Oh4e500

Hi everyone, this one has been plaguing me for a while, hoping for some pointers. 2004 e500. The leak started a few months ago, would notice little drips/puddles behind both front wheels every morning. Sometimes it'll come through the front wheel well liner (where a screw is), sometimes appears to be draining somewhere behind the front wheels on the bottom of the frame there. It can be either side or sometimes both, and amount of liquid really varies too. Some mornings its just a few drops on the ground, other days it might be maybe an ounce of liquid. The loss was slow enough I figured I'd do some basic troubleshooting on my own before taking it into the shop, based on what I could google out there on the interwebs:

No obvious signs of leak inside the engine bay. Water pump area, around the reservoir, radiator, etc. All bone dry as far as I can tell.
I replaced the coolant reservoir cap, in the off chance that it was a bad cap and not properly relieving pressure.
I rerouted/bypassed the windshield wiper heater element entirely, in case it was leaking into the windshield washer fluid tank.
No wetness around the heater control valve.
No white puffs of smoke out of the exhaust.
No milky substance in the reservoir to indicate any sort of mixing with oil.
No wet spots inside the car front footwells.
No coolant/sweet smell coming out of the air vents.

I'll also note that the car runs perfectly fine, no issues that I could detect. Engine temperature is always around 90 Celsius or so, never had a problem with overheating.

Anyway, I finally gave up and brought it to a local shop where they performed a pressure test. An hour on the lift, and they couldn't find any leaks! They actually suspected a washer fluid leak based on the location of the leak but I knew I was topping off coolant and not washer fluid, but wasn't going to get into it with them. Pressure test showed no problems, so that was that.

I brought the car home and topped off the reservoir and decided to see how long it'd take before the coolant check level warning would come back, and today I have my answer - almost exactly a month.

But now what? I could bring the car back to the shop and ask them to take a closer look but I imagine it's going to start costing a lot more than what they charged for a pressure test. Has anyone run into something like this before? Where else might I check for leaks like this? Help

bbirdwell

FWIW, on my car, it was a failed o-ring on the heater control valve. Nothing in the engine compartment but would drip from the driver's side fender liner after following the channel under the windshield wiper mechanism. Took me a month to trace it down.
Again, for what it's worth...

Sandygear

Get your own pressure tester, get a coolant uv die, wait for the drip, follow the drop using the uv light. Add expanding foam to seal the leak. Leak not fixed, add more foam. Leak still not fixed, take to mechanic and become a start on Just Rolled In.

Let pressure tester sit overnight.

https://youtu.be/yssbHgc293Q?si=3d2G4XvDDfwA8OBT

sand8

You didn't mention if it is water or antifreeze. It may be condensation from the a/c system which drained around there. If the drain plugged with debris then it may take some time to drain and if it is overflowed, the carpet in passenger side becoming wet.

bbirdwell

I had to pull the windshield cowl to find my leak. Found the leak immediately as soon as the engine got up to temp. Be prepared to have a replacement cowl on hand if you go this route; mine was original and in sad shape.