Engine Coolant System - Radiator Caps

DanPerryy

Well-Known Member
The stock cobalt has a closed coolant system with the pressure relief cap in the coolant reservoir tank. There is no radiator cap on the radiator.

The cap on the coolant reservoir tank has a pressure relief valve to vent for excess pressure AND a vacuum vent to draw air in for low pressure. The Goblin's radiator has a radiator cap. This cap too will relieve excess pressure. However which of the two caps (radiator, reservoir) relieves pressure first is a guess. That is not a problem BUT the "extra" vacuum vent on the radiator cap is a problem. Which of the vacuum valve actuates first is a problem. If the radiator cap activates first you have just put an air "bubble" in your sealed coolant system and the "bubble" has to be "sucked" through the system to the engine water pump and then the bubble can be discharged back into the reservoir through the small hose from the top of the head ( the little pipe running parallel with the fuel rail).

The simple solution is to close the coolant system like the stock Cobalt. This is easily done by capping off the little overflow vent on the radiator so it can't expel coolant or draw air into the radiator. As a closed system, it does not matter the direction for flow through the radiator (top to bottom or bottom to top). The Cobalt flows from side to side of the radiator.
 
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JSATX

Goblin Guru
Can we just replace the cap on the radiator with one that only has a pressure vent function?
 

DanPerryy

Well-Known Member
I unsuccessfully tried to find one. I also unsuccessfully looked to find just a cap, no pressure or vacuum relief. I found a Nissan that used a cap like that but was not sure the size was correct.
 

BAR-AIR

Well-Known Member
Hey Dan,
Just a quick question with regards to the radiator cap. I'm running a catch can style reservoir without a relief cap on it. That with the standard Goblin kit radiator with the relief cap I should be good correct.
THX, BAR
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DanPerryy

Well-Known Member
The hose from the top of the engine that runs to your catch can is to let any air trapped in the system exhaust to the reservoir bottle. If you look at the reservoir bottle when the engine is running there is a constant stream of hot water going into the bottle. The place where the line hooks has been designed to be the "top" of the cooling system where any trapped air will go. That air as well as the coolant is pumped out of the system into the reservoir. Of course the air goes to the top of the bottle and the liquid settles into the bottom. The fluid in the bottom of the bottle recirculates back into the cooling system without any air as long as the reservoir is not empty.

Effectively all the air in the system is trapped (and held) in the bottle and the coolant in the bottle is recirculated. If the engine overheated and developed an excessive pressure the cap on the reservoir would release the pressure (and vapor from the top of the reservoir). Upon cool down the coolant system would create a vacuum and would need to allow air to come into the system to equalize the pressure. This is also done through the cap on the bottle, a separate valve in the cap. This valve in the cap can be more easily seen in a regular cap. The big rubber gasket in the cap is pressed to the radiator neck by a large spring. the large spring is set to release at about 13 PSI (I think). In the middle of the large gasket is a little disk the is held by a small spring. This is also a valve that releases with vacuum. Inside the reservoir cap is the same arrangement.

If we just use the radiator cap to release pressure and to release vacuum the release of pressure works just fine but the release of vacuum will suck air back into the radiator. This air however is "trapped" in the top of the radiator and the bubble (possibly very large bubble - like a radiator half full) would need to be sucked down to reach the coolant tubes in the very bottom of the frame. I don't think that the water pump will suck this bubble out of the radiator. I think we need to keep the radiator a "sealed" system just like a stock engine setup using the radiator cap only to purge any air trapped in it.
 
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