Good evening friends,

Looking for some help and professional opinions here as I can not find the source of my slow coolant loss.

Car: 1985 Mercedes 300D Turbo



Mileage: 650,000KM+ (400,000 miles)

Notes: Engine never rebuilt, original engine/transmission/diff, original headgasket, original timing chain/tensionor

Maintenance: very very well maintained, never overheated, all suspension replaced/rubber mounts, bearings, flex discs, hoses etc replaced, check ups at indy twice a year, oil changed every 5000km, transmission every 30, coolant every 25k using mercedes blue oem fluid, new water pump/housing/thermostat, no leaks whatsoever from this area or engine/frost plugs. I've checked 100 times.

Thoughts and opinions by professions of the W123: pressurize the system, look for leaks, if you cant find anyway its a headgasket or warped head slowly letting coolant into the combustion system.

Thanks in advance for reading this, I really need your help and opinions and thoughts on this. It is driving me bonkers. Is it time to tear into it and replace the head gasket and timing chain/tensioner? My guess after everything done and tested out that I've pointed out below, the outstanding causes are headgasket or cracked cylinder head but let me know your thoughts. I have no idea how I would have this without ever ever overheating the engine? I glue my eyes to oil pressure and engine temp.

I've been trying to chase a very very slow coolant loss over the past few years and I have been unable to whatsoever. Pretty much the car is my daily driver that I use 5 days a week travelling over 150km in Toronto, Canada.

The car is very well maintained, no rust, all fluids maintained and OEM/Fluids etc used. Below are the symptoms I'm having and what I've ruled out.

-Every 2-3 weeks I'll have to add half a liter of coolant (approx 2000km)
-The vehicle has never ever been over heated
-New rad, new upper/lower rad hoses, new water pump, new monovalve, new thermostat, clean thermostat housing and hoses, new heater hoses, no leak in car, no coolant smell at all, no drips inside the cabin, no visual leaks, no leaks at monovalve, no leaks at sub pump, no leaks at any of the side hoses (i've checked 100 times with engine hot, cold, everything)
-There are no visible signs of leaks at all whatsoever not even dampness
-the car has been pressure tested with a coolant pressure to 15-18lbs, no leaks anywhere. I left it for hours and not one single drop
-there is no oil in the coolant, there is no coolant in the oil, there is no white smoke out of the tailpipe, there are no bubbles in the coolant
-the car starts and runs very well, like new, no problem idling, no issues accelerating, drives 120-170KM on the highway daily.
-excellent fuel economy

Cheers lads,
Marco

__________________
1976 Mercedes 240D (Sold)
1985 Mercedes 300D Turbo Diesel: 500,000KM
1986 Mercedes 300E (Sold)
1988 Mercedes 300E (Sold)
2002 Mercedes C240 (Sold)
2008 Mercedes C350 4matic

A great site for purchasing industrial rubber products!

Industrial Rubber

To check for a coolant leak into the combustion chamber is to do a leak-down test. Not easy on a OM617 since you have to have your balancer marked at TDC, 72, 144, 216 and 288 degrees.


could also be a small water pump leak only when running, check evap drains or floors for a heater core leak

Thank you, I'll take mats out again of the front seat and look for any leaking fluid into the drain hole where the evap is. I've seen nothing here though and t here is never ever a smell of coolant within the cabin. Very dry and clean.

What do you think of this product? Apparently it will tell me if there is a cylinder head leak or head gasket.

https://www.amazon.ca/UVIEW-560000-Combustion-Leak-Tester/dp/B000NPDL76/ref=sr_1_2?keywords=Lisle+75500+combustion+leak+detector&qid=1588646200&s=automotive&sr=1-2

__________________
1976 Mercedes 240D (Sold)
1985 Mercedes 300D Turbo Diesel: 500,000KM
1986 Mercedes 300E (Sold)
1988 Mercedes 300E (Sold)
2002 Mercedes C240 (Sold)
2008 Mercedes C350 4matic

A great site for purchasing industrial rubber products!

Industrial Rubber

Consider doing an oil analysis, it will detect coolant in the oil not visually detectable.

Also study under the hood with a flashlight at night, occasionally a coolant leak will leave a stain that glows.

My experience with this type of situation is the leak is most often an intermittent water pump leak.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Pricey, This price would easily cover a small air compressor here in the states.

Buy some UV dye with a UV light and you can check your engine and stuff after dark.

Check the monovalve and auxiliary water pump for signs of weeping. My 300SDL had a slow leak I absolutely could not find and it turned out to be the thermostat for the wiper defroster (not present on 123's). I never saw drips, never saw pressure in the cooling system, but once a month I'd have to add about a quart of coolant.

