Hi Guys,
2006 X350 3.55L 166000km
Done about 14000km since I bought it
I'm currently trying to find a coolant leak / loss
No puddles under the car
No coolant loss when standing cold (up to a couple of weeks)
Only looses coolant when running and hot
About to try a coolant UV dye
Engine oil is clean and clear
No odd smells from exhaust pipe(s) or white smoke.
Is there any test that can be made to check for coolant in the exhaust
Then I could rule that out (or in)
Cheers.

 

You can visually check the coolant overflow tank for bubbling from exhaust gases or more expensively and more accurately you can get a test kit that includes a liquid that will change color when exhaust gases are present in the coolant.

 

The reason, why it leaks when engine is running can be small crack or pin hole in many different components. The crack develops and leaks only under pressure. UV die in coolant is best way to locate leaks. Just make sure the light source you use is same wave length as trigger in the UV dye.
Many folks report that in our cars this will lead to chasing next weakest point:
you fix leak you found put everything back together to find another leak; take stuff apart repair that leak put everything together; next weakest point develops leak.
Folks that went those circles recommend to replace everything at once:
all rubber houses
Thermostat
Expansion tank
water pump
valley hose(?)
Radiator, at least check that it holds the pressure.
There us some wisome in this: car is 22 years old and probability that plastic or rubber is at EOL is really high. As long as you there, you may fixed them all.

 

The coolant reservoir itself can be the culprit

 

I had cracks in thermostat housing, coollant reservoir and valleyhose.
I had to put more pressure on the coolant system, than recommended, to see the cracks.
The valley hose requires some boroscope to see it leaking.

 

For exhaust specifically, there is a tool that will pull the gas from the coolant tank and a liquid you put inside will change color if it detects gases.
https://www.autozone.com/p/oemtools-...64b6d933e06681
You will still need the fluid but any local parts store that sells the tester will sell the fluid too.

A poor mans way of checking it is to let the car get warm, then later when it is completely cool squeeze the hoses. If the hoses are still firm then there is residual pressure that is being generated somewhere. Additionally, you can open the coolant cap and if you hear hissing you are getting pressure inside the cooling system from somewhere.

 

Thanks for all your suggestions Guys.
I have plenty of info to go with now.
My first move is to determine if the missing coolant is in fact heading out through the exhaust.
If so, an engine pull down is called for and all the other parts you have mentioned will be inspected / replaced during that.
If not, then I'm on a leak chase. So thanks for your tip Big Koshka -- Having found one -- keep looking!!
Many thanks to all. If and when I solve this, I will post back here.

 

I would think it's very unlikely that you're losing coolant out the exhaust if you have no white smoke or steam on startup. I had a car years and years ago with a fairly slow coolant loss, and it produced a significant steam cloud from the tailpipe on startup. Turns out there was a crack in the cylinder head in one of the exhaust ports, which allowed coolant to settle in the port after cooldown. I really can't think of any other way for coolant to get into the exhaust.

On the other hand, a head gasket failure would easily let exhaust gases into the coolant, which would pressurize the cooling system beyond the normal operating pressure that comes with coolant being heated as the engine runs. I think you could check for that simply by removing the pressure cap, starting the car cold, and watch for bubbles in the recovery tank as the car warms up enough to open the thermostat. This would be unlikely unless the car has been overheated, which can warp the heads.

There are a million much more likely places to lose coolant. See the thread in my signature for my own experience.