The mystery of the Lost Coolant

I was convinced I had a head gasket replacement on my hands. Now I'm not sure I do.
I was actually looking forward to it. (Engineers for ya)

​​​​​​Short story long, It overheated once (or twice? It was a few beers ago) due to an incontinent radiator - which was infuriatingly hard to pinpoint - the join between the bottom tank and the core was leaking behind the AC condenser. Radiator was replaced and all was right with the world for a while. 
Fast forward approx 5 years to just recently and it's drinking so much coolant I nearly called an intervention. It started off slowly, it's hard to put a timeframe on it, but I usually have had to put a trickle of water into the radiator on my weekly look over and somehow its got to the point I'm refilling a decent amount every time I drive it. ("Overflow" tank level seems constant) This is the point I had convinced myself the head gasket must have been compromised when it overheated all those years ago and it had finally given up. It only seemed to happen warm, while driving, and never was there any wet spots under the vehicle. No water leaks anywhere. I had come to it that water was somehow being sucked into the engine gradually and being sent out the exhaust. Engine oil stays suprisingly clean between changes and has never had any hint of water in it, not even under the fill cap or on the dipstick. 
Over the Easter weekend i had a long, hard look at it. Something in the back of my head was beginning to doubt it was the head gasket. I removed the radiator (its only 5 years old, remember) checked it for blockages and leaks. All clear, no problems as expected.
While I had the radiator out, I looked at the water pump and figured I was half way there I may as well take a look at that. Out it came. It was a little "squeaky" when turned, like the seal was a little dry (I know that sounds silly as of course with no water present it literally is dry) but bearings were firm and it turned smoothly so I called it good. I inspected the original sealant and it seemed to have been properly applied and no sign of leakage. So I reassembled the water pump and had another think. In my infinite wisdom I decided to check the thermostat. I have an OBD scanner and had noticed it seemed to like running about 94°C, which seemed a little hot to me but the dash temp gauge was a little below half way. (I know I don't trust it but seeing as thats what is supplied you've got to go by something, right?) 
Most of an hour later and with basic hand tools as I'm at home and not the workshop (how hard can it be to remove a thermostat, said no jimny mechanic ever), i have the thermostat in my sticky hands and proceed to check it in a pot of boiling water. Sure enough it opens at mid 90's. So it's junk. I mean it works, but opens way too hot.
But that doesn't explain where the coolant is going. 
So I decide to pressure test the engine. (Yeah, that thought was late to the party). Reassembled the thermostat housing with just an o-ring to seal it and capped the lower rad hose. Filled with water and cobbled up a contraption to pressurise the top rad hose. Pumped up to 15ish PSI and left it for a while. Came back expecting to see water on top of at least one piston ( looking down the sparkplug hole). Nothing. Dry as a bone. Water level was down a little, but my caps on the lower rad hose and the throttle body connections where dripping slightly. (Still have the intake manifold off of course). Upped the pressure to 20 psi. I'm running the 20psi pressure through a regulator from a compressed air system set to 60psi to keep constant pressure on the cooling system. Still nothing. Not a drop. And yet I've been losing 250 odd mls everytime I drive it!? (at least enough to get it warm). 
The radiator cap doesn't give me the warm fuzzies, so the only theory I can think of is the dud thermostat is letting it get hot enough to boil coolant slowly out the cap, and by the time I got home again, it had evaporated.
The dilemma I am facing, is whether I go to all the effort of reassembling everything to test this theory? Or do I just bite the bullet and pull the engine, which I am prepared to do, I just don't like wasting time and money unnecessarily. Head Gaskets for these engines aren't cheap, and I'd feel like a pillock removing the head to find a perfectly good gasket. 
Has anyone had this before? A head gasket failure that passed a pressure test yet somehow still loses coolant? I'm kind of a bit confused so if someone could shed some light or past experience that might help make up my mind I would appreciate it. 

Apologies for the rather lengthy post. 
FYI this is a 2007 model. (Gen3) M13A engine (vvt)
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While not exactly the same situation I recently had over heating problems caused by the electric fans not working..how this does relate is as it got hot it was loosing water somewhere, I did suspect the radiator cap but it was dry by the time I got out and checked it.
I wondered if there might have been another reason for the water loss, but after getting new fans it's back to normal.
That does make me think that could be water coming out of the cap as it heats up.
You don't really have anything to loose but a bit of time by putting it back together and seeing. One thing I learnt as a maintenance fitter was not to go fixing things unless you knew what the problem was.
Preventative maintenance is one thing, but just replacing things in case they might have a problem is an expensive fix to a non existent problem.
Fix what is broken, or what's on its way out, but as they say if it's not broken then don't fix it.

Have you done a sniff test on the coolant and checked for any mayonnaise inside the oil filler cap?  Have you changed the radiator cap in case it's over-pressurising?

Also, slight leaks when the engine is hot can dry up instantly, so is there any blue staining?

