ECS probably has a replacement for you, or morristown.

Here's basically what you do:

Pull the splash shield off the front nose of the car. There are a few little bolts that hold it in place. Put it aside and place your coolant catch bucket under the place where the tstat is for later.

Take off the air intake snorkel. It's held in with four plastic rivets. Then take off the clutch fan. It is left hand threaded, and you need about a 1 3/4" wrench - I forget the actual metric size - and a screwdriver to get that off. You wedge the screwdriver between two of the mounting bolts for the pulley, and then give the wrench a whack. It should pop free and spin off. Let it sit in the fan shroud - no need to take it away from the car.

Now you can see the tstat and the hoses that go into it. There are two of them. They are held in place with metal retainer rings that you must pull out. Pull them out and wiggle the hoses off. They are on tightly, so you will have to pull. Try not to damage the rubber part of the hose, or it's off to the dealer with you.

When you have disconnected those hoses, coolant pours out...into the bucket you put under the car earlier. Once it is done, unbolt the tstat housing from it's spot in the motor. There are a handful of metric bolts that hold that in...no big deal. Once it is loose, pull it out away from the block. Wipe down the hole. If the gasket (built into the tstat) stuck on the hole, clean it off or your new gasket will not seat. Lube the new gasket with a little new coolant. Slide it in, and put everything back the way it was. Don't forget to put some anti seize on the clutch fan nut, or you will never get it off again.

You will need to add coolant once you are finished. Don't button the splash guard or the snorkel back on until you inspect for leaks.

At 53k you should be ok with everything else unless something is showing symptoms.

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2001 E53 3.0 5pd