Cooling System Done

For a while now, I’ve been chasing a coolant problem on the Mercedes. You can drive the car as much as you want, and never loose any significant amount of coolant (more on that later). However, on occasion — sometimes every night, sometimes not for a week — it will dump anywhere from a half pint to a quart of coolant on the driveway as it sits overnight. Never when it’s hot; only when it cools down.

The shop I talked to said it was likely the radiator. There’s an aluminum radiator with plastic tanks, and as the materials cool down and contract at different rates it can open up a marginal seal and leak coolant. OK, I can see that happening. Radiators aren’t expensive; a good quality, brand new one can be had for under $175. I had plans to replace mine.

I have, however, had the cooling system open. While replacing the coil packs and the air/oil separator, I had to remove the thermostat housing. From what I have read, it’s virtually impossible to get the cooling system completely full and purged of air without using a vacuum fill tool.

How could this be something so simple as air in the system? After refilling the coolant, I ran the car for quite a while with the front and rear heat on full, and spent some time squeezing the upper radiator hose to encourage any trapped air to exit. Well, the other night I went to check the coolant level, and found it overflowing the expansion tank when I removed the cap. Slowly squeezing the radiator hose resulted in some gurgling and the coolant level dropped when I released it, so there was definitely trapped air.

So the question becomes — is this all just because there’s air that needs to be bled out? Or, is it sucking air IN through the leaky O-ring as the engine cools, perpetuating the problem? I guess we’ll find out. I have the tool to fill and bleed the system, and today I took a look. After draining the coolant, I pulled a vacuum on the cooling system. I was able to get about 0.65 bar of vacuum, but when I shut off the air it was obvious there was a leak. I decided to take a look at that leaking O-ring. It was worse than I remembered, with evidence of coolant around the area. I think it had been sucking air in during cooldown and wreaking havoc.

I did see what the shop referred to as corrosion on the driver side turbo frost plug. It was just a little evaporated coolant; the plug is directly below the coolant lone that was leaking. I managed (after a couple hours of cussing and all) to get the O-ring replaced and the line secured. Now I’m able to pull .8 bar of vacuum, with no leakage at all after a couple minutes sitting with no additional evacuation. I refilled with coolant and will watch what happens, but I think that will fix it. I found zero evidence of any other problems — no leaks around the radiator or anywhere else.

Shop estimate to repair cooling system: $3281.23
Parts cost: $7.88
Tools & Materials: $101
Shop time (service bay rental): $120.00
Total spent: $228.83
Total Saved: $3052.35

And that includes the cooling system fill/test kit, which I’ll no doubt use many more times.