Thanks to everyone for your responses. Please know that I am reading everyone’s replies and taking in the information. I truly appreciate it.

@Bio300TDTdriver - The engine has not overheated to my knowledge. Although there is oil in the coolant reservoir, the temp has managed to stay at 80° steady even with the AC running. And I agree, since my dad’s mechanic seems unwilling to work on the car, we would need to find a trustworthy shop knowledgeable with working on this engine. That might be the most difficult thing for us. As long as it is prudent, I can come up with the funding for the repairs. Key word being prudent.

@Jeremy5848 – If it were to be repaired, I would not mind keeping it for 5 years or more. I am a fan of the styling of W126 chassis and am astounded with its overall durability after 25 years “in the family”. Also very impressed with the efficiency of the diesel engine, over the years we’ve experienced up to 30mpg with this big boat of a car. That is good especially in these times of high gas prices.

@BoiseBenz – I will check the fuse out tomorrow. I did watch the youtube video from mercedes-source which explained the kickdown switch causing no upshifting transmission on his 300sdl a few days ago and afterwards I did shoot a bit of spray lubricant on the kickdown switch behind the pedal. It did not change anything that I could notice. In my own opinion and mind you, I am not at all that knowledgeable on these transmissions, I just have a suspicion that it is vacuum related. Not that there isn’t any vacuum, but more that it is not getting complete vacuum, maybe a slight leak? I don’t know if that makes sense or not. But the fact of the matter is: it doesn’t happen all the time. There can be weeks between occurrences, and then sometimes it can happen twice in one day. To be more specific regarding the problem, it only has a problem “at times” upshifting from 1st gear into 2nd, meaning the car gets stuck in 1st. And it doesn’t happen all the time only sometimes, not necessarily frequently. When it happens, I just pull over, put the shifter into park, then put the shifter back into drive and it resolves itself and then it is ok for the rest of the trip. As long as the car shifts into 2nd there has never been any issue with shifting through the rest of the gears. I take the car on the freeway and it drives nicely at 75, 80mph and higher no problem.

@vstech – yes, the trap oxidizer was replaced by the dealer early on…. I believe we were part of the first wave of recall notifications… I wish I could get a free head from the dealer on a 25 year old car, that would be something.

@sleepstar – yes the engine stop valve lever was replaced around February 2012, but the transmission issues were prevalent months before that and they continue to persist after. One day it just didn’t want to shutoff after the key was out of the car; however, the lever was able to shut the car off. My dad brought it in and they diagnosed it as a faulty stop valve lever, it was replaced and the car shuts off fine now. Things like the vacuum central lock work flawlessly.

I elaborated a little more in my response to BoiseBenz above, but we have not experienced any slippage with the transmission, it is not upshifting from 1st to 2nd, this is from a stop. See my clarification above.

@ Junkman – Hopefully I can find a shop here that can give a proper diagnosis. When and if, I will cross my fingers that it is an inexpensive fix. I agree with you about the guessing, my first step in trying to figure it out is starting the dialogue with you nice folks here on the forum.

Thanks again everyone, please keep the dialogue going.