I searched the site and found a few good ideas about discovering slow coolant leaks, but I thought I would describe my symptoms and someone might suggest a direction to explore.

Cooling system history:

New radiator in 7/00 (installed by MB dealer)
New thermostat 12/00 (installed myself)

Leaking history:

In late August coolant level light comes on for first time (cool night drive to Boston).
Over the next four months (4kmi) I've added 1 gal of MB coolant and 1 gal of clean water aiming for 50% mix.
I believe that I've seen fluids leaking, but not while I was watching for them. Some ran off the under pan when I jacked up the car to change to snow tires.

Other things:

This month (12/01) I've smelled a strange odor from the heating system. This seemed to happen when I was stopped on a steep decline.
The dashboard gurgles loudly (but I don't believe this is new), but I learned from the diesel mailing list archives that this is a "feature" of the W124 heating system.
The system maintains pressure even after sitting over night.

Thanks,

__________________
-Mike
'87 300TD 304kmi (RIP)
'95 Toyota Camry Wagon 125kmi

The smell of anti-freeze inside is not a good sign. However, can you find any traces on the garage/driveway? I had a leaky heater core and the symptoms were coolant dripping alongside the transmission. This was traced to a drain from the heater core housing. There is a drain hole in the housing that leads to a rubber drain that empties along the transmission. (was that redundant?) Look also on your floor for coolant.

I think I noticed the smell only when the heat was on and in bad cases, you will notice that the heated air is moist and can condense on the windows.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Michael,

Thanks for your observations. I will look in those places you suggested. I'm guessing that the drain you refer to is the same that drains the condensation from the evaporator when using the AC in humid conditions.

The heater core doesn't sound like an inexpensive or enjoyable replacement given it's location in the dash. What kind of expense should I plan for if this is indeed the problem.

Thanks,

__________________
-Mike
'87 300TD 304kmi (RIP)
'95 Toyota Camry Wagon 125kmi

If the W124 is the same as the W201 you should check the heater control valve for leakage. On the 87 W201 this is a slide valve with lever arm that has a seal. Mine was leaking and dripping out next to the transmission. It was a very easy fix. The valve is located on the W201 under the leaf screen below the wiper.

__________________
Tom Savage
Vienna, WV
1984 300D Euro
1995 S320

Mike:

Have you replaced your radiator cap (its on the coolant tank)? If not, it may be bad, and allowing the coolant to spill out of the recovery tank under the fender behind the front wheel.

My heater gurgles once in a while too, very annoying. I've not replaced the cap on the 300D yet, but did replace the one on the 300TE last spring due to coolant loss -- boiled over some when hot, especially if just pulling off the highway. New cap fixed it.

Another other thing to check is the circulation pump up by the battery -- it can both go bad and leak, and leakage won't show up in the engine compartment.

The water pump can "weep" very slow when the seals first go, and will, I think, drip straight on the ground, not into the pan. Hard to see until it gets bad.

Let us know what you find -- my 300D is also slowly consuming coolant, and I fear a cylinder head gasket is in my future. I'd much rather it be a water pump!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I have had to replace both as well; the circulation pump by the exhaust manifold and the heater valve under the leaf screen. They were both leaking. Thanks for bring those up.

I centered my attention to the inside of the car as Mike said he could smell coolant.

I missed the part of his post about the coolant on the pan under the car.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Hey Mike,

Did all of this start happening after you replaced the thermostat. If so that may be your problem. My car has a slow leak I suspect since changing my thermostat. It is just a bit too cold up here in chicago to play with it at this time. I will take it out soon and drill a small hole in it to promote removal of air. Also my thermostat was not OEM so that may be the problem too.

Anyway I know how you feel.

bob

All,

Thanks for the responses.

I replaced the thermostat and radiator cap just over a year ago. My recollection is that the gurgling started in the summer and the leak (coolant level low condition) began in late August. So, I really can't say that this began when I changed the thermostat. That was the first place I looked for the leak, but none is apparent there.

Check out this other thread too.

It seems to have info relevant to coolant leak problems.

__________________
-Mike
'87 300TD 304kmi (RIP)
'95 Toyota Camry Wagon 125kmi

Well things aren't going so well. I went to the parking garage without calling first so that I could see the car cold. First thing was the large amount of coolant under the car. Next was the pressure in the coolant system. Then I released it and confirmed that the pressure builds up promptly upon starting the car. No rough gurgling under acceleration, but quick pressure build up.

So I decided to take it to my mechanic, Nick. They carefully washed the entire engine and inspected it for all the leaks. When the belly pan came off we had all three fluid types in great quantities: engine oil, ATF, and coolant. This was a dramatic change from 2500mi ago when I had the pan off to change oil and had small quantities of ATF and oil.

Now for the bad news. The coolant comes out of the head at a place above the head casket on the rear cylinder (I saw the fresh coolant on the clean head too). Nick says that in his experience this nearly always proves to be a cracked head. The oil leak at the casket and front seal is pretty heavy too. And the transmission seal leak is too bad to ignore any longer. I also have a rear bearing that needs replacing.

My wife and I have decided that we like the car well enough to continue to invest in it.

I'd appreciate any opinions on the following prices. Also anything I should be sure to do while I have the head off?

Labor to remove and replace head and front seal: $1300.
Labor to replace the transmission seal & rear wheel bearing: $500

Nick recommends a new head only. Nick gets his for $1850.

Thanks,

__________________
-Mike
'87 300TD 304kmi (RIP)
'95 Toyota Camry Wagon 125kmi