Tightness on Cap on Side of Coolant Reservoir?

Ive never seen such a thing. WTF? Thats like asking for a major leak.... almost as if darwin designed that with Murphy's encouragement.

 

THat thing needs to be tight. Otherwise it'll vibrate loose. And you'll have a coolant leak.

 

FL explained it to me but that was years ago. So I don't even remember what the reasoning is.

 

So that coolant reservoir is actually a two part deal. Surge tank and reservoir. So the top area is the reservoir. There is a small capillary tube that runs along the outside seam (rib) and connects it to the surge tank. Overflow from expansion goes into the surge tank. The top cap is for filling only. It needs to be tight. The side cap is your pressure cap. It is sealed with o-rings. Technically it does not need to be tight. The o-rings seal it. Realistically it should be tight so that it doesn’t vibrate off.

If there is an air gap in the top of the tank with fluid in the bottom area you have a coolant leak. Most of the time it is the top fill cap not sealed but have seen other leaks cause the condition.

 

I want to also add, you can check both caps when the engine is cold. Take an air gun and blow back through the drip hose, be careful. Your only looking for 20 psi (ish) of pressure in the system. It should leak down to roughly 15 psi then quit. Spray soapy water on the top cap to confirm you have it sealed (no bubbles). Give it a few minutes then loosen/remove the side cap. If it still has pressure then your cap is most likely good.

 

Should be hand tight.
From what i got in training classes if that helps