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Old 12-24-2000, 05:04 PM
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I have been running an '80 300SD for 10 years and now I bought an '87 300SDL (6cyl Diesel). What do I have to do different, if anything, to maintain the SDL? Are the frequent valve adjustments necessary? I will be using Mobil I 15-50 oil because it kept my older SD working great.

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Old 12-25-2000, 09:45 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
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These newer diesel engines don't require valve adjustment. Frequent oil changes & coolant changes are key to long engine life. NEVER allow this engine to overheat. The aluminum cylinder head won't last 20-30 seconds at high temps. If the engine runs hot shut it off. Have seen engines like this with over 500k mi & never apart. Take care & enjoy the car, as the W126 chassis is one of the best cars MB ever made.
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Old 12-26-2000, 03:53 PM
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I realize that the 6 cylinder Diesel engine is a completely new design compared to the 5 cylinder which was an update of the 4 cylinder that went back to about 1960.

What was changed that the intake and exhaust valves don't need adjustment anymore? What was the design idea for the aluminum cylinder head, just to save a few pounds of weight? I never did understand how you could bolt an aluminum cylinder head on a cast iron block and not have problems because of the fact that aluminum has three times the temperature coefficient of iron. Something has to slide and that doesn't sound good. Seems like the head wouldn't have to warp much to tighten up the cam bearing surfaces.
It also seems like the valve guides would loosen up when the aluminum head expands. Are they a press fit like the 5 cylinder or are they screwed in or attached some other way.
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Old 12-26-2000, 07:11 PM
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PEH, I don't have that particular eng. mine is a 617.950. What I do know is that usually what car companies do to get away from valve adjustment requirements, they install hyd. lifters. This is what Porsche recently did. So maybe that is what MB did. As far as the aluminum head/iron block issue. I too was wondering how they do it. I had a 1985 Ford Escort with a iron block/alum. head. I also had a Ford Taurus 3.8 V6 with the same arrangement. My Vw Passat 1.8, 4cyl, 20 valve head is the same way iron/alum. So companies have been doing it for years. This type of arrangement is probably the reason the cooling system is real critical these days.
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Old 12-26-2000, 09:04 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Yes they did go to hydraulic lifters. Gas engines & aluminum heads seem to be a natural, but the diesel motors run a much higher combustion temp so overheating is BAD. I'm sure you keep an eye on gauges so don't worry & enjoy the car!!
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Old 12-28-2000, 10:46 AM
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Higher temps

I to just went from a 83 300D to an 86 300SDL, and it seems to run hotter, and there is significant fluctuation in the normal course of running, either to work or on a trip. Is it wise to change out the thermostat as this seems to be a concern in my mind. Also any recommends on coolant??

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