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”

Continue reading “Cold starting your Mercedes Diesel – facts and fiction”


Most of the hoses for the cooling system of the 300SEL appear to be original, or quite old.  I wanted to replace these hoses with ones that looked original, or at least were the current Mercedes part.  Searching through the EPC to gather part numbers was rather useless.  There were many different part numbers for hoses that appeared to be the same, and nearly all of the part numbers turned out to be bad.  With the help of the classic center, I was able to come up with some length of hose in the necessary sizes.  For future reference:

  • 38mm: N 900271 038025.  This 38x46mm hoses is used on the thermostat housing.
  • 18mm: N 900271 018038.  18x27mm hose used for the heater connections
  • 12mm: N 900271 012018.  12x21mm hose used for several connections, including the injection pump and fan coupling thermostats.
  • A 340 501 02 82.  The upper and lower radiator connections use this short funny hose with a large bulge in the middle.
  • 46mm+: The connection from the water pump to the radiator is larger than the 38mm hose I had.  I still need to figure out the correct size for this one.
12mm Hose
12mm Hose, old and new

Today I managed to remove all these hoses and replace most of them.  You begin to appreciate just how complicated the M189 when you work on it.  There are a lot of hoses.  There are several very short pieces of 12mm hose associated with the fan coupling thermostat which are very difficult to get to.

I ran into a snag when I realize I didn’t have the right size hose for the large water pump connection under the manifolds.  Since I had already removed the old hose and the car would now be laid up until I could get the right size, I decided to take things a little further.

Looking through the radiator cap, I could see the core was pretty crusty looking.  So I pulled the radiator out and will have it cleaned up at a local shop.  While its out, I’ll have easier access to change the belts and and address the oil leak at the front of the engine.  Still lots of work to do!

Getting ready for spring

Yeah, its been a while since the last update.  There has been one major issue with my 1967 Mercedes 300SEL which I might have mentioned before:  I didn’t have a title.  This is a fairly major problem for a New Jersey resident.  But I finally found a solution and the car is now titled and registered in my name!

Title and Plates

This opens the door to get the car on the road by the spring.  In the last year or so, its been sitting in a very nice and dry garage.  I’ve been starting it occasionally and driving it around a bit to get it up to temperature and the car continues to sit on its own airbags without falling significantly for a few weeks.  Its now suffering from having old gas in the tank (but luckily its only 1-2 gallons) and has become more difficult to start.  It really needs to see some regular use.

From what I can see up to now, the following things need to be addressed before the car is roadworthy:

  • Change all cooling hoses
  • Change belts
  • New Exhaust system
  • Fix oil leak in the front of the engine, which appears to be related to the fan coupling.
  • Change power steering hoses, and change the oil in the rear differential
  • New Tires, and an alignment
  • And cleaning… lots and lots of cleaning, inside and out.

Its not an insignificant amount of work, and will require a lot of time and money.  Hopefully I can make it by spring!

Front suspension

After a month break, I’m back to work on the W109!  I’ve got three days this week and hope to make some good progress.

The most important work to be done is the front air bags.  The driver’s side bag is leaking badly and the passenger side isn’t far behind.  This job is one of those that goes much faster after you do it once or twice.  The order in which you remove things is pretty critical to making things go smoothly.  This is well documented in the factory service manual, but sometimes I need to figure things out myself first!

So, heres how it goes:

  1. Disconnect the tie rod from the steering knuckle.  A proper popper (thanks Daniel) makes it possible to not destroy the boot on the joint.  I can’t see how you can get the air chamber out of the car unless you get the tie rod out of the way.
  2. Disconnect the air line.
  3. With a piece of wood (I used a short 2×4), press the bag up and away from the piston mounted to the lower control arm.  Do this before the removing anything else!
  4. Once with piston is free from the bag, disconnect the 5 bolts holding the piston to the control arm.  There are 5, not 4 as I initially thought!
  5. You should be able to pull the piston out now.  You might want to disconnect the sway bar to keep it from holding up the control arm to give you more clearance.  Or, it helps to have both sides of the car jacked up with the wheels off.
  6. Now, remove the three bolts at the top of the subframe and the assembly will fall out.  Remove it from the back.

Thats it!

Actually, thats not it.  Now you need to remove the bag.

112 320 00 17

To start, I use a wire brush and some penetrating oil to clean up the tops of the screws and the nuts.  The locking tabs (A 112 327 01 73) need to be bent down to expose the nut completely.  Do this carefully, and you can reuse them.  I ended up replacing 10 of them, since this is what the dealer could get me when I originally ordered up the parts.  There are 24 total on the car.  To remove the nuts, start with a good quality thin 13mm wrench and get the nut to turn.  Once it moves, use a nice #3 philips screw driver to remove the screw while holding the nut.  I didn’t find it necessary to hold the chamber in any special way when doing this… I just put some cardboard down on the floor to avoid scratching the paint.

