Tom's 300SDL has had a slow coolant loss since he bought it last year , initially it also ran a bit too hot , I did a heavy Citric Acid flush on it as the coolant tank showed rust silt inside it , this seemed to halt the collection of rust silt and afterwards the cooling only got a bit warm when running the AC and going up hills on the Freeway .
Recently we took a cross country trip from California to Maine , often we'd have to shut off the AC when ascending hills as he'd get nervous when it reached 100* C , the coolant loss increased at the same time although we couldn't find any leaks and the system pressure checks O.K. .
Some times there was a slight streak of something on the right leading edge of the hood , Tom thought it was coolant , as it never made even a droplet , just a dark stain of something that collected dust , I couldn't tell .
After we arrived in Maine , needing a pint of pre mix coolant every 800 miles or so , I was looking under the hood as he bought new coolant in a NAPA Partshaus , I thought maybe the 7 MM coolant return hose from the radiator's top tank to the plastic surge tank *might* have a similar looking stain right where the hose connects to the radiator's nipple ~ the hose was also looking tired , clearly the original 1987 one as it felt too soft to the touch and was swelled up near the clamp so I decided to replace it .
I went inside the NAPA Store and asked for some 7 MM coolant hose , they had it in stock , cut me off a carefully measured piece and then is when the real fun began .
Upon removing the hose from the surge tank I saw a rusty chrome pipe sticking out of the tank's nipple , when I removed the hose from the radiator's nipple I thought I saw a bit of rust flake so I used my toothpick to try and scrape it out ~ instead I gently teased and coaxed out a 10" long string of red RTV sealant ! (picture attached) apparently some prior repair had been poorly done elsewhere in the cooling system and when it was filled with coolant and run , the excess RTV collected in the top outlet nipple of the radiator , blocking the coolant flow to the surge tank .
This is fairly common on thermostat and water pump repairs : folks use a bead of sealant instead of carefully using the fingertip to smear a super thin coating of RTV , when the parts are re assembled the excess RTV mooshes out , not only outside but inside as well where it travels and causes havoc wherever it winds up ~ often in the oiling system , causing catastrophic engine failure .
This is why I like to take things apart : I know from 50 + years of working on machines that often there are hidden issues ~ Tom hates it when I take his cars / engines apart but when dealing with older machinery there's always some little thing that's not apparent ~ a loose screw or gone missing washer , some old gasket materiel in the sump that'll get sucked against the oil intake screen and reduce oil flow , a little thing that's just waiting to do you dirt .
I'd wanted to remove the entire radiator as experience has taught me that whenever there's a cooling issue , there's almost always crud hidden in it somewhere ~ in Mercedes , it's usually in the lower radiator tank where no amount of in car flushing will get it loose , but easy to flush out with a garden hose and your fingers when the radiator is off the car .
I also often find strings of RTV trailing out the thermostat and into the top tank on a regular basis , even on Dealer Serviced vehicles .
Just _looking_ in the upper water jacket is often educational as the coolant may be clean looking but if there's a red coating of silt inside the hose nipples , your cooling system is not able to properly transfer heat away and this same rust will create ever MORE rust and damage , water pump wear etc. as you drive along .
Take the time to sweat the small things and your 40 year old Mercedes will remain trouble free and inexpensive to operate .
-Nate
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