The leak on ours - which was external - was a  combination of the wrong size hose clips - although I gave the supplier the reg no! - and a rad cap that wasn't relieving pressure soon enough.  It took about 30% more force to open than the replacement did.  Been fine for two months now.
 

A while back my Zafira started using water and I couldn't find the leak. Pressure testing for an hour or so didn't show up anything conclusive. Over time it slowly got worse and I started suspecting head gasket until one day after a long run I spotted a tiny dribble of water on the floor under the radiator. It turned out to be a pinhole leak near the top of the radiator and the coolant was evaporating before it reached the bottom.
One thing I had spotted before and discounted was some oily looking stuff on the cross member under the rad. It turned out to be pure antifreeze with the water evaporated out of it.

Have you also looked at the viscous fan?

The viscous clutch is fine, it has a fair amount of resistance when cold, however I haven't tried it warmed up. When driving at highway speeds the fan is kind of along for the ride anyway, as ram air through the grill will push more air through than the fan ever will. Electric fan works fine, I believe it only comes on with the A/C pump anyway. 
​​​​​​A pinhole leak lesnewell describes is the exact reason my rad leaked originally, so i was keeping a sharp eye out when I pressure tested the rad separately and there appears to be no leaks, and certainly no physical damage to the fins. 
As mentioned above the oil has never had the faintest hint of milkiness anywhere, even under the fill cap.

One thing I learnt as a maintenance fitter was not to go fixing things unless you knew what the problem was.

My background is Mechanical Engineering, so I 100 % agree, but I also don't like doing work twice  . 
I already have a new thermostat and Rad cap. I'm thinking I might just put everything back together over the next week or so in my spare time and see how it goes. I will reassemble everything as best as I can and hopefully along the way I fix whatever it was that was causing the issue. If it's still not behaving, then at least I've isolated the issue, and its off with its head. 
At least with the engine out it's easier to see and work on. I'm not too worried if it comes to that, I also have an HD clutch to go in that would be easier to do outside the vehicle. 
Thankfully I have a second vehicle so it's not too much of an inconvenience.

Thanks for your input guys.

 

Also, has anyone bypassed the coolant lines to the throttle body?
I accidentally made a hole in one of them with my hook tool getting them off.
I could shorten it by 10mm or so to get rid of the damage and it would still go on and work perfectly, but it occurred to me just now that as it virtually never gets below freezing here they don't seem to serve any useful purpose.
I thought it might stop the IAC valve freezing at idle as it does have small passages with high velocity air passing through, but I looked through the service manual and can see no reference to it needing to be heated.
I know the performance gains from not having a heated throttle body would be negligible, but these things need every help they can get!! 

I'll take a bet with you, bypass it and you'll find the engine stays at the "cold idle" speed longer or possibly all the time.

But just make sure it's a bypass and not simply block it off.
I once got given a car by a mate that was sick of it always loosing water, it would heat up, over pressure and come out.
The heater had died and one of the mechanics at work simply removed the heater and blocked it off.
After looking at it, I unblocked it and fitted a hose instead and it never had that problem again.
That's a tiny hose and probably won't have the same problem, but why risk it.

Quite a few of the vehicles on the roads now have the thermostat on the return (cool) side of the loop, and these depend on circulation of warm water through the heater core for the thermostat to open, both the M13 (Gen 3) and I believe the K15 (Gen 4) are like this, the G series (Gen 2 & early Gen 3) don't.

fordem wrote:

I'll take a bet with you, bypass it and you'll find the engine stays at the "cold idle" speed longer or possibly all the time.

I had wondered that, but the IAC circuit I don't think has any temperature measuring capability. The ECU already has that info from the coolant temp sensor, seems pointless measuring the same thing twice just for the idle speed. The incoming air temp is measured at the MAF sensor which is before the throttle body, so shouldn't affect that either. I believe some other vehicles do have a mechanical (I'm guessing Bi-Metallic Strip) system in the throttle body to manually drop the idle speed when warm, in which case you are correct, the idle would never drop to "warm" RPM. 
Also, I believe the idle speed is increased when A/C is turned on, which it wouldn't do if it was a completely mechanical system.

I haven't touched the heater system, just looping the throttle body line from the thermostat housing back to the block, bypassing the throttle body.

I've basically got all it back together apart from the lower rad hose, so will find out what happens I guess. I just thought I'd give it a shot, doesn't sound like anybody has tried it. 

Many vehicles have a "wax based" cold idle mechanism in the throttle body and typically these have that coolant circuit that you are considering bypassing - I've never looked at the M13 to see if they are built this way, but I'm willing to bet that they are, simply because of that coolant loop.

Moving on, the IAC is not a "completely" mechanical system, the idle speed is controlled by the ECU, the ECU controls the IAC valve which controls the air bypassing the throttle plate and so controls the idle speed.

Typically, the a/c compressor, the HVAC fan, the powersteering pump and any one of several electrical loads will trigger the ECU into increasing the idle speed.