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I have a 1999 Pontiac Montana Van. The radiator fluid is backing up to the over flow container.

So,what can be my problem?

I put new thermostat in and flushed the radiator.

Are there any additional troubleshooting steps I can enact to discover what the problem is?

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    It could be the radiator cap. These are pressure rated (16 psi in your case) and when they get worn they can allow it to overheat and/or bypass to the overflow. This is a cheap troubleshooting items since they are usually under $10 and can solve a lot of cooling issues. – CharlieRB Aug 11 '16 at 20:15
  • @CharlieRB - I'd put my $10 on a radiator cap first as well. A heck of a lot cheaper than jumping right to a leak/blown gasket. Good call. – Pᴀᴜʟsᴛᴇʀ2 Aug 11 '16 at 22:22
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More than likely it is a head gasket leaking or a cracked cylinder head.

You can use a combustion leak detector to find out if it is the exhaust gases leaking into the cooling system causing your issue.

https://www.amazon.com/Lisle-75500-Combustion-Leak-Detector/dp/B0007ZDRUI

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