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2008 Toyota Tundra with a 5.7L V engine

I noticed last month that my coolant reservoir was well below the 'low' mark. I filled her up, only to find out last week that it was back to the same spot (about an inch above dead empty). What could be causing this behavior?

An internal leak would completely use the remaining coolant, then, when depleted, overheat my engine. But an external leak (a crack in the reservoir itself) would be obvious, and I would've noticed a huge puddle under the truck the next day.

I had the dealership replace a bad water pump two months ago, at about 80k miles, so that could have something to do with it. Before going back though, I'd like to at least diagnose and understand the issue. Any thoughts?

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    Have you refilled it more than the one time? Ay smell of coolant? – mikes Jan 3 '14 at 21:31
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    I was thinking along the same lines as "mikes" ... the system could just be "burping" itself - getting rid of air pockets in the coolant system after the water pump change. – Pᴀᴜʟsᴛᴇʀ2 Jan 3 '14 at 21:44
  • @mikes No, but no smell of coolant. Never heard of the burping phenomena, so I'll go with that theory...assuming it drains again, I'll take it in and respond with the diagnosis. Thanks – Rubber Duck Jan 6 '14 at 16:04
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As both 'mikes' and 'Paulster2' thought, the radiator system was simply burping.

It took much more coolant than I expected, ~2 refills to the middle of the coolant tank. Might have been some combination of the rapidly cooling weather coupled with the water pump replacement, but at any rate, coolant level is perfect now without any intervention other than adding a little now and again.

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