Cooling system leak / pressure test question

[mrdctaylor]

Hey guys,

I've had a coolant leak in my son's 2003 Santa Fe for months. The reservoir goes from the full line down to below the low line in around a month. There was never any evidence of it being in the oil and it wasn't a fast enough leak to make it to the ground and pool up. A couple of months ago, I took it in and my mechanic did a pressure test and couldn't find anything. He said it held pressure over night. He replaced one hose clamp where there were a few drops of coolant, but other than that he couldn't find anything. Fast forward to a couple of weeks ago. I took it back in because it was still losing coolant regularly. This time he drove it around and got it good and hot before putting it on the lift, since a pressure test hadn't shown anything previously. This time he could see some coolant leaking from the water pump. Fortunately for me, this was still under warranty because he'd had to replace it this past summer when the original water pump went belly up. I guess we just got a bad pump since it had pretty much had a coolant leak since then--we just didn't realize it immediately. It looks like that finally solved the leak.

So my question is this: Why would this leak not show up under a pressure test? Why would it only start leaking when hot? I mean, it sort of makes sense, but I just like to understand exactly what's going on with my vehicles so I'll know what my mechanic is talking about.

Thanks in advance!

 

[Billbert]

Nearly everything expands when heated, different materials expand at different rates and clearances change. Could have been a tolerance stack up problem.

 

[mrdctaylor]

Originally Posted By: Billbert
Nearly everything expands when heated, different materials expand at different rates and clearances change. Could have been a tolerance stack up problem.


Right. That's my thinking. But by the same token, a pressure test seems to be THE test for cooling systems. Maybe that's a misunderstanding on my part. Unfortunately, at least in some cases it gives a false reassurance that there is no issue. :-/

 

[Trav]

+1 heat expansion.

 

[LeakySeals]

AKA Expansion tank.

 

[bullwinkle]

Originally Posted By: mrdctaylor
Originally Posted By: Billbert
Nearly everything expands when heated, different materials expand at different rates and clearances change. Could have been a tolerance stack up problem.


Right. That's my thinking. But by the same token, a pressure test seems to be THE test for cooling systems. Maybe that's a misunderstanding on my part. Unfortunately, at least in some cases it gives a false reassurance that there is no issue. :-/
It is THE test-but many times it's better to test them hot. I had a Ford company van with the 4.6 ('12 E-250) that had this exact issue with both head gaskets-no leak cold, but there were telltale coolant tracks running into the block valley from both HGs. Ford dealer didn't feel like fixing it under warranty, they finally dumped in enough stop leak that the leaking stopped...

 

[Leo99]

I had the same symptoms on my Rav4. Losing coolant. Overflow tank emptying. It was the radiator cap.

Hope your leak is fixed. You're lucky to be vigilant and notice the leak before you lost your water pump and had to be towed or worse and overheated the engine and blew a head gasket.

 

[Trav]

Originally Posted By: LeakySeals
AKA Expansion tank.


?? The point was the metal expands not just the coolant.

 

[mrdctaylor]

Yeah, I'd already replaced the rad cap with an OEM one. I'd hoped that was it. He's driven it quite a bit since it was repaired and the level hasn't gone down any. I just hope this aftermarket water pump doesn't start [censored] out like that last one.