Weird cooling system issue

[trac209]

I'm trying to track down an issue I've never seen before. My cooling system even cold will seem to build a slight pressure(just enough to let some coolant out when cap is released). I thought maybe head gasket was letting combustion gasses into the system. So I first checked all plugs,all look great. Then I pressure test cooling system. It holds pressure and vac. Ok now I'm stumped. The truck had the rad cap off for a day while I was doing other stuff,the spark plugs were out too so there wasn't any compression in the engine whatsoever.

So I put the plugs in and capped the rad, did not run it and came back to it couple hours later and pulled the cap off and a bit of coolant comes out again??? The truck never runs hot whatsoever runs awesome but I've never seen this happen before,at least I didn't notice it before. I can't understand how it pops a little bit of coolant out when it could not possibly have and pressure source to do so due to it not even being turned over let alone run. I'm stumped. The only thing I can think of is some sort of chemical reaction in the coolant???
Posted via Mobile Device

[SS Tim]

Sounds like you may be overfilling the radiator. The spec. is 3" below the bottom of the filler neck cold. Are you running any sort of catch can or a full recovery system? The factory installation is a overflow tube which really shouldn't most of the time.

More information in this thread we just did.
http://67-72chevytrucks.com/vboard/s....php?p=6027978

[trac209]

Yes I have an over flow . I do have a air gap at top when cold that's how come I don't understand how it spits some out magically after sitting without even being run again before checking.
Posted via Mobile Device

[cleszkie]

I see you are in Winnipeg. Has it been cold enough to freeze? Maybe some ice in the system causing enough expansion to force out some coolant/water?

[trac209]

Originally Posted by cleszkie

I see you are in Winnipeg. Has it been cold enough to freeze? Maybe some ice in the system causing enough expansion to force out some coolant/water?

Lol no it's coolant good to minus forty and its 25 Celsius out there. Funny you mention temps though as when the temps drop at night so does the pressure in the system. As the day warmed up the hoses got harder. And the truck has not been driven period. So it's some kind of chemical reaction in the system either because of the aluminum rad and iron block. I've been reading up and it can happen. Weird but true apparently.
Posted via Mobile Device