So I've got a coolant leak that I need help with. I know where it's coming from but I don't understand why?? If the car is off, it doesn't leak. If the car is running, I'm getting coolant leaking from the top hose on the reservoir tank. It's almost like there's pressure building up in the tank and ends up leaking past the little hose on the coolant tank that goes to the radiator. The water pump and thermostat housing are roughly a year old. Any info would be great. Hoping for a cheap fix? Thanks!

 

When the car is running, do you watch the hose seep?
AND do you mean the return line to the top left corner of reservoir that is held on with a pinch clamp?
How is that even leaking, as your reservoir volume should.n3ver reach that height.
when car is cold, Try loosening the reservoir cap a few threads and start car and bring to warm temp to see if that reduces pressure...

 

When the car is running, you can see the hose seep like its seeping past that nipple and hose clamp and seems to come out the end of the hose. Coolant tank is filled to max level and when the car sits shutoff, it doesn't leak. As soon as you fire it up, it takes a few seconds and you can start to see it seep past and out the little hose on top of coolant tank. I thought maybe the hose or nipple was bad but they both look just fine? I don't know. I'm confused to be honest. Doesn't make much sense to me but before I start throwing stuff at it, I figured I'd check here? Thanks for the reply.

 

All I can think is there is a blockage somewhere, and the coolant is exiting the radiator through that little hose...

Can you see coolant coming into the overflow tank through that little hose?

 

I'll have to check. I was trying to diagnose where it was coming from so after it sat overnight and everything was 'dry', I fired her up and watched around the expansion tank for leaks and it's almost immediate that it seeps out the little return hose. I pretty much shut her off at that point and started scratching thee ol noggin. I'll have to start her back up and see if it's going into tank or not? Strange?? I was thinking of replacing that little hose on top and see if that solves it. Or possibly a new tank cap/cover??

 

Here is the functional layout of the cooling system, showing the flow paths:


If coolant is coming out the little top hose, I might be tempted to believe either the lower hose has a blockage, or the thermostat is blocked somehow.

 

Try to switch out the hose clamp to a worm clamp, then I would try to pressurize the the system and first check for any leaking. With the worm clamp on there it should hold 15 psi.

 

You can borrow a coolant pressure testing kit from your local auto parts store. If they don't have one available then you can order one on Amazon for very little. Usually 30 bucks or less. You can put the pressure on the cooling system and see where the coolant is leaking from without it being hot.

 

I have used it to bleed my brakes and after a good cleaning used it to pressurize my coolant system.