Im Having overheating issues on hills and bought a new t-stat to try and solve the problem. I can't tell which radiator line goes to the t-stat housing, is it the upper one or the one lower on the passenger side of the block. Thanks

The lower.

X2

It's NOT on top of the engine. There is something there that looks like it, that is where I broke a stud off the block.

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08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D

It's just above the alternator. BTW, the alternator in a 126 with a 603A comes out in about four minutes, making access to the thermostat easier.

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James Marriott
2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
www.engineeringworks.biz
1987 300SDL junker 170k
1982 300SD junker, 265k

In a 124 removing the 50*C temp switch just ahead of the thermostat makes it easier through by no means easy to replace the thermostat. Removing the temp switch is a puzzle in itself unless you have a wide assortment of tools.

Sixto
87 300D