coolant coming out of reservoir cap

[Musick17]

So last week my truck went in and got a new heat control valve because it went bad and dumped my coolant all over a gas station parking lot.

I noticed a couple days ago that I was low on coolant, which definitely should be since it all just got replaced with fresh and should be topped off accordingly.

I took it back to the mechanic that did the work and they pressurized the system and it held for an hour, so they called it good. I ended up just taking my truck back and saying Id keep an eye on it.

Now today, I was out heading to the mississippi missouri confluence to find some new fishing spots and it was running hot. Nothing critical, just a little above normal so I got to where I was going and popped the hood. Once I shut the truck off there was fluid being pushed back into the resevoir tank, which I believe is normal. But there was so much dumped back into it that it filled all the way up and started pouring out under the cap. So that solves the mystery of why I was low on fluid.

But what could be causing so much fluid to be pushed back into the reservoir tank? I dont think replacing the cap would make a difference if it held the pressure of the system fine when they did it in the shop?

Any advice if appreciated. Thanks Fellas

I have a video of it coming out under the cap on my iphone, but dont know how to get it onto the forum.

 

[RonD]

They sometimes use their own rad cap with fittings to read pressure, so maybe they didn't test your rad cap.
Rad caps have two valves, the larger one(big spring) is the caps rating valve, so 15psi(or whatever), this allows coolant to expand when heated and pressure in the system gets above the rating(15psi), the expanded coolant is sent to the overflow tank.
Normal operating temp of the coolant system is 200-220deg, the internal pressure helps prevent boiling at 212deg, if rad cap can't hold the pressure then coolant can boil which causes more coolant to be sent to the overflow and steam to form in the engine, this can produce an air lock as the water pump can not circulate steam/air.

The smaller valve is inside the larger, it allows coolant from the overflow tank to be pulled back in as the pressure in the rad falls when coolant cools down, if this valve fails you would notice larger rad hoses looking like they are collapsed when engine is cold, the coolant got out but can't come back so negative pressure in the cooling system collapses larger hoses.
This part of the system only works if overflow hose is air tight, no cracks, and its end in the overflow is debris free, if overflow tank has gunk in it it can block return flow.


Just to see......with engine cold remove rad cap, start engine.
Watch for coolant bubbles or coolant bubbling out, this is a sign of a leaking head gasket.

A cold cooling system has no internal pressure, the pressure in a cooling system only comes when coolant is heated above 180deg., which takes about 5 minutes.
Only place pressure can come from in a cold cooling system are the cylinders.

Cylinders have 600+psi when they fire, so a 15psi pressure test might not ID a small head gasket leak.

How did the heat control valve fail?
Broken fitting?
Extra pressure in the cooling system can blow off fittings, it finds the weak spot for pressure and....pop.


If in doubt, with cold engine running and rad cap off, put a latex glove over rad cap opening, pull off overflow hose, hold latex glove tight around the opening and put finger over overflow outlet.
If glove starts to inflate you have a head gasket leak, if it doesn't then all is well.