I have successfully fixed the problem with my 1969 280 SEL. It ran extremely rich and would resist all attempts at tuning. Clean spark plugs would foul after 20 seconds or so of running, so....let's get to it. The problem was in the housing for the plunger that is activated by the pump thermostat. It was seized up solid which kept the pump in the full rich position. The good part is that you don't need to remove the whole pump to fix this. The plunger is accessed by removing the two screws that hold onto the thermostat. Once that is removed you can see the plunger and push down on it with a screwdriver or some other probe. The reason that it seized up is because the sensor inside the thermostat housing wasn't seated properly allowing water to get in and muck things up. Just free it up (if you can) or get another housing from a used parts place, replace the sensor/bulb in the thermostat and you're all set. It took me two months to figure this out so copy and paste this post for future reference and you will save yourself a world of trouble. By the way, this is my first post here so hello everybody. If you'd like to see this car and the project it has become just go to... http://www.geocities.com/thompintello/MercedesProject.html
Cheers,
Thom

Hello Thom.

Your maiden posting is a wonderful insight.

Your project looks rather successful and sharp! Amazing restoration! :)

I have written more than once about this problem. The story is in the archives but one point should stay attached to this post.

It is very easy to evaluate this problem by removing the small air filter at the rear of the injection pump. This is for the additional air used during the warm-up cycle. The valve for the air is moved by the same action as the enrichment. When the car is cold placing one's finger over the airfilter hole will stop the motor. When warm the same activity may allow ones finger to feel vacuum but closing off the hole will not result in engine speed difference.

I always include this simple test as the first step in mixture evaluation on all those mechanically injected motors. If the plunger is seized or if the t-stat is dead (the more common problem) there will be no change in the airflow, cold to warm.

1969 280 SEL running way too rich
 

Thanks ymsin, I'm having a blast with this particular vehicle. Even though it was awful to look at in the beginning I tried to keep focused on how good a shape the powertrain was in. It only had 80K on the motor and the automatic trans had been completely rebuilt two years before it was put into storage. In a couple of days I'll have an additional page up at the web site which will bring things up to date. The interior is finished now and the whole project is in the home stretch.
Steve, thanks so much for the insight and lore. I'll have to bookmark this thread as well if it keeps being this productive. The frustrating part was that there was precious little information out there on this area of the car. The so-called workshop manuals were useless so the info that I got was obtained by talking to Mercedes mechanics and paying them for their time. Nothing pisses off a mechanic more than someone that wastes their time. Now I have to sell the second injection pump, hopefully to the company up near San Francisco that rebuilds them. Thanks all.
Thom

Thermostat
 

stevebfl,
I know a Mercedes mechanic who mentioned that he changed the heat feeler in his thermostat when he noticed that his gas mileage started dropping to around 13 mpg. He replaced it and it shot back up to 20 mpg. Ordinariliy, what is the lifespan of one of these heat feeler/bulbs? Just wondering. Thanks in advance.
Thom

It is hard to say on the lifespan. Since the measure criteria I have given is strictly subjective and MB doesn't give much in the way of operational criteria, I would replace the t-stat as often as I would replace the other t-stat for the radiator. The wax bulb that operates the two are very similar.

The criteria I gave for determining how far the valve has shut does a pretty good job as long as nobody has messed with it. In other words if the air valve makes it shut when hot, the t-stat is probably working as the thing fails in the rich direction. The wax bulb grows as it gets hot and drives the airslide down shutting off aux air and reducing the cold running enrichment with the same movement. Since the device has two different sets of shims within, the airflow method can be messed with giving different characteristics with the t-stat working properly.