T-Stat Failure?
 

The temp. gauge never reaches the mid-point and stays cold. Now its been super cold here in CT (contributing to my CCV failure. See other post) but it seems odd even at highway speeds it never gets there. Matter of fact when I get on the highway the temperature gauge gets colder closer to the 1/4 mark.

Obvious T-Stat failure in the M54?

Sounds like it is stuck open.

Any codes (not check engine lights, but fault codes)? That might give you a good indication.

I didn't have any scans done.

I read in an E39 post that there could be air in the cooling system?

Could be air. Bleeding with the nose higher than the tail helps get the air out of the system.

Many auto parts stores will read the codes for free (they want to sell you stuff). Autozone does. Don't know what stores you have there in the People's Republic of CT, but see if you have a stored code for low coolant temp.

Do the M54's have the electronic thermostats like the N62's? Mine threw a code and a CEL on my trip back to Vegas from DC. Fortunately, it wasn't stuck closed (or open), but rather partially stuck open; enough so we could easily drive home in December with no problems.

T-stat is open.

The t-stat should be closed until the engine warms up, then it cycles open and closed to maintain the engine temp within a narrow range. Your t-stat is stuck open and the engine never gets warmed up.

It appears in at least one thread in the on-line community that I can get away with NOT draining all the coolant to accomplish this as a DIY. Just take out the cover underneath the engine and capture what will come out of the hose/t-stat with a tray/bucket. Any comments on how much coolant is lost in this DIY?

How hot should it run when fully warmed up?

If the coolant is clean, you can capture what comes out and put it back in. If the coolant is not clean, then it does not matter what comes out, you have to replace it all. In the 3 Series cars, this engine takes 11 or 12 quarts -- 3 gallons.

You want a 50/50 mix of coolant and water, but it's not rocket science so you can be off quite a bit without creating any problems.

While the t-stat is out, you can run your garden hose through the engine until it runs clear, then if you refill with 5 or 6 quarts then top off with water, you will be at the magical 50%.

You can buy 50/50 premix, but strangely it costs more than a gallon of straight coolant.

Another strategy is to drain what you can, then add a gallon of straight coolant so that you have an empty bottle, then use another full bottle to make your own 50/50 premix, then top the system with that. (This method prevents you from flushing the block with your garden hose.)

To drain the system, you can open the petcock on the bottom of the radiator. I like to disconnect the lower hose at the radiator and get the coolant out of the top half of the engine -- when you open the t-stat, this will drain anyway, so you may as well control the mess.

It should be something in the range of 90 degrees C, which is somewhere around 200F. The needle should point straight up. A needle width or two to either side of straight up is okay, if the needle is at the 1/4 mark the engine is cold, and if at the 3/4 mark, it is hot.

Why?