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Old 05-12-2018, 07:11 AM   #1
apollo's 940
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Default green vs black coolant cap

1992 940t

for the coolant expansion tank --- what is the difference between the black and the green cap for the tank ?

I think my stock was green but ive swapped it many times with some old ones that I have forgotten which was stock.

now i just want to buy a brand new cap along with the new tank to get it done and over with.

any advantage of one over the other?

thanks in advance

andrew
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Old 05-12-2018, 07:53 AM   #2
InTheWhite
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The safetyvalve opens with different pressure between the two. You should have the green one at your car.
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Old 05-12-2018, 10:39 AM   #3
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Black cap will keep the cooling system at 11psi, the green one at 22psi. Higher pressure means higher boiling point. volvo started putting the green cap on the later turbo cars. Unfortunately, the higher pressure shortens the longevity of many cooling system components. Many people have had success using the black cap on a turbo car, just depends on where you live and the rest of your cooling system. I prefer the black cap, if your car has the right fan and thermostat.
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Old 05-12-2018, 04:38 PM   #4
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I too like to use the black cap instead. If your are using the green cap. The main thing is to be sure your plastic heater valve is good and other plastic is in good shape like the radiator end tanks, and expansion tank.
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Old 05-12-2018, 08:45 PM   #5
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What about the grey caps? Mine had a green one I took from an 850 long ago, never had any problems with it but now I'm using a grey one
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Old 05-13-2018, 12:28 AM   #6
dl242gt
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Green and gray cap are the same. If i recall correctly the gray cap was used on the PRV engne.
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Old 05-13-2018, 01:35 AM   #7
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Also, on cars that use the 240 style radiators, if you're using a 3-row rad, use the black cap. The green/gray caps apparently allow the pressure to be too high for them, at least, according to iPd back in the day.
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Old 05-13-2018, 04:54 PM   #8
apollo's 940
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thank you for all your input

very very helpful

I now know that my stock cap is green (1992 b230ft 940)

BUT I WILL NOW PURCHASE AND START USING THE BLACK FOR LOWER PRESSURE

because I want to be easy on my system

come to think of it, while using the green cap all these time :

1. my water pump is now leaking
2. I have coolant smell inside the car --- maybe heater core, maybe heater valve. so I bypassed the heat hose into the firewall back to the engine.

andrew
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Old 05-17-2018, 09:22 AM   #9
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Originally Posted by dl242gt View Post
I too like to use the black cap instead. If your are using the green cap. The main thing is to be sure your plastic heater valve is good and other plastic is in good shape like the radiator end tanks, and expansion tank.
Same here. I've always used a black cap and never had any issues. I also run a step cooler thermostat and a bottle of water wetter.
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Old 05-17-2018, 11:04 AM   #10
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I too installed a black cap on the 740. The gray cap was causing our heater control valve to leak pretty bad at idle. New valve and 10psi cap and no leaks yet.
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Old 05-17-2018, 12:08 PM   #11
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i have heard bad things about the grey caps failing alot.

Saw it on my lemons car. Always go green over grey.
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Old 05-18-2018, 10:21 AM   #12
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Black cap in the Florida summer over heat all day and night. Sure more stress on cooling system but I rather replace water pump and hoses instead of head gaskets
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Old 05-18-2018, 12:17 PM   #13
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Black cap in the Florida summer over heat all day and night. Sure more stress on cooling system but I rather replace water pump and hoses instead of head gaskets
The boiling point of a 50/50 mixture is well over 212F. If your car is overheating with a 10psi cap you have other issues.
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