"Tweaking" 1987 300SDL - 175k

[ehm]

I recently acquired an '87 300SDL. It has 171k on the clock, which has stopped working, but I have every reason to believe it still only has about 175k as I use the same mechanic that serviced the car over the years. It sat for several years (4?), with very infrequent driving - not tagged during that time so pretty infrequent I imagine! I have replaced all the 'engine compartment' fuel filters, of course oil change, and some other parts such as calipers all around and lower brake lines, and tie rods, 1 of which were definitely bad. It runs great and just seems to keep getting better. And the mileage seems to be getting better but it is still less than 25 I think, and it seems it should be better. Also, in town, especially if not driven in a week or 2, I see more smoke from a takeoff from a light than I'd like or expect. Are there relatively simple things I might check, perhaps like some type of air/fuel adjustment etc? I had a leak on the return fuel line and fixing that definitely helped! Thanks as always.

[Diseasel300]

2 things worth checking:

1: Check the ALDA on top of the injection pump. Try pulling a vacuum on it. If you can't pull a vacuum, or if you can and it quickly bleeds off, you may consider re-sealing the unit. The ALDA senses boost pressure and enriches the fuel mix to spool the turbo. If it leaks, it'll be pretty doggy until the turbo spools then suddenly have a burst of power.

2: Check boost pressure. SDL's should be running around 13-14PSI when in gear and pulling >2000 RPM. If boost is low, or builds late, you'll be smoking more than you should AND you'll have lower horsepower (and greater fuel consumption).

It is also worth checking the port in the head for the boost sense line to the ALDA. They frequently get blocked with soot/tar/oil.

For what it's worth, I have my IP tuned pretty tight (though it comes from a 3.5L engine). My SDL does ~24mpg in city traffic with the A/C running. Highway driving depends on the weather (A/C or no A/C) and speed. 70-75mph with no A/C (late fall, winter, early spring) I can pull 29-30mpg. 75-80mph and A/C running is more like 25mpg. Remember, this a barge of a car with no lockup torque converter.

[ehm]

Thanks! I think I did get about 25 doing about 75 with the air on, so maybe I'm more or less where I should be in that regard. Speaking of air, would you be concerned at all if the temp needle went a little above half-way with the air on (and more down around a 1/3 of the way up with it off?)

[Diseasel300]

Ideally the needle should stay right in the center of the gauge (85-90˚C). Mine will climb a bit with the A/C on and pulling up hills on the interstate, but comes right back down once the hill is climbed. Temp never goes above ~1 needle width under the 100˚C line.

If the temp is staying low, consider replacing the coolant thermostat. It's easy to get to and relatively cheap. See if that helps both high and low temps.

If you still see the temps climbing on the highway, consider upgrading the cooling fan and clutch to the ones from the '98-99 E300D. The 11 blade fan moves quite a bit more air than our 9 blade fans and seems to really help the temp stay put. Helps the A/C out a lot in city traffic too.

[ehm]

Great idea; thanks!

[Maxbumpo]

If your thermostat is not working and the engine doesn't get up to full temperature, your fuel consumption will increase.

Test the new t-stat in a pot of boiling water to make sure it starts to open at 80 deg C and is fully open by around 90.

Get the proper MB spec coolant, can be either blue or gold colored now. The blue is the most recent version, the older gold is called Zerex G-05 on the aftermarket, usually around $15 to $20 per gallon at Napa. Other FLAPS may also have it in stock.

[Diseasel300]

Originally Posted by Maxbumpo

Test the new t-stat in a pot of boiling water to make sure it starts to open at 80 deg C and is fully open by around 90.

I can't agree more with that suggestion. I went through an ordeal with an MB OEM thermostat that was bad out of the box on my SL. Was pulling my hair out trying to figure out why it was overheating until I tested the thermostat. Was open maybe .5mm at 100˚C. No bueno. Testing first would have saved my hair.

[Maxbumpo]

I've only had one T-stat that was marginal out of the box, it was opening early. I ran it for a couple years and then changed it with a coolant flush. Now my engine runs about 5-10 degrees warmer, and mileage has improved by about 10%.