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Originally Posted by fcffab
I was given an 1989 300SE. It had been sitting for several years and had a dead battery. I replaced the battery with one I had and after some trying it started. When I got it home I replaced all the fluids and filters, cap, rotor, and plugs. I filled the tank with gas and a can of injector cleaner and drive it for about 275 miles. Now when I start it in the morning I have to hold the RPM at 2000 to 2500 for a few seconds and gradually bring it down to idle or it will stop running. After this first attempt it refuses to start untill the next morning when I must repeat this procedure. Once its warm and running it starts normally.
Any help or ideas will be appreciated.
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The idle air bypass control may be stuck in one position. It's at the front of the intake manifold with two hoses attached. Try cleaning it as it may be full of crud. Also, check for vacuum leaks, such as the two hoses previously mentioned.
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Originally Posted by jackcmail
I have a 1991 300SE that has been sitting for over a month. Now it will not start. The fuel pump is not being energized when the ignition switch is turned to the start position to crank the engine. The fuel pump does make some noise and the measured votage varies but for the most part it is not running while cranking. The fuel pump does run if I connect 12V directly to the pump terminals. I did verify that there is spark while the engine is cranking.
I have ordered a new overload protection relay and a new(used) MAS control unit. Hope they solve the problem.
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You've narrowed it down to the fuel pump relay.
It's not an overload protection relay, it's an overvoltage protection relay. If the voltage exceeds 22 volts, the fuse blows because a zener diode conducts to ground causing the fuse to blow. The ECU and ABS computers are protected in this way.
http://mb.braingears.com/126_DISC1/program/Engine/103/07-121.pdf
http://mb.braingears.com/126_DISC1/program/Engine/103/07-165.pdf