Strange coolant loss

[baylock]

This is kind of a continuation of:
http://www.navara.asia/showthread.php?t=25233

but i want to ask something in a different way.

here is the situation;
A little while back i put the fan through the radiator, i have replaced the radiator (Koyorad dual core, so not a stock radiator), cap, and top hose, and flushed the system several times... the problem im left with is this:

with the radiator full, i run the ute for a trip, and as normal the coolant expands, and flows into overflow bottle, but it does not flow back into the radiator.
a 20min trip uses about 1L of coolant, and a 1hour trip also uses about 1L of coolant, both with no sign of overheat, or further engine trouble. I set up a camera to monitor my exhaust, there seems to be no steam, and practically no smoke (A little soot occasionally when taking off from a dead stop), and the motor is giving me no problems.

My questions are:
Why would the coolant flow out when hot, but not in while cold?, the cap seems to work properly.
If the unit is sealed enough to build up the pressure to push the hot coolant out, then where is it getting the air from when the coolant contracts again?
The manual says the ute takes 11L of coolant total, but with an after market radiator would this change? (It took 11L to fill it up from the last time i did a complete flush.)

It has been mentioned that it is likely to be the head, or a head gasket, but these symptoms, and the lack of any other problems make me wonder a little.

Also, if i remove the cap just as the motor cools, the overflow bubbles, that would mean there is still positive pressure in the radiator, but how can there be positive pressure if it is pulling in air from somewhere?

Any clarification welcome, and thanx in advance.

 

[Old.Tony]

If you think there's a problem with the head, don't waste any time tinkering with anything else at first, take some coolant straight to a radiator specialist and ask them to test it for combustion gas contamination. Positive test = damaged head gasket at least, or cracked head at worst. Negative test = likely to be ok. It's a small investment to not only give you peace of mind, but to take one possible (obvious) cause of symptoms away.

It doesn't sound like the head to me - it sounds like there's a one-way valve in the overflow! There might be a split in the hose between the radiator top and the overflow bottle. The hose should be soft and malleable, if it's hard it could be cracked or holding itself off the surface of the connecting tube. This would be more prevalent at the radiator side of course, so it's an easy check. A split might not let much coolant out (coolant is a lot denser than air so it might not make it through the gap) but it might allow air to suck in, so coolant goes to bottle and never returns.

That's where I'd start. Also examine where this hose mounts to the radiator, in case it's got a hairline crack around it (same deal as a damaged hose).

 

[baylock]

So, i did a little experimenting, this is what i got:
http://www.youtube.com/watch?v=ASxIJESICgg
Would this be the confirmation of a blown head / gasket i have been looking for??

 

[Old.Tony]

Nice experiment.

It certainly could be. Take some of the coolant and have it tested, you'll know for sure.

It is possible that it's just air escaping (that's being sucked in and trapped in the upper reaches of the motor). Filling the radiator once is a thing of the past - with these convoluted things, you need to do it, run the engine, do it again - and if yours is sucking air back from the overflow bottle pipe, it'll never get rid of it all!

 

[nathand22]

IMO the air is coming most likely from exhaust gas entering the system due to the crack or head gasket,
it would also explain why the coolant dosnt come back once it reaches the over flow, it cant come back against the pressure caused from the gas constantly pressurising the system and it is that pressure that is forcing the fluid into the over flow so quickly.

 

[Old.Tony]

Yep, entirely possible. That's why the first suggestion is to get the coolant tested, which will point the finger right at the gasket/head if either is damaged.