Where is the Coolant Going?

[onecrazyfoo4u]

I have an 05 GMC 1500 with the 5.3L and 157k miles.  I've noticed over the last few weeks that my coolant is disappearing.  I'll fill up the reservoir to the correct fill line, then after driving it the next day or two, it'll be down 1 inch under the fill line.  But it never goes lower than 1 inch below the fill, even after several weeks.  It just disappears over the first day or two, and then holds the low level.  I've checked exhaust at startups and there is no white smoke.  I've looked all over for leaks and there are none.  I park in the garage so it would be obvious.  There's no burning sweet smell at all.  I'm at a loss as to where it's going.  And it's so weird that it doesn't keep draining, just goes an inch low then holds.

What do you guys think?  Could it be a bad head gasket?  How would you tell for sure?

[Doug_Scott]

If it only goes down that one inch, and never goes any further I wouldn't worry about it.  It sounds like it simply has a level it wants to settle at and it does just that.  If the head gasket was causing it, the level would not stop going down, ever.  The reservoir is just that, a reservoir.  Does the reservoir have more than one line on it?  For example, does it have a "cold" and "hot" line?  Or maybe a "Min" and "Max" line? 

 

It may be as simple as once the engine gets up to operating temperature the coolant has expanded enough that it vents some out the overflow hose.  Once the engine cools back down, you see it is down an inch.  Next time it gets hot and coolant expands, it does not make it overflow because it started out at a lower level.  You could verify this by attaching an empty beer can below the reservoir overflow hose to catch the coolant if it comes out that hose.  

 

[onecrazyfoo4u]

9 minutes ago, Doug_Scott said:

If it only goes down that one inch, and never goes any further I wouldn't worry about it.  It sounds like it simply has a level it wants to settle at and it does just that.  If the head gasket was causing it, the level would not stop going down, ever.  The reservoir is just that, a reservoir.  Does the reservoir have more than one line on it?  For example, does it have a "cold" and "hot" line?  Or maybe a "Min" and "Max" line? 

 

It may be as simple as once the engine gets up to operating temperature the coolant has expanded enough that it vents some out the overflow hose.  Once the engine cools back down, you see it is down an inch.  Next time it gets hot and coolant expands, it does not make it overflow because it started out at a lower level.  You could verify this by attaching an empty beer can below the reservoir overflow hose to catch the coolant if it comes out that hose.  

 

Yes, I believe it does have a min/max line.  I fill it up to the cold max (when the engine is cold), and the next day it'll be just below the minimum line.  I guess I'll just watch it to make sure the level doesn't drop anymore.  Hoping for the best!  Thanks for the input, it makes sense.

[Jsdirt]

You could be in the beginning stages of head gasket failure - heat may be sealing the leak back up again, when everything expands.

 

Only way to know 100% for sure if that's what is happening, is to take a 4 or 5-gas analyzer, and stick it in the radiator fill, or coolant overflow - anything over 100ppm is a leaker. Those start at $4,500, so best to find a local garage willing to put their expensive sniffer in there WITHOUT sucking coolant into it.

 

Those fluid testers that change color won't pick up a leak that small. Also, if it's ONLY leaking from the coolant jacket side, this test won't work.

 

Another area to check is the weep hole on the water pump - get underneath after the truck has been sitting all night, shine a flashlight up on that weep hole, and look for coolant traces. This also can leak cold, then seal up again with heat expansion. You'll see the streaks of evaporated coolant.

 

Hopefully it's the latter ... but if not, you could always toss a can of Bars Leak in there, if you don't want to spend any money on the truck, or if the body is going to completely rust out in a year or 2. One bottle won't hurt anything. I used the stuff for 13 years on my Grand Marquis, until I finally repaired the leaking timing chain cover. Car has 250k on it and still runs like a top.

[Doug_Scott]

I agree with Jsdirt's post, provided the coolant level continues to drop over time.  You may want to take a sharpie and mark the current cold level once a week to see if the level continues to drop after the initial one inch drop. 

Your truck is in the "Castech head" zone.  There is a TSB on this from GM that describes the issue and how to determine if your engine has Castech heads. 

It is TSB #06-06-01-019B: Information on Gradual Coolant Loss Over Time with No Evidence of Leak Found - (Jun 12, 2007).

It is very likely on this site in one of the many posts on Castech heads. 

Again, this only applies if the coolant level continues to drop after the initial one inch drop. If the coolant is weeping through porous castings into the crankcase it would normally accumulate in the oil, and since oil and coolant don't mix, it would be the first fluid to drain from the oil pan on your next oil change.  Some will evaporate provided the oil temp gets higher than the boiling point of water/antifreeze solution. 

[onecrazyfoo4u]

Thanks for the input guys.  I'll keep watching it.  There's definitely no leak through the water pump weep hole.  And the level does stay steady at the 1 inch low level.  I'll keep watching it to see though.  Hoping it's not the worst!

[onecrazyfoo4u]

Well after a few weeks of watching I think I'm in the safe zone.  The level hasn't budged, so as mentioned earlier I guess it just has a sweet spot and doesn't want the level any higher than that.  Yay!