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green vs black coolant cap

A

apollo's 940

Member
Joined
Jun 12, 2006
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
 
The safetyvalve opens with different pressure between the two. You should have the green one at your car.
 
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.
 
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.
 
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.
 
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
 
dl242gt said:
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.
Click to expand...

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.
 
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.
 
i have heard bad things about the grey caps failing alot.

Saw it on my lemons car. Always go green over grey.
 
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
 
eeaves42 said:
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
Click to expand...

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.
 
I just finished up a head gasket job on my 93 940T and it's got me scared to drive it. Head was planed, all new hoses, new radiator, and bypassed the heater. Didn't have coolant so I filled it up with water to test it, drove with a black cap a mile and it boiled the water out of the cap when I parked it. I let it cool down, pulled the lower hose to empty the system, then filled it with 50/50. Drove it a mile twice with the black cap, once no a/c, once with a/c. Fluid level is ok, upper and lower hose temps seem ok (160F-190F), but the coolant is just starting to boil.

Any advice on where I should go from here? It's a pretty hot day in Florida but it's only getting hotter from here. I was going to use water wetter but they no longer sell it at the parts stores and I figured I'd try 50/50. Is the green cap the way to go from here?

Edit:

Drove with the green cap, level is fine, hose temps 160F-170F, no boiling. Gave it WOT and now have idle surge, but I think something else is going on, mode switch for a/c isn't working but was before head job. It has a new 87c thermostat, but I'm going to order an 82c
 
Last edited:
Sounds like that black cap is bad. Make sure you installed a thermostat with a jiggle valve otherwise it’s hard to bleed the system of air
 
The black cap is good, or at least it works on my n/a 240. Thermostat has the jiggle valve and is installed properly
 
Remember folks, the cap pressure rating is for the SUCTION side of the water pump. Engine block & head will have the higher pressure provided by the pump. There is pressure drop across the tubes of the radiator due to the length/size of the tubes.

The "work" done on the fluid by the pump increases the pressure...... then the radiator resistance drops the pressure. You do require a minimum pressure to stay above the flash point of the coolant/water at the suction side of the pump (where your tank & cap are installed).
 
So where can we get the black caps? I ordered a few for some 7/9 cars over the years, but have run out. Ran it on my 90 745T and a few other NAs with no issues. Ran it on a 95 945T and it would keep blowing coolant...but that was a headgaskst issue.

Alan Chung
91 945T
 
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