A Sneaky Coolant Leak Could Be the Slow Death of My Mercedes E-Class

After flushing the cooling system, I’m still concerned a corroded freeze plug will ultimately kill this car.

byLewin Day|
A Sneaky Coolant Leak Could Be the Slow Death of My Mercedes E-Class
Lewin Day

Since I bought a Mercedes that's old enough to fight, drink, and swear, I've mostly enjoyed comfortable, reliable motoring. However, the last week saw things turn for the worse, with a nasty rusty puddle leading to plenty of head scratching. I think I've narrowed down the cause, and if I'm right, things are potentially a lot more serious than I'd hoped.

Lewin Day

The Drive's Australian office is neatly located in my living room, so I don't commute much. However, some errands had me doing laps across town recently, and suddenly my Mercedes threw a warning on the dash—"Coolant Level Low." I'm a busy journalist, so I threw a liter of water in the expansion tank, saw the error cleared, and went on with my week.

I limped the car around for a few days while keeping a close eye on the temperature gauge, hoping it was no big deal. The warning reoccurred, sadly, but I pressed on during a busy day of many appointments. Only when I pulled into a store to pick up some more coolant was the extent of the problem revealed.

"Is that one yours, mate?" asked a random man as I left the store, gesturing to the brown liquid that had gushed out underneath. Miss Mercedes had brought things to a head.

No, it's not supposed to look like that. Lewin Day

I topped the car up with water and noted that while it was running, there was a healthy leak splashing out fluid on the driver's side. The crowded engine bay meant I couldn't identify the precise location. I don't have Bezos bucks, so a tow was out of the question. Instead, I threw some bottles of water in the back of the car and limped it home for further investigation. About halfway there, the coolant warning sounded again, indicating I had a definite ongoing leak.

I was quietly confident I'd sort the issue quickly. Enough water had been coming out that the leak should be clearly visible with a little detective work. Things are rarely so simple, however.

Despite its fancy Euro breeding, the W210 engine bay isn't actually that bad to work on. The airbox pops out without tools in mere seconds. Even better, the expansion tank was easy to remove, too. That became necessary when I topped it up with fresh coolant to try and make the car leak. The bottle had sat in the sun too long in my garage and literally fell apart, dropping bits of plastic into the cooling system.

With everything fished out, I had the cooling system full and got the engine running. I was greeted with a smoothly running engine with no obvious leaks at all. I'd suspected a hose worn through or a broken fitting, but neither were apparent. Even as I got the car hot and circulated coolant through the heater core, the driveway remained dry. Well, except for all the coolant I spilled while working.

Try as I might, neither a healthy throttle application nor a drive on the highway helped shed light on the problem. The coolant stayed in the engine, and no errors popped up on the dash.

The only clue I had was a tiny little wet patch on one of the engine's freeze plugs. I couldn't get a clear look, as it was buried down behind the exhaust manifold. However, it was looking a little corroded, and also like it had a tiny wet patch. It was also in the rough area I'd seen the leak before.

My suspicions are that this freeze plug may have corroded, thus making the coolant that ugly rusty brown color. This could cause a pinhole or hairline leak which only passes significant fluid when the coolant system is overly hot and thus highly pressurized. When I drove around with the coolant warning on for several hours, this could have caused the leak.

It's pretty rare for such a leak to magically go away. Either the freak problem has gone, or it'll continue to get worse to the point that it's obvious what's going on. I'm okay with that. I'll keep the coolant level topped up and hope that doing so will prevent any major leakage events for the time being.

If my hunch is right, the problem does turn out to be a leaky, corroded freeze plug, that won't fix itself. And seeing as my E240 is not quite nice or special enough to justify pulling the engine out to deal with the problem, it may spell the end of my time with the car. In any case, I'm waiting and seeing.

Got a tip? Let the author know: lewin@thedrive.com