Changed T-stat, now I'm losing coolant (sorry long)
 

I changed the thermostat in my '81 240D. I was not losing any coolant prior to the change. My engine temp just never got over 40. I did the install no problems, arrow up, put the new thermostat in the same direction the old one came out. Snugged the 3 bolts in the housing.

I test drove the car, temp came up and I was happy, for a couple of days.

Now, I smell hot antifreeze. I check after I drive and all is dry around the thermostat housing, the water pump and the heater hoses. Where I do see moisture is the overflow line from the top of the radiator. I moved it from the hanging position so that it was directed to flow on the wheel well. I also put on a new radiator cap. After driving I saw nothing, dry. After sitting, wet. When I am done driving the upper and lower radiatior hoses are hard.

Last night, I topped off the radiator, secured the new radiator cap and drove the car. When I parked it at the house, I did not smell hot antifreeze. Top and bottom radiator hoses were hard. This morning, when I left for work, I opened the radiator and it was low on coolant again.

Is my thermostat bad? I have purchased another thermostat, but have not installed it. I'm at a loss, any help is appreciated.

You may have had some air trapped in the system. Very typical. Keep a close eye on fluid level to see if air naturally works its way out.

This is a 240D without a coolant tank (just an overflow hose), correct? Is it possible you are overfilling it? The coolant level should cover the top of the radiator tubes, but there should be some air space below the cap. If you add too much coolant it will expand and go out the overflow hose when the engine heats up. When you found the radiator "low on coolant," how low was it?

Correct, there is no coolant tank on this car. The coolant was right at the top of the internals of the radiator after 1 day of driving. After a couple days of driving it took about 1/4 gal of coolant to fill. Prior to the t-stat change, the coolant was always right at the bottom of the radiator fill.

I keep my 240D filled to a little above the top of the tubes when it's cold (maybe 1-inck below the neck). If I fill it all the way to the neck it will overflow when hot. I also agree you may have air still in the system. Are your temperatures now normal? I would just keep an eye on it for a few days.

My coolant level, is just above the internals of the radiator. If I stick my finger in, just the tip of it gets wet as it touches the the top of the internals.
The temp is a little below what it used to run. Currently it runs about half way below the 80 and the line below it, I assume that is 70c. Prior to the old t-stat going out, it used to run just below 80c.

I'll just watch it for a couple of days. Thanks for the info!