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i have a '82 300 SD with 374k miles on it. i recently went through it, put a new timing chain on it and also replaced the glow plugs. in the last week i have twice experienced a strange happening while driving on the freeway. both times after driving 50-60 miles the engine suddenly began to race and it was like the car was shifted into neutral and the pedal was put to the metal so to speak. the first time i did put it into neutral after completely letting of the accelerator then as the car slowed to 40-45 mph i put it back into drive and never had another problem. the second time it was like the cruise control suddenly kicked in and it started accelerating. after pumping the brakes several times the car went back to normal. both times there was a good deal of black smoke behind me, the cruise control has never worked since i bought the vehicle. i have noticed also that when idling it sometimes acts as if it has dropped a cylindar, idles roughly and the typical rattly diesel sound is much more pronounced. any ideas what these problems might be?
 

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It sounds as thought there may be different issues at work here. First is the transmission slipping. Either the transmission itself is slipping (not surprising for such high mileage), or the linkage is slipping (which would feel like a loose shift knob) and you need need new linkage bushings.

The other problem has happened to me (I think), where I let up on the gas pedal, only to have the car continue accelerating. This was the result of a throttle linkage binding a gainst the intake on my car. This was readily evident by the shiny wear pattern on the manifold. Look for something similar in your car. The fix was to just staighten the rod a bit.
 

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Oh, yeah. Back to the transmission, the B-2 piston is also known to fail. There is an update for this that does not require a rebuild, but can cause slippage. Do a websearch for "mercedes b-2 (or b2) piston" or similar variations for better details.
 
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