Leron,

The W140 is the greatest Mercedes, but with all great cars this car has quadruple the parts count of your average car.

One part in particular can ruin the engine, and this part has ruined many engines. The famous "water tee fitting". Replace this plastic fitting, and buy the tee from a Mercedes dealer only-no ebay parts, aftermarket parts, and no old plastic Mercedes. You want a fresh one. They are good for 70 to 80k miles.

I drove for 1k miles with a broken one after my purchase of my W140

The tee is located on the drivers side of the firewall by the back of the valve cover.

Here is a wonderful story of why you should replace the tee, and pictures of what it looks like.

the evil that lurks under the hood... is really the devil himself - Benzworld.org - Mercedes-Benz Discussion Forum

Best of luck,

Martin