Mercedes: fuel system, mass air flow sensor, mass air flow

Question
Hello. My 190e 1990 2.6 has a problem with the fuel system. When I try to start it, it starts and then after running for a few minutes it gets sluggish and starts to blow black smoke. I believe this means it is running rich. Please correct me if I am wrong. I checked the pumps as another shop told me they were bad. If it is running rich until it quits could it be the fuel pressure regulator? I need some help.!! The car is only worth between two and five thousand, the dealer wants $220.00 just to evaluate. The dealer says that it could be three or four thousand to fix it if it is the fuel injectors. I'm respectfully asking for some good advice. I really need to get this fixed. I loved this car while it was running and I would hate to have to junk it or something.

Answer
John
What is the mileage on this vehicle? What Pumps? When was the last time you changed spark plugs. You may have contaminated fuel.
You may have a bad Mass air flow sensor. Or MAP sensor.
Your catalytic converter may be stopped up. Make sure you have not added leaded fuel.
Make sure your air cleaner filters are new.
Here is the deal the engine is an air pump. The fuel mixtures is determined by the amount of fuel atomized inside the cylinder with the correct amount of air during each cycle.
The injectors are controlled by a computer that regulated the length of time they open. The computer looks at incoming air pressure to decide on how much to add.
If the computer thinks your are getting more air than necessary it will add fuel. As the density altitude changes  i.e. rises through increase moisture or driving to a higher altitude the sensor tell the computer to wean off the fuel flow.
Now after combustion, if the fuel is incompletely burned then carbon contaminated your 92 sensor and the converter. If the flow out is restricted then the computer changes the mixture.
It sounds to me like you have fouled plugs.
I would remove a plug and check it out. It could be one or more cylinder. They could be fouled by something in the gas, blow-by of oil on the rings. The smoke will usually have a blue/ gray cast if it is oil related.
The injector could be staying open too long. IF possible I would remove the fuel in the tank and pour some Techron black bottle injector fuel system cleaner in the tank and fill it with a regular grade fuel. Try to keep it running.
If these conditions started during the recent cold weather season, it also could be the cold start injector which is use to enrich the mixture during start up. However, if this injector circuit is hung up it may be continuing to operate after the vehicle warms up and that could cause the black smoke.
Yes if the regulator is providing excessive pressure to the injectors you could get too much fuel in the cylinders. Because that could be an extensive fix, I would look for some additional possibles. I would locate an 'independent' mechanic and ask if they will run a code check for you. They are generally much less expensive that a dealer.
I hope I have not overloaded you with possibilities. I would start with the least expensive and the easiest first. Good LUck
Robert