Combustion gas in coolant - can it be smelt?

Apologies if I've put this in the wrong category - but as it's a non Fronty question thought this would be the place to ask.
My normal road motor is my beloved 2001 2.9 diesel Kia Sedona bus/mobile shed and just lately it's started to pressurise the coolant system.
What is weird is that the coolant is clean, as is the oil with no cross contamination. Engine still runs sweet as nut and still pulls like a train.
Did have blue smoke evident on initial start up but then cleared very quickly, so I thought perhaps valve stem sills. Also, it doesn't get used very much - not unusual for fortnightly lapses between starting it up.
To date, I've not been aware of any of the dreaded white smoke clouds from the exhaust.
All that seems of happen is there is back pressure being released from within the engine when the coolant cap is undone the day after it as been fired up. Also, if I start it up with the coolant cap removed, air bubbles can be seen in the coolant.
So my question is, if these air bubbles are being caused by combustion gases from the chambers, will there be a distinct exhaust smell that should be detectable with the nose? I've had a good sniff and can't smell anything apart from the normal coolant smell.
The coolant system on the Sedona is different to that of the Fronty by means of a pressuried cap on top of the thermostat housing. When the pressure builds above the cap's 9-10 psi pressure level, the pressure pushes the spring load cap valve up allowing the pressurised coolant to escape through a hole in the neck, down a pipe attached to the plastic expansion tank. Guess if this didn't happen, I would end up with an exploded radiator and pipes.
I have thought about buying a leak test kit but they are coming in at around the £50-£60 mark so I'm going to try and find a friendly independant garage that might do the test for me for half that money cash in hand.
Last but not least, I have been told that cylinder heads can go porous on early Sedona's. Would this allow pressure from the piston chambers to penetrate into the waterways without coolant being sucked back into the chambers?

I seem to recall reading an article on the Steel Seal website about tiny head gasket leaks that would allow the smaller gas molecules into the coolant, but the larger water molecules couldn't pass back.

I've got a block tester kit if you want to borrow it, not sure if it's got any liquid though. I could post it down if you wanted?

Cheers,
Sym.

Cheers Sym - I would very much appreciate that.
If it helps, I could arrange a FedEx remote courier collection at my expense from yours.
Thanks again for the offer.
Pete

Pete, if you get stuck I've also got a "Sniff Test Kit" you can borrow.

Might also be worth checking that the Waterpump is operating correctly and also the Thermostat ?
Is the top hose of the Rad getting hot and is the whole of the radiator actually hot to the touch as if there are "cold spots" the rad core may have started to collapse or got blocked which can produce similar symptoms to H/G failure.

Cheers Mark
There are a few things going on with it that as got me bamboozaled.
The coolant capacity is 9.4 litres - when I drained it out at the lowest point of the radiator only 4.5 litres came out.
I then refilled it with clean water and a 400ml of Forte cleaning/flushing agent. Could only get 5 litres including the Forte stuff in. Instructions on the Forte stuff was to run it for 30 minutes at fast idle from when it reaches normal running temperature then flush it all out until clean running water is evident.
The Forte flushing agent turned the water into a thick brown colour. There were no rusty lumps present only dirty brown coolant.
When it was idling, top hose, bottom hose and front heater hoses were hot so that suggested to me that the coolant was circulating OK.
I took the rear cabin heater hoses off and flushed out and then they in turn ran hot as well.
The proper Kia workshop manual that I've got gives instructions to leave the radiator cap off, run the car until the top hose is hot and then fill with new coolant. Reckon this is to do with allowing the thermostate to open so that the new coolant can make a complete cycle around the engine and heaters. I could NOT do this because the coolant was bubbling up and spurting out of the rad cap housing. What I did was to take the hose from the heater to the thermostate housing off and back filled with fresh coolant. Did this in both directions so I'm confident that the new coolant ran down past the water pump, through the block, into the bottom rad hose, through the rad and back up via the top hose to the state housing. I could hear it gurgling pushing air out of the radiator cap housing.
In every instance of back flushing, definately had clean running water coming out.
On the final refill, I could only get 6 litres of coolant in. Privately, I'm hoping that the heater matrix will be accountable for some of missing coolant capacity and perhaps the rear cabin hoses would not have drained out because they have to return up to the front heater matrix.

If you could arrange a Fedex collection then that would be excellent. Do I need to wrap it or do they have their own bags and labels? It's boxed.

Cheers,
Sym.

Massive thanks to Syms for loan of tester
Just repeating what I P.M. to Syms - hopefully it as saved me a massive lump of money or alternatively the bus was going to have to go in a compressed state to it's final resting ground.
Figures involved are £450 in parts alone, and that's not main dealer. Kia parts are not cheap - timing belt kit over £200+, cylinder head gasket set £120+ and cylinder head bolts £50+ and thats not including skimming the head or waterpump.
I tested all 3 of my diesels, 07 Focus, 2.8 Fronty and the Kia and all 3 stayed the same colour blue - so fingers crossed the cylinder head is OK.
Had a talk with a guy who's a VW mechanic and he said they have the same problem with the VW Sharan MPV. Massive air lock due to the amount of coolant piping required to reach the rear heaters - plenty of places for an air lock to form. In fact, the dealership where he works have actually bought a pressure machine to force the coolant in purely for forcing the air out of the coolant system. He's gave me a few pointers to try and hopefully get rid of any air locks so fingers crossed this will sort it.
Thanks again for the loan of the tester Syms

Good result mate...if all else fails you could try bypassing the rear heater stuff to prove a point I suppose...
You dont sit 6 rows back when you are driving so wont get cold anyway! :lol:

This sounds like a similar issue that I had. Luckily I knew someone who took a look at it and fixed it for me.