Coolant was leaking down over the oil pan, mostly on the front and passenger side. The pool on the ground was definitely more on the passenger side of the engine. Never noticed it when the engine was running, only after it had been sitting overnight. It got progressively worse over a week or two. (at first I thought it was just AC condensate.)

After searching around here, it seemed like a water pump failure so I ordered one and got the radiator and fan out Now that I can see everything and I am better oriented to the front of the motor (my first coolant adventure with a w123), I actually think that the leak was coming from the thermostat housing.

So tomorrow I am going to go searching for a thermostat gasket.

Here are my questions:
1.) will NAPA have the gasket in stock? I can also call the dealer, but thery aren't stocking any w123 parts anymore - it seems. No one else seems to have them in stock either - all special order from AZ, AA, or Oreilys.

2.) should I go ahead and replace the water pump? I am so close now, but not sure that there is any weeping at all from it.

3.) No gasket sealer is needed for the thermostat, right?

Thanks gurus!

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

If you are going to replace the thermostat housing gasket (not to be confused with the thermostat gasket), you might consider replacing the bypass hose at the same time.

That's the little 2" piece, right?

Do you think it is the housing gasket rather than the thermostat gasket?

Edit: I see now - it is definitely the housing gasket - there is no gasket there - just some black goop.

So should I replace the water pump, too?

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

That's correct.

There are several possible culprits in that area. I would suggest that you take the time to positively identify the source of the leak before you shop for parts.

If your current pump is original, replacing it would certainly would not be a bad idea.

Thanks tangofox - I am 90% sure it is the thermostat housing gasket.

I have to special order that gasket unless I want to cut my own from gasket paper.
I will also replace the thermostat and seal while I have it all apart.

Do I use gasket sealant on the thermostat housing? There are no instructions in my FSM!

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

In my opinion, just about any paper gasket will benefit from a thin layer of sealant. Permatex makes a RTV sealant (part #22071) specifically for use on cooling system applications. Permatex #2 also is a good choice.