Heator blowing cold air at highway speeds
 

1984 Mercedes 300td

This has started happening about 2 months ago never really paid attention to it until I was on the highway. The car would be 80 degrees Celsius blowing hot air on the road until I would merge in a highway going 60 miles an hour the heat pump blow warm air. I also noticed a puddle of coolant on the floor the next day coming from the passenger side. Can someone give me some insight what could be the issue. I checked the coolant resivor and it was low so I topped it off with more

Coolant leak allows heater core to be empty, therefore blows cold air. Find and fix the coolant leak and your heater will work better.

Definitely fix the coolant leak! Next read about the monovalve cartridge.

Sixto
98 E320s sedan and wagon
02 C320 wagon

9/10 it will be the monovalve. Diaphragm in mine just tore the other day on a very chilly morning. 5min swap with a spare and I'm toasty again.

I will raise those stakes to 9.95/10. Cold air at highway speeds is the classic symptom of a torn monovalve diaphragm OR inferior aftermarket monovalve cartridge.

Heater Leaks
 

Oh, boy ~

You neglected to tell us where you're located but in any case, puddles on the carpets almost always mean heater core leaks, this can be serious as it's a major job to change .

OTOH, if or once, your cooling system is squeaky clean you can often use "Silver Seal" to plug the leak and not the radiator .

My 1980 300CD had thick red sludge in the entire cooling system when I bought it, I had to replace everything (!) including the exterior coolant transit pipes, thermostat housings, hoses, valves, FREEZE PLUGS :eek: on and on, one it was all done the heat was fine but after six months I began smelling coolant then noticed some coolant in the passenger side foot well .

The long and short was : the heater core was too rotten to hold the coolant under pressure .

Instead of taking the ENTIRE DASHBOARD APART I added one tube of Silver Seal and a 1949 Chevrolet radiator cap ~ that cap isn't pressurized yet the car never leaked again and didn't overheat when running it flat out in Death Valley with the AC on in 120* F heat .

No matter what else you find wrong, you're going to need to address that coolant leak first and foremost .

Don't panic and don't give up just yet .