Hello,
I have a 1981 300SD with 245k miles driven often and nearly 100% highway use. Recently (6 months) I've been having to add coolant. I have to add usually 2-3 quarts per 2 tanks of fuel. I cannot find any external leaks, it only seems to lose coolant while running. I've never had any issues with overheating (solid at 82 degrees, occasionally climbs to 85 on a long hill), have no coolant in the oil, and although my exhaust is more visible than I'd like it doesn't smell like coolant. Expansion tank was new in 2013 and hoses new last year. It's worth noting that the PO had a used head put on the car at 200k miles after an overheating incident. (Found out after purchase by looking at receipts).

The car runs excellent otherwise, so I'd like some advice.
-Is it dangerous to keep running it in this condition?
-How is it possible to have a leak but not an overheating condition or water in the oil?
-Is a head replacement a relatively garage friendly repair? I'm not short of normal tools but don't have many M.Benz special tools.

Any other advice appreciated!
Thanks

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-1981 300SD 242k, stock - sold @ 250k
-1994 Toyota Pickup 4x4 R150F 5 Speed, 5VZ V6 swap.
-2004 Volvo V70-R 2.5T

You have not mentioned an aproximate of how much coolant per miles is getting used up.

Fill it properly with Coolant and take the Car for a run getting it nice and hot and park it. and 1-2 hours later squeeze the Upper Radiator Hose and see if it feels inflated. If so it is likely combustion gasses getting in there.

But tiny tiny holes or seeps under hoses can also evaprated when the Engine is hot and leave no obvious wet liquid.
Under Hose connection area (especially the coolant bypass hose area also on a W123 the Heater Hoses are connected to steel tubes), areas of the Coolant system where there is a gasket and the Freeze Plugs.
Depending on the coolant used you can sometimes find white to greenish colored poweder where tiny tiny leaks are.

Pressurizing he Coolant System may show up external leaks.

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84 300D, 82 Volvo 244Gl Diesel

The cylinder head needs to be removed with a lot of care because of the need to keep the timing chain in time. However, with the exception of the tool for the Head Bolts and a good quality torque wrench it is no dofferent from chaning a gasser cylinder head.

There should be some how to do its in the below links:
Repair Links
Fast navigation http://www.peachparts.com/shopforum/diy-links-parts-category/146034-fast-navigation-do-yourself-links.html

It is not unusal to find deep pitting under the coolant hoses on the Aluminum parts. I fond that on my Bypass Hose Nipples and the same on the Thermastat Housing.

Note that sometimes the Bolts that hold the Coolant System together break off during removal. Make plans with that in mind.

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84 300D, 82 Volvo 244Gl Diesel

I had a similar problem with my 83 W123. Eventually I traced it to a crack on the top radiator hose inlet and the small tube on the left hand side leading to the expansion tank. I replaced the radiator and no more losing coolant. You W126 uses the same radiator. The radiator is prone to fatigue and develops cracks in these 2 places. How old is the radiator? Do a pressure test on the cooling system. It may give you some clue.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Pop over to Autozoo or its ilk and borrow a pressure tester. Use it to rule out any external leaks.

Murray Temperature Control 9300 - Cooling System Pressure Tester | O'Reilly Auto Parts

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Your antifreeze may flouresce under UV - if so a UV flashlight will help find external leaks as well

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

Harbor Freight has a pressure tester at least as nice as the parts store if you get tired of having to rent tools and have room in the tool box. Mine was leaking from the 2" hose - bypass hose I think.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Look over the plastic expansion tank well. They are known to crack at the fill line.

Also, inspect the radiator cap. They crack internally and allow coolant out the weep hose.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Thanks guys! I'll definitely look into a pressure test asap.

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-1981 300SD 242k, stock - sold @ 250k
-1994 Toyota Pickup 4x4 R150F 5 Speed, 5VZ V6 swap.
-2004 Volvo V70-R 2.5T

I just resolved the same issue on my '85 w123. It was a pinhole in the hose that runs from the back of the head (the one that gets in the way when you change the oil).

I can't explain why, but it didn't leak consistently. Nor did it leak when parked so I never saw a puddle under the car.

I happened to stop to fuel up on a very hot day, opened the hood and voila noticed a little puddle on the concrete of the gas island. On the w123 it's a straight shot down to the ground from the section of the hose that leaked on mine, so it wasn't showing up as a wet spot under the hood either.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22