first problem with the glow plug light is one of your glow plugs is out. the car might have some trouble starting if you have more than one out, so maybe youve noticed that, its easiest to replace them all. the only way to find out which one is out is to test each one, ive never done it so i dont know how. if youre going to replace them yourself, youll have to take the intake manifold off.

the second thing doesnt sound like a problem at all, 80 degrees C is actually very cool. 100 is equal to 212 F, so 80C is fine. mine usually runs at 90C, but lately at about 100C, because of the summer heat. you only have to worry about the temp if its over 100, adn at 120 you really have a problem.

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350