Mercedes diesels from the 1970s and 80s were equipped with block heaters, although the power cord wasn’t installed in cars delivered to warmer climates.  The heater is very powerful… if left plugged in for a long time, the engine can actually heat to the point that it will be warm to the touch.  This isn’t harmful to the engine, only to your electric bill.
  • Do not use ether or starting fluid.  This could cause engine damage.
  • And of course your engine is going to need to be in good condition in the first place.  Low compression will make the engine difficult or impossible to start in colder weather.  Your injectors should be in good condition.  All glow plugs have to be working properly.  And so on…

5 thoughts on “Cold starting your Mercedes Diesel – facts and fiction”

  1. Thank you. This is helpful.
    What are your thoughts on running bio-diesel in the winter months?

    1979 W123, 240D

  2. IN THE 1970S Frank King wrote a wonderful article in the Star magazine titled “Cold diesels will start”, I remember him writing, in extreme conditions you could run the car at idle all night long, it would consume about 4 gallons of fuel

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