'90 300SE - Where is my coolant going??

[tinypanzer]

A few months back when the weather warmed up, I took the opportunity to get rid of the green coolant in my 300SE that the PO left me with. I went out and got two gallons of MB antifreeze, and flushed out the system completely. I then refilled it with distilled water and MB antifreeze mix.

Everything seemed fine for a while. A few days later, I checked the coolant level and found it low. No big deal, I thought. There must have been an air pocket in there........

My dad then borrowed the car for a couple weeks while I worked on the 560, and when he brought it back it had just started overheating, which he hadn't noticed until I pointed it out

So, I wound up putting about a gallon of coolant into it. I then discovered that the hose fitting at the top of the thermostat housing was loose and weeping a bit. So, I cleaned up the connection and re-tightened the hose and thought I had fixed the problem.

This weekend I discovered it low again, by about a gallon!

I can't find any place where it appears to be leaking.... All hoses seem tight, the radiator is less than 3 years old and is genuine MB.

The thing NEVER leaked a drop when it was running the green coolant. Now all of a sudden, it is mysteriously losing about 1/2 gallon per tank of gas.

Where could it possibly be going? No sign of coolant in oil, no signs of leaks!


Ideas?

[latief]

Seal between pump and the block, or the pump weep hole

[macdrone]

Thermostat or maybe the coolant cap isnt holding pressure? The little nipple off the coolant reservoir sometimes gets cracked and doesnt hold pressure. Water pump weeping? Cooling fans not coming on? Viscous fan going bad?

[mbzman]

Have you done a pressure test of the system yet? If there are no signs of leaks in the engine compartment or on the ground then it is very possible that you have a head gasket that is leaking and allowing coolant to be burned off. Has the head gasket ever been replaced before?

[mbzman]

Another things I've notice about the factory MB coolant is that is leave a crusty white residue when it dries up. Have you recently noticed any white deposits on any coolant parts or hoses?

[cliffmac]

if you are losing that much coolant and there is no external leak the only other answer is your cylinder head and/or gasket is shot...probably the head gasket needs replacing, it's one of those unfortunate $2500 issues that surface now and again... but for the price you all kinds of other things as well, you can't do a valve job without fixing a bunch of other little things along the way....

[JohnM.]

Sometimes you can be leaking while driving at speed, but when you pull over and look for a leak there is nothing. The Benz coolant does leave the white crusty residue wherever it leaks and dries. If you were burning that much coolant there would have to be white smoke out the exhaust.

A gallon is quite a lot to be low in such a short period. I have a very slow coolant leak that is driving me nuts. Maybe 1/4 gallon every 6-8 months of heavy driving.

The rad cap or o-ring between block and water pump are my guesses. Did you also check the rubber hoses behind the battery area? I had a nasty leak back there once.

mbzman offers good advice here. Most any radiator shop will do a pressure test for little or nothing. That is definitely where you should start.

Not only could it be a head gasket, but if it hasn't had regular coolant changes it could very easily be water jacket corrosion. If that is the case, when the head is off they can weld it up and fix it.

[tinypanzer]

Yikes. I guess I will start with a pressure test. I sure would hate to pull the head off, but it may be time.

I can't tell if the gasket's been done before or not. Sloppy record keeping on the part of the PO. There is a busted off lug on the exhaust manifold, which means that the head was probably pulled at some point. Only 135K on this thing, I would suck if it were already due for a second head gasket.

I can see no sign of any leakage near or from the water pump. I keep my motor pretty clean, so I'd noticed that dried-up spittle that the coolant leaves behind. There was evidence of it before I tightened that one hose, but not any more.

Even with cruddy valve stem seals, the motor just runs so nice. What a pity if I have to pull the head for want of a little water.


I will report back my findings.....


Thanks for all the suggestions!

[tinypanzer]

So I went and bought a pressure tester this weekend and it seems that my cooling system is up to snuff. I only tested it at 15 pounds (this is a 20 pound system, right?) but I left it pressurized for a good hour and I didn't see any sign of pressure loss or leakage.

I tested the cap, and it wouldn't hold any pressure at all. I'll feel pretty silly if that was the only problem all along. However, sometimes used caps don't seal well in anything other than the fitting in which they were first installed, so I'm not 100% sure I've found the problem, but it's a cheap enough place to start.

This is a 20 pound system, right?

One other thing - Can I use a generic AutoZone cap, or do I need a German one?

[lsmalley]

If you are gonna buy something that may work and that you plan on keeping, I would suggest MB/German cap. If it was something that you wanted as a temporary fix, then go for the Autozone/Generic brand.

[tinypanzer]

Yeah, I bit the bullet and shelled out $15 for a cap at the dealership. It's so hot, I just didn't want to wait another day.

I guess time will tell if that was the problem all along.

[S-Class Guru]

Well, if you have good system pressure with the motor not running , then it might certainly point to the head gasket. Motor runs, expands the head, and the gasket leaks. (Of course, water pump seals can act that way also). Seems heads usually leak to the exterior of the block, on the right rear, rather than into the engine (thank goodness).
If you are getting that qty of coolant into one of the cylinders, then you might be able to verify it by looking at the spark plugs. If one is very clean compared to the others, then that might be the one getting steam-cleaned.
No joking, water ingestion will really clean a plug/cylinder.
I've had the head gasket leak, and pretty good water pump leaks, and neither dripped a drop on the garage floor. But the tell-tell white residue was there when I searched around under there.

One more remote possibility: if the rad cap is bad, and coolant passes out through the overflow tube, it will route to a quart-size bottle in the right fender. Then, when the bottle fills, it drips out on the ground near the wheel. That might happen and you'd never see it. You must remove the inner fender liner to see the bottle.

DG

[tinypanzer]

Dumb question - This one's really gonna make me look dumb - Is that quart sized bottle necessary? I thought this system was a closed system and didn't have a slosh bottle. My hose to that bottle has been broken (rotted) for a while, and I just assumed that it was a drain hose for overflow. So I just ignored it, figuring it was just for an 'orderly' egress of fluid from the reservoir to the ground. This system isn't supposed to recover from that bottle is it? If so........


Confused.....


BTW - The coolant pressure tester available at AutoZone for $75 was very well made, and it fit on my W126 without any adapters. My new cap tested good to 20 pounds, unlike the old one which wouldn't hold any pressure at all.

[S-Class Guru]

Yeah, I was kinda surprised by the catch bottle in the fenderwell. I concluded it's just typical W126 techno overkill (gotta love it).
Only time any liquid could pass to it would be if the underhood coolant tank was overfilled and then the engine overheated and passed some fluid out the pressure cap. It couldn't be drawn back into the coolant tank because the hose to the catch bottle does not go all the way to the bottom of the bottle, to suck back up the fluid. When I found mine a few years back, it was dry and appeared to have never had anything in it.

So, it's probably designed for the rare instance when Hans wrongly filed the tank all the way up, then ran the car up the Alps and overheated it. Then the excess coolant would burp past the pressure cap and into the overflow line. Now, we don't want evil antifreeze dripping from der Mercedes, and possibly getting on the rear tire and causing a skid, so we'll just catch it in this neat little bottle, and let it slowly evaporate over the next few years.
Ya, das wunderbar.

DG