Coolant leak

Hello fellow motorists.


I've been trying to find a coolant leak in my engine bay to no avail.


Here's what I know:
-The leak is external - a pool/puddle on the ground is visible after the engine cools down.
-The leak is small - it doesn't loose a measurable amount until the engine starts to cool.
-The leak is finite - once the system depressurizes it seems to stop leaking.
-It is on the rear of the engine (flywheel side) - near the outlet flange (Page 19-16 in your service manual, item 9 in the diagram).
-It drips down the back of the engine - no clear path is indicated (I should probably steam clean my engine bay, but I lack such equipment at home and I'm a bit nervous about breaking pigtails and stuff).


Are there any common failure modes I should know of? I spent like 30 minutes looking under there to no avail. I've cleaned around the flange and am planning to leave a paper towel under there to gain some hints on altitude. All the hoses were secure and showed no sign of leakage at the connections.


Thank you,


300k_logician

Try coolant dye and a UV light...

The temp sensor is back there. Might be that. Get a mirror and strong light and take a look after you shut it down.

The CTS is a prime candidate, yep.

I had the same style issue with mine it seemed like it was from the engine but mine was where the water pipe feeds the heater core through the firewall. Check there when the engine is running it would be a very slow drip the connectors are known to fail very cheap and easy to swap and fix.

My 2004 BEW suffered from similar issues. The first time it was the CTS O-ring and the second time it was the coolant outlet flange. What I’ve been noticing on my 17yo Jetta is that as I fix one weak link in the cooling system it causes another weak link to be exposed (i.e. I fixed the coolant outlet flange and then found that the coolant ball was leaking at the level sensor). Don’t be surprised if you find/fix your leak and the develop another shortly thereafter. Just my 2¢ - YMMV.

As stated by cptcrnch ^^^

1. CT’s o-ring not flexible anymore.
2. O-ring under coolant flange to head..... same problem, becomes brittle.
3. o-ring in connectors to heater core..... same problem, becomes brittle.

Many times o-rings become permanently deformed - flattened. Not really brittle in loss of elasticity, but they just don't spring back to seal, and therefore they leak.

Cheers,

PH

Hey guys, thanks for the suggestions. I left a paper towel beneath the outlet flange the night I posted, and it was bone dry when I got to work the next morning. After I finished up, I checked it and it was soaking wet with coolant.


The CTS is not original, I had that fail about 2 years ago, and replaced it, so I doubt that that is what's leaking.


At any rate, the outlet flange O-ring is the number one suspect. I've gotten a new O-ring and plan to tear it apart tomorrow. Also it randomly didn't leak last night. Gotta love those intermittent problems. I'll let y'all know what happens.

Bingo.


Thanks guys.

Nice ...glad you found it .

Powder Hound said:

Many times o-rings become permanently deformed - flattened. Not really brittle in loss of elasticity, but they just don't spring back to seal, and therefore they leak.

Cheers,

PH

Click to expand...

The flattened nature is actually abrasion wear due to excessive rocking motion of the engine. When I find them leaking here, I'll usually find a dogbone mount that needs to be replaced.