For quite some time now the water level in the cooling system of my 300E drops gradually. I've been topping it with about 0.6 litres of water every 2-3 days. I've taken the car to a mechanic who, after thorough investigation, could not discover any leakage point - I've even poured some colorful coolant to see if that could help detect the leak point (if any) - but no success.
* What could be the possible causes of this problem and what solution shall I apply?
* Could water be evaporating from the cap - the temperature guage never gets above 90 degrees

Thanks in advance for your suggestions.

Oge.

300e 1986

Do you see oil in the coolant tank? Or white smoke coming from the exhaust? These would be symptons of a bad head gasket seal.

88 230E (W124)

Check your waterpump - mine had to be changed

There may be a leak in the heater box which might show on the carpets or it might drain via the AC drains. Look under the car after it's been running for a while and is fully warmed up. The HVAC system in the 300E is very complicated and has a lot of potential leak points. Be persistent, you will find it. BTW if your head gasket was either going or gone, it would most likely cause an over pressure situation in the cooling system that would be pretty obvoius to you if it existed.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

Pop your hood and listen (in a really quiet area) after driving... any hissing? Look at the hood pad...Any spray? Touch under the water pump...any wet greasy feel? If its your heater box you should smell it with the heater on. Look at the auxiliary water pump. Bend all the hoses...should find it..Pressure check the system, thats an easy one.

On occasion, we have to pressure test some cars over night to allow for expansion and contraction of the hoses and seals. If this persists, just have your mechanic pressure test all night.

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Benzmac:
Donnie Drummonds
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Oge,
This is just a "shot in the dark" because as previously stated, the cooling system is complicated. I had a similar problem awhile back and added some dye to the radiator that glows when subjected to UV light. No leaks could be seen. The dealer said I needed a new radiator and new water pump. I didn't jump at the chance to spend hundreds of bucks when I saw no leaks in either....
It turned out to be a 80 cent O-ring in what I believe is a heater hose. Pull the air cleaner and look on the left side of the engine very near the firewall. The hose comes thru the firewall and does a 90 degree turn into the engine block. There is a bracket held by two bolts and has an O-ring seal. It was leaking a very small amount but has not leaked in over a year and the radiator and water pump are still going strong.

Imran, robi99, Benzmac, Surfnvet, Jeffsr,

Thanks a lot for all your suggestions, hope the mechanic will find out the cause this time around.

Yal,
I can't notice any oil in the reservouir, however every morning when I start the engine, I usually notice some white smoke and some water vapour coming out from the exhuast. This stops ( or rather it's unnoticable) as soon as the engine is fully warmed.


Regards,
Oge.
300e '86

Oge,
That sounds fine. The smoke is just condensation in the pipes. When my waterpump went the leaking antifreeze was burnt off before reaching the ground. No leak was visible when cool. It wasn't until it got really bad that I found the leak.
Do you smell the sweet coolant smell when you turn on your vents? As you can tell from all the above responses the nature of the cooling system makes for a tough diagnosis. Even the benz dealer was fooled with my waterpump!? Have it pressure tested. Failing that, check the whole system hoses and all particularly the heater core. There are some good suggestions above hope this all helps.

yal
88 230E (W124)

Since getting my expansion valve replaced, I've noticed a faint smell of antifreeze when I switch the system from AC to Economy prior to shutting off the engine. Is there anything in particular that I should look for? Thanks,

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Robert W. Roe
1984 300SD 170K mi

Could it be your freeze plugs?

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'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)

Re. the dropping coolant leak, I had the same thing in my 280E for a couple of months. A mechanic traced it to a leaking water pump, which fixed 50% of the drop. But the leakage continued, at a slower pace, until the head gasket blew.

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1977 W123 280E
286,000km