[matthewstryker]

2008 Tribeca with 170k miles. After a recent trip from the Bay Area to LA (in extreme weather) I noticed that the aux fans were frequently running at full speed (with AC off, during light flat street driving etc) when the cooling shouldn't really need electric fan assistance.

I assumed that the thermostat must be out, so I replaced the thermostat (using OE) and radiator cap (with a generic - this was because the old radiator cap had some beat up gaskets beneath it). I used a burping funnel when adding new coolant and squeezed the hoses until air stopped being pushed out, I then ran the engine at 4k for a few minutes and the thermostat finally opened making the lower radiator hose hot. I didn't use the strangely placed bleeder screw on the heater core hoses (because access is a pain), but I'm pretty sure I got the majority of air out and the car seems to cool perfectly (I was unable to overheat it driving up hills in high gear during testing).

Here is the issue: for good measure, I attached a pressure tester to the radiator fill cap. When I pump it up to 13psi (when cold) the pump indicates that pressure drops to 12psi after about 5 minutes and 10psi after 10 minutes etc. I pressurized the system (both hot and cold) with the splash guard and top radiator plastic covers off and used a flashlight to try and find the leak... but I can't seem to find a single drip of escaping coolant (it's possible that air is escaping so it wouldn't be visible). I'm thinking that either:

• I have an internal leak that I cannot see (water pump or head gasket)
  • There are no bubbles in the overflow tank when running hot
  • While the overflow bottle is very dirty, it isn't filled with oil
• My bleeding wasn't perfect, so the spot where the pumped in pressure escapes is where air is already trapped so there is nothing to see
  • I could spray soapy water on the hose connections to rule this out
• I have a defective pressure testing pump setup (it's this one)
  • I plan to try it on another one of our cars tomorrow and see what happens

Any other ideas? As this is the 3.6 my understanding is that head gasket issues or water pump issues are uncommon (thankfully). Any suggestions are appreciated!

-Matt

 

[jimihimisimi]

To recap, engine is now running cool, no signs of a leak, but pressure test is no good?

Reservoir looking OK? Line to it looking good as well?

May be worth taking a look at the hoses going to/from the radiator and seeing if they have ballooned at all .

 

[matthewstryker]

To recap, engine is now running cool, no signs of a leak, but pressure test is no good?

Reservoir looking OK? Line to it looking good as well?

May be worth taking a look at the hoses going to/from the radiator and seeing if they have ballooned at all .

Hi Jim,

"engine is now running cool, no signs of a leak, but pressure test is no good" < exactly

I did clean out the expansion tank as well as I could (it was a pain to remove so I filled it with water and pumped it out multiple times which removed most of the crud). The hose between the radiator cap and expansion tank looks okay and isn't hard. There is some minor ballooning in a few of the hoses, however if I leave the pressure tester connected for 90 minutes it drops all the way to 9psi so I doubt that a hose stretching under pressure would get to that point.

I did the replace the two top radiator hoses (LINK 45162G and 45162GG here) and I still need to replace all of the bottom hoses by the thermostat (45162H and the little ones next to it are pretty beat up).

I did notice that almost every nipple has some rust when I remove hoses (I guess the aluminum used isn't a resistant enough alloy to withstand 15 years of wet/dry and hot/cold cycles). It might also be possible that a few of the metal tube sections in the cooling system are painted or powder coated steel... no idea??

In any event, I ordered some UV dye and a blacklight. They should arrive tonight, I plan to dye the coolant, run the system for 20-30 minutes and see if I can illuminate a leak. If this fails we will simply use the car and keep an eye on the coolant level to see how quickly it drops.

 

[matthewstryker]

Jim / @jimihimisimi ,

It looks like you were correct to suspect the expansion tank. I'm seeming what looks to be dye on the bottom of the expansion tank (it's possible that it is the UV light shining through the plastic, but I don't think so). I'm also seeing tiny amounts on the wiring cover below the expansion tank and the lower radiator hose below that.

There is none on the top of the expansion tank.

Photos: UV dye in Subaru coolant

 

[jimihimisimi]

Look at that! Armchair mechanics on the internet know everything, right?

Keep us posted

 

[jamesbonnar]

I had a similar problem with my 2008 Tribeca, two problems in fact.
1. Losing coolant through perished rubber hoses on the heater crossover pipe (both ends). This showed up as coolant pooling on top of the block, and
2. Loss of coolant from the reservoir tank (as you have shown in you dye tests). This article describes the problem and the fix.

Subaru: Coolant Comes Out of Reservoir – UnderhoodService

If you encounter a vehicle that experiences coolant coming out of the coolant reservoir tank, it may be caused by the radiator cap pressure valve being stuck open by debris sucked up through the reservoir tank hose.

www.underhoodservice.com

Good luck!