FYI, depending on your radiator and cooling system, your 240 could have come equipped with a variety of cooling system pressure caps. High pressure caps can avoid/delay boil overs while operating in extreme conditions (towing a trailer over a mountain pass on a hot day), but this produces a lot of pressure in the system. In non-mountainous areas, you will probably be well beyound overheating temps before you would see the steam venting from under your hood. (overheating a 240 engine typically results in a warped engine head and should be avoided at all costs)

However, for the rest of us, a low pressure cap puts the least amount of strain on the cooling system. Since many of us are using radiators that are passing 15 years and the upper water pump seal is a potential problem, a 75 kPa cap would provide the least amount of pressure in the system. 75 kPa equates to about 11 psi, which is typical for an American made passenger car (usually 9 to 12). My '87 240 wagon still has its original Blackstone radiator, and was originally fitted with a Green pressure cap. I've since switched it to a Black cap in order to provide the longest possible service life.

Here are the various color and the cap pressure ratings:

grey = 150 kPa
green = 150 kPa
white = 100 kPa
black = 75 kPa

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 230K