What should the coolant system pressure be. Or what presure cap should i use on expansion tank? Peachparts offers 1.2 bar for radiator but no spec on the expansion tank cap.

Yes, but reduce the pressure requirement by 3.247% for every 100 meters below sea level.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Are you thinking he lives near the Dead Sea ?

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Nah, merely pointing out the absurdity of trying to guess what model of chassis this is supposed to fit.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Mike, I was referring to the ' below sea level ' in your post....

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

I know you were. The point I was attempting to make is that a nonsensical request deserves a nonsensical reply.

Okay, let's try this again.

What model and year?

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

I don't think this is what you're trying to ask, but it's what you're asking.. the cooling system pressure should be little to none (i.e. atmospheric) until the engine is at or above 100C. So the pressure rating of the cap is really only relevant above that temperature. Which specific caps you need depends on which car you have.

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

The radiator is considered a part of the chassis system? Ok. W123 .

the 120 cap will be fine.

there are different caps for different applications, but if 120 causes a problem, you likely have other issues to deal with.

something is wrong if you have a cap on the radiator and the expansion tank. they might be misnaming them, only one should be offered for one particular year and model.

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Absolutely wrong. Coolant will expand 5% between ambient and 100C. Air in the pressure tank will be compressed accordingly, the pressure cap has to resist this pressure. The attached graph may surprise you. The manufacturer selects a cap based on the volume of coolant, size of the air pocket, expected heat rejection, coolant chemistry, etc. It can't be too high, because you don't want to exceed the limits of they system's weak points. It can't be too low, because you don't want to blow off coolant unnecessarily. If you have the manufacturer's specification (which you can obtain from your dealer or from EPC), that's what you want to go with.

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