Mystery coolant leak, or not? 2010 Jetta

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Feb 8, 2006
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305
Location
VA
Sorry, kinda,long...
I got a 2010 Jetta from a family member a few months ago with a variety of issues. It had been setting for about a year before I got it due to a shifter cable issue. When I got it to my house, it had an empty coolant reservoir.
Over the last 4 months, I have usually started the car once a week, sometimes moving it around a little on my property. Each time before starting it, I would top off the reservoir. There was a coolant leak somewhere in the front. It was a very heavy drip that was dripping from the front bumper/skid plate area. I was never able to visually see the leak. The car would continue to leak when I shut the car off and would empty the resevoir bottle. Im pretty sure got it up to full temp, but cant be sure.

Yesterday, I finally got the car road worthy, and was able to really test drive it. I drove around the area for about 15 mins thinking the reservoir would be half empty when I returned. To my surprise, when I pulled into the driveway, the car wasnt leaking. The reservoir was at the exact level as when I left the house. This morning, it was still full. I drove around about 30 miles today, including 15 mins idling while I ate lunch, and the car hasnt leaked a single drop. The car has fully cooled for several hours and not a single drip lost as far as I can tell. I inquired about any coolant leaks previously, and was told that one day they noticed a big puddle on the ground under the car while it was out of commission. What could cause a coolant dump while setting for months, continual leaks when it was idling, and now no leaks at all?
 
Sorta SWAG, but:

Heat makes things expand, so possibly a seal had dried and contracted enough to leak. A thorough heat-soak expanded it enough to seal. It may start leaking again if it sits long enough. I would go around the cooling system and check the tightness of all clamps - on the rad hoses, etc. Leaking from that area, probably the rad hoses. Make sure they are secure, not brittle, cracked, or bulged.
 
modern radiators are usually plastic end caps crimped to metal tubes. They corrode at the crimp points.
There may have (already) been some stop leak in there too & you distributed it by driving & heat cycling .
 
Without identifying the source of your leak it's purely speculation as to what may have happened. Watch the coolant tank and make sure it remains full and keep your nose open for the all-too-familiar scent of coolant. It'd be a shame to ruin the engine because of an O-ring or other miniscule item.
 
modern radiators are usually plastic end caps crimped to metal tubes. They corrode at the crimp points.
There may have (already) been some stop leak in there too & you distributed it by driving & heat cycling .

Besides where crimped, the actual plastic tanks themselves can develop microscopic cracks that open up when cold, but seal when warm or hot.


Rent/borrow a pressure tester and look at that Jetta coolant system. Anything that leaks typically doesn't stop on it's own(unless stop leak was involved as vw7674 suggested above). You may or may not have to move plastic cladding to get a good look at everything.
 
Get some dye and a UV light and pressurize the system, these things can have a lot of plastic parts in the cooling system that are prone to cracking and sealing surface failure.
Depending on the engine there may be aftermarket aluminum replacements for these parts
 
Was the outside temp cooler when the leak was happening? I’ve seen hose connections that only leaked in the wintertime.
 
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