After all the hardware is removed, pull off the bag.  It might help to blow some compressed air into the chamber if its really stuck.  Don’t pry anything between the ring and the chamber… this is part of the sealing surface!

The hardware just after removal is see here.  I cleaned everything up to get all the dirt and grime off.  I also lightly cleaned the air chamber, ring, and the sealing surfaces of both.

As you can see, the old bags were pretty beat up.  This one is from the passenger side that wasn’t leaking!  Its also interesting to see the bag rests in a position it would be in with the suspension completely collapsed, as it sat for many years.

Installation is reverse of removal.  Getting the new bag on the ring is a bit of a challenge, but some soapy water helps.  I tightened each screw and nut slowly to minimize the amount of distortion on the ring before it was fully seated.

I installed an air valve (A 112 320 06 58) and filled the chambers to check for leaks.

Here is the chamber filled with about 50psi.  I submerged it in water to check for leaks, then reinstalled it in the car with new seals at the air line.

While the air chamber was out of the way, I cleaned up the front suspension.  Years of old grease was caked up and covering most of the components.  I started with a large screw driver to pull away the larger pieces, then used a wire brush with some WD-40.  Heres a picture during the cleaning:

I can do more here, but the goal was just to clean things up to make greasing easier, and to help the guys when its time to get an alignment.  I greased everything with good chassis grease when I was done.

And finally, before reinstalling the air chambers, I replaced the bump-stops on the lower control arms.  Since the car was sitting on them for 20+ years, they took a bearing.  These are part number A 112 333 02 65.  Removal wasn’t so bad.  A pry bar with a 2×4 sitting on the control arm removed them with only a little effort.

While I didn’t get the finish the job completely today, I did get quite a bit accomplished in 6 hours.  I still need to clear up the driver’s side suspension and reinstall the air chamber there.  I’ll post pictures of the “before” state to show how dirty it is tomorrow.

Fine tuning

Now that the car is somewhat drivable, I can pay attention to getting the engine running well.  First step was to install a Pertronix electronic ignition kit to replace the dual-points in the distributor.  I’ve used these kits in previous cars with excellent results, so I was happy to see a kit (model 2865) exists specially for the M189 and it’s dual-point distributor.  Along with the pertronix, I changed to a Bosch Red coil (A 000 158 49 03) and 1.8ohm ballast resistor (A 000 158 17 45).  New wires and rotor are on order, but did’t arrive yet.  I won’t replace the cap due to the high cost, but the one on the car looks like its in good shape.

With the new ignition parts the engine ran better, but still not well.  I checked the cold start valve for leaks by removing the small bolt on it, as described on the old site.  No fuel came out after some time, so the valve doesn’t appear to be the source of the rich running.  I then disconnected the throttle linkage with the engine running and operated only the butterfly on the intake manifold.  The engine reved up quite a bit, confirming that its running very rich.  I shut it off and let it cool down.

Next step was to adjust the valves.  I’ve heard its possible for the intake valves to recede into the head quite badly over time, so I didn’t want to run the engine much more without checking them.  All the valves were tight, but not terribly so.  Adjustment was tedious, but I finished them all with the help of a friend in about 30 minutes.  A 14mm crows foot and good quality screw driver are a must for this job.

Now back to the fuel system.  According to several postings on the M100 message boards, it seemed likely that the rack in the injection pump is hung up from old fuel varnish that developed after years of sitting.  To free it, I used a bulb syringe to push acetone into the injection pump through the fuel line.  I used an extra fuel line that I ordered that normally goes on the output of the electric fuel pump, A 111 476 09 26.  This fit to the input connection of the injection pump and make it quite easy to fill it with acetone.

After soaking it a bit, I removed the small cover at the front of the pump to access the end of the rack.  This is a very difficult job because of its location.  Removing the distributor made it a bit easier, but a collection of long tools was needed to get in there.  I screwed a long M5 bolt into the end of the rack and check it for movement.  It move fairly easily, but didn’t return to its center position on its own until I moved it several more times.  Then I took this video to show its movement:

You can see it moves quite nicely here.  I put everything back together and ran the engine again.  Big improvement!  The engine reved up very easily now and idled much smoother.  However, once it warmed up, it would stall at idle again.  I think the cold start valve might still be an issue, so I’ll order up the seals and take it apart this week.  Just to be sure the injection pump rack can move freely, I filled it with acetone again to let it soak over the next few days.  This certainly can’t hurt!

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.