while re-fitting that overboost line today, i noticed that my coolant reservoir was bone dry. i'm not sure when or how this could have happened, because it hasn't been overheating and i've been driving it daily. well, as i drove it out of my neighborhood, i noticed the temp gauge going above 100, almost all the way up to 120. the weird thing was, it would fluctuate a lot. within a minute, after hitting the turbo a few times, it was almost back down to 80 oC. obviously there is a leak somewhere, but there's no coolant on the road where i park it or anything. where are the obvious places to look first?

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1985 300D Turbo "Eunis"

First and most importantly, check your oil. If you see any sign of liquid, if it looks creamy or bubbly, don't drive it any more, even to the shop. If not there, smell your exhaust. A fair leak will produce a coolant odor. In any case, very likely you need a head gasket.
ben

You might want to check your coolant recovery bottle cap to see if it needs to be replaced. If its leaking out on the road you won't see it on your drive way.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png

I've had this problem as long as I’ve had my car, about 3 years. I have no water in my oil. My car runs great. I changed the cap, but no change it still slowly runs out. My only thought is it might be the heater core, but it doesn’t leak... there is no water in the car, but it dose have a moist smell some times. My leak is very small where ever it is, because it takes about a week for the reservoir to go dry.

There are so many places coolant can seep out of I can't think of how to cover it except clean around your engine compartment, add some coolant to your reservoir and start looking. You'll probably need to add coolant several times.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Mine has been doing this for 3-4 years.. it's a slow leak and completely disappears.

There are white traces on the radiator, near the seams. I think the radiator is leaking slowly, the coolant goes into the fan, and blows all over the engine and under the car. So there is no residue. And it's not getting worse, so I haven't gotten around to swapping in a new radiator.

But your first step would be to check for leaks and traces of white stuff. Look at hose connections, etc.

Second step is to swap the radiator cap because it's probably leaking slowly. Don't bother testing it.. if you don't remember replacing it in the last few years, just get a new one.

If the problem still happens, get (or borrow) a cooling system pressure tester and test out the system.

We really need to know how fast it's burning (or leaking) coolant to take a good guess. Fill it up to the full line, then wait till it drops. Fill it up again and measure how much you put in this time. Tell us how much it burned over a certain period of time and/or certain number of miles.

Once we know this, we can help out with whether it's a slow leak (small gasket leak, crack in head, etc.) or something major. Since you don't specify (or maybe don't know) how low the coolant was before, we don't know if it has just emptied the entire tank or if it was just a bit.

Neal

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)

There are coolant lines that go to the fuel pump area to heat the fuel.
On my sdl the coolant also takes a tour thru the windshiel washer reservoir.

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82 300D....went to MB heaven
90 350 SDL....excercising con rods

1984 300d Turbo 137k
Your Water Pump Veep Hole Botton Or Top, Replace Water Pump,
The Bearing Seal Leak Under Pressure, Use A Radiator Pressure
Tester Diy 2-3 Hrs

Really?

Any chance you could provide a picture?

Wow that really sounds like a head gasket. I'd check the oil but even if it looks good send it out to be tested. See if the lab red flags coolant.

You will wipe the bearings out of that engine if you run it with coolant in the oil.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200