Leaking coolant, can't find it.

[AdrianD]

Hey everyone, my diesel Jeep is slowly losing coolant. After the head swap it lost quite a bit, traced it back to the water temperature probe (almost invisible, I had to feel around for wetness ), tightened it and most of the leak went away.
But now I can't see any visible sign of a leak, can't smell coolant like before and it's very slowly losing coolant. No extra pressure in the overflow bottle and overflow return flow is nice and constant so I can rule out a cracked head. Head gasket is MLS so a crack there is unlikely. Oil level is constant and oil looks good. No white smoke out the exhaust and it doesn't smell any different that what I'm accustomed with. No lost power, works the same as before head swap (same speed up the test slope, same EGT and boost).

What the funk is going on and what's the best way of finding the leak? Pressurizing the coolant system? UV dye (can't find it in shops here)?

[745 turbogreasel]

some will disappear if the reservoir/cap don't seal 100%.

[AdrianD]

Didn't think about that, it's a new cap though but was installed when I had head problems (big pressure, overflowing). Time to put the old cap back on.

[TylerDurden]

I rekon UV dye is the best way to find a small leak. Someplace local to you must have a kit.

There are so many places it could be leaking - rad pinhole, WP shaft, etc etc.

[theman53]

If it is just recently running I have had them burp out air pockets for a good bit. Especially when there is another leak and it doesn't pressurize correctly. Maybe keep a real close eye on it and see if it levels off?

[CRSMP5]

depending on the antifreeze.. i think it glows under uv... also depending on antifreeze.. give it a week or 2.. the small leak should leve a faint white/pink/green trail/stain or residue type of remain...

[AdrianD]

Rad pinhole is definitely on the list. A 17 year old copper radiator isn't in tip-top shape...

I've installed the old overflow bottle cap, if it doesn't cure it I'll pressure test the system.

[bajacalal]

This thing has the VM diesel engine right? I'm not familiar with that engine but common places for the gas engines to leak on these were the heater control valve (looking at your pictures it would be under the coolant bottle I think), the water pump, thermostat housing and radiator tanks.

I've also seen hoses that wouldn't seal up anymore after you "disturbed" them.

[ORCoaster]

I second Bajacalal on the hoses.  At times they get a memory and re positioning them on the same pipe without moving the clamp to a new spot can have them leaking real slow or at high pressures once the hot engine is turned off.  That is when I find my leaks to occur.  The extra bit of heat soak ups the normal system pressure just a tad.

 

[AdrianD]

Yes, it's the VM engine, I'll double check the heater hoses.
I'm going to flush the cooling system (never did after the head swap and the heads were washed in diesel to clean all casting slag inside) and I'll go over all the hoses I can access. 

[AdrianD]

Yesterday I could smell a hint of coolant after parking in the garage after a drive. So it's leaking outside, not in the cylinders. It's a start 

[air-cooled or diesel]

do you have a block heater? the seal on this can give out and drop coolant, its under the intake,  freeze plug.

[AdrianD]

No block heater, this thing has glow plugs. Still haven't gotten around to pressurize the coolant system. The old overflow cap will be butchered to install a valve.

[air-cooled or diesel]

you can have a block heater with glow plugs, most (if not all)vw's have block heaters(mk2's, and earlier) so its possible you have a heater, you might want to check. if you have a wire that plugs in to 120v at you right side of engine well, that you've been wondering what its for; that's what its for. if you don't have a wire you may want to check, If I recall correctly its the middle freeze plug, reach back under intake and feel freeze plugs, if you find one that's not a standard freeze plug than you may have found your problem. the block heater 'plug' has electrical connections, so it wont be depressed like a standard freeze plug. (its not a standard outlet connection at plug, but wire is a 3 prong standard outlet connection)

[TylerDurden]

... most (if not all)vw's have block heaters(mk2's, and earlier) so its possible you have a heater, you might want to check. if you have a wire that plugs in to 120v at you right side of engine well, that you've been wondering what its for; that's what its for. if you don't have a wire you may want to check, If I recall correctly its the middle freeze plug, reach back under intake and feel freeze plugs, if you find one that's not a standard freeze plug than you may have found your problem. the block heater 'plug' has electrical connections, so it wont be depressed like a standard freeze plug. (its not a standard outlet connection at plug, but wire is a 3 prong standard outlet connection)

This is contrary to my experience.

None of the many VW diesels I have owned had block heaters, except one (which has a crate engine). Block heaters are not stock, as far as I know.