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”
I’ve been thinking a lot about tuning the engine, and specifically, why it suddenly seems to run pretty poorly as of this past Wednesday. I had let the injection pump soak with acetone again for a while, which didn’t seem to do much. The engine was difficult to start, then would stall after it warmed up. The idle was low and adjustment of the idle speed didn’t do much.
Out of concern for the problems it can cause, I first took apart the cold start valve. Taking it apart was quite easy… just two screws hold the solenoid to it, and three bolts remove it from the manifold. I confirmed that it wasn’t leaking fuel and the needle inside moved freely. I also confirmed that its a very good idea to replace its seals. They were in very bad shape. The small seal (A 001 997 75 40) is very expensive for a little o-ring, list price is $22.50. The larger seal (A 002 997 52 45) is a value at only $3. This is well worth the peace of mind.
Next started looking toward the thermostat on the injection pump used for warm-up. It was clear that it didn’t work correctly since some air was being pulled through the little filter after the engine was warmed up. I installed a new thermostat (A 001 203 95 75) and experimented with adding shims below it and it didn’t make any real difference in how the engine ran.
After thinking about it, the problem was quite obvious: ignition timing. When I removed the distributor to fiddle with the injection pump rack, I only eyeballed its position. It turns out that it was WAY off. I set it correctly and the difference in the way the engine ran was quite remarkable.
I ended up adding about 0.5mm of shims under the injection pump thermostat and it seems to work well now. When running at operating temperature, no air is pulled into the warm-up system. The idle speed adjustment screw works correctly and the engine idles smoothly at 800rpm now. Throttle response is also much better.
To celebrate, I took the car on another trip around the parking lot:
As you can see, the car runs very nicely now. There is no hesitation and it seems to have very nice power.
After this drive, I changed the oil. It had already started to turn dark and I was worried it might be diluted with gas. I also noticed that one of the front air bags began leaking, moving this job way up on the the list of things to do.