My 350SD was losing coolant very early in my ownership and it was the shaft seal on the auxiliary water pump. It has since been deleted and has not used a drop of coolant since.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

My guess is the short coolant bypass hose between thermostat housing and pump. A leak at that location is never a gusher but is a weeper that evaporates quickly thus undetectable unless you catch it in action.

It is such a PITA to change that it is often overlooked. Mine developed a leak at about the same mileage as yours.

I had a slow leak from the rad at one of the transmission cooler nozzles. Only found it when the nozzle retaining nut split and leak got a lot worse.
On another car, I had a small leak in rad. Hardly any drip, but likely leaked while driving.
May be some like this. Just leaks while driving. But you would think this would show up on pressure test.

ADDED: Forgot to mention the water pump. Over the years I had 2 small leaks. One from weep hole in pump (on 107) and once on 300D from water pump flange gasket. Both very small. Probably vaporized while driving, but dripped over night. Discovered by placing large piece of plastic coated cardboard under car and checking it in mornings.

The coolant tester you provided the link for works well for combustion leaks. For external leaks leaks you could try a coolant pressure tester. After that, an ultraviolet dye in the coolant and a black light. I am sure you have tried all the easy options, so unfortunately it looks like you need to buy stuff to solve your problem. I have used all of these techniques with success.

Another place coolant could be going to is the transmission, via radiator ATF/coolant heat exchanger.

Doubtful with that big of a coolant loss and still perfectly shifting tranny, but one place to check and hopefully scratch off the list and narrow things down some.

I just did a makeshift repair on the plastic coolant reservoir on me 84 300SD. There were no obvious visible leaks but when checking after it reached operating temperature I discovered several tiny hairline fractures (you could see bubbles when I splashed water on it but noting when dry). As your car is original, it is something that you might want to check. You might also check the return line nozzle on the radiator.

regarding the combustion leak detector... they work but realize there is a specific test fluid for diesel engines and one for gasoline. i use one and it seems pretty sensitive.


-dan

From experience with other M-B models, I would check the coolant pump first. A 1987 300D (in-line six) had a similar problem and the techs finally found a crack in the engine block because the car only had 20,000 miles! (Way over my head technically, but that was the diagnosis.) M-B replaced the engine under warranty, apologized, and sent Christmas cookies until the warranty expired. Those are extremes, but both types of problems were difficult to detect. When was the coolant pump replaced? The last time I had this type of difficult to detect coolant leak, we determined it was simply time to replace the coolant pump on a 190E 2.6. Since then, we have been more proactive about replacing coolant pumps such as when other maintenance made it easy to get to the coolant pump. We had a 1982 300D and wish we still had it. That five-cylinder engine is amazing. I saw it recently and it does not look like it would if I were still taking care of it. That model was comfortable, efficient, easy to maintain, and very practical. Hope the input helps!

Please do an oil analysis and look for the presence of water, glycol, Na and K in there.

Napa sells kits for $14 or so.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

I have been following your thread in hopes that you are able to track down your mysterious leak with no serious issues. Also hoping that you do not encounter what I did which has gotten me moving to a head gasket/head replacement.

My 1976 240D started and ran great, no smoke, smooth. I too had a coolant loss issue, less than a liter every 2,000 miles, no drips on the ground. I could see some very light weeping near the bottom of the radiator (original) but no pooling and no "leaks"; I put off chasing it down, everything was "ok". After removing the shroud as part of the replacement of the steering pump it became clear that the cooling fins in about 25% of the radiator had been weeping for quite some time, I assumed that to be the source of my coolant loss. I decided to clean out the oil pan and re-seal it, the first 500ml were pure coolant.

That led to removal of the head. My thread is in the Vintage Forum, search "coolant in oil" if you are interested. In post #19 there is a link to some photos taken immediately after the head was removed from the block (the head gasket remained on the block) you will note just how wet the areas of oil and coolant were. Also on the head, post cleaning, you will note the pitting ("cavitation") in the #2 and 3 combustion chambers, cracks near the exhaust valves and a low spot of .003" in the same area. My point is that my 240D was a great running machine but the photos indicate what was going on internally; I would think that it took a good while for the cavitation to occur, those chambers were being exposed to a low amount of coolant for a considerable amount of time.

I really hope that your coolant loss surfaces in an easy to repair area. The great comments from others about testing oil, coolant, etc., may save you some frustration and you may be able to rule out the slow degradation that my trusty 240 experienced over the past few years.

Best of luck,

Steve