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My car is leaking coolant onto the garage floor under the radiator, but my mechanic couldn't find the leak even with a pressure test. He said it held pressure for 20 minutes. So he replaced the radiator cap in case that was the problem. But it's still leaking coolant. What's the next step?

  • Did he run the test with the engine hot? That might make a difference. – MooseLucifer Jan 12 '18 at 18:02
  • Is it leaking while it's stopped and cold, or just when it's running? If you don't know, that's the next step? – GdD Jan 12 '18 at 18:52
  • @MooseLucifer That's a good question. I'll see if I can find out. – mrog Jan 12 '18 at 19:37
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    Add some UV dye to the system. – Ben Jan 12 '18 at 23:59
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    The overflow hose and bottle are not under pressure. They are exposed to the atmosphere. The hose doesn't have to be clamped because the test adaptor seals below the hose. FYI, if you have an expansion tank, that is under pressure. – Mobius Jan 15 '18 at 17:45
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Have you checked the hoses? I had an issue like this years ago with a tiny (and I mean tiny) hole in a pipe close to a clamp which when the engine was running and hot the amount coming out would just evaporate, as it cooled though then it would form a puddle.

So, how much do you find under the car - a spoonful, a cupful or a gallon?

  • I usually see a small puddle (maybe a quarter cup or less) on the floor when the car leaves the garage. The overflow reservoir was completely empty when I checked it today, and it was filled two weeks ago. So I wonder if it's mostly leaking during driving. The hoses seem dry on the outside. My mechanic says he checked them. – mrog Jan 12 '18 at 19:33
  • I think I have the same thing going on. The leak is intermittent, and only seems to happen on longer drives. That might be why the pressure test couldn't find it. I added some UV dye and checked on it a couple of days later. I can't see the leak itself, but based on where I'm seeing droplets in the engine compartment, it seems to be coming from the place where the upper hose connects to the radiator. I suppose I could throw a second clamp on to see if that helps, but I bet I need a new hose. – mrog Jan 21 '18 at 16:33
  • I've never replaced a radiator hose before, but it doesn't seem like it should be hard. Since it's an upper hose, can I get away with only draining half of the coolant before swapping the hose? Is it safe to reuse the coolant I remove? If not, how do I dispose of it? – mrog Jan 21 '18 at 16:37
  • If it is an upper hose then you only need to drain to a level just below the hose (sometimes I have managed to change hoses by blocking holes quickly) but you still need to bleed the systems properly... As for disposal - you need to follow the regulations for where you live - but down the drain is not a good solution... After all this will you be accepting my answer? – Solar Mike Jan 21 '18 at 17:02

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