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I've got a 95 Acura legend with cooling issues. I recently replaced a cracked radiator and the thermostat while i was at it, but the coolant is boiling before the thermostat opens so I can't burp the system. The temperature gauge on the dash reads just under half, but the system is boiling over.

My first guess was a busted head gasket, so I checked for exhaust in the coolant with a chemical block tester but it came back clean, I have no coolant in my oil, no white smoke/moisture in the exhaust and the cooling system also hold pressure like it's supposed to.

Any ideas? I'm at a complete loss on what to do next.

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    Are you sure you purchased the correct thermostat? Is it fitted the correct way round? Thermostats usually have a small hole designed to allow any air to pass anyway. – Solar Mike Dec 21 '17 at 8:09
  • Exactly, it is even recommended on some engines to drill a small hole on top of the thermostat. It will allow the trapped air to release and won't change the cooling characteristics. It also could be that the thermostat is defective. You can take it out and test it in a boiling water. You can also test run the engine without the theromstat to rule out its influence. – Jan Dec 21 '17 at 11:56
  • That's a great idea. I'll do that this weekend and update – superjax Dec 23 '17 at 4:39
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Turns out it was a bad head gasket. I tried Jan's advice and removed the thermostat, which kept the coolant from boiling. This made it a lot easier to take a good chemical block test reading, which came back with evidence of exhaust fumes. Darn...

Though I hadn't thought it relevant, I also replaced the EGR when I replaced the radiator because it had gotten clogged. However, there is this awesome post on the Legend forums that describes what I think happened on my vehicle. Turns out that Legends aggressively advance their timing (up to 10 degrees) and decrease fuel by about 5-10% when the EGR is active, but there is no sensor to detect EGR flow. So, the EGR valve opened, but there was no EGR flow, and detonations likely occurred at high RPM where they are hard to hear. This is likely what blew the gasket on cylinder 3, and the radiator. Crazy. Looks like I need to keep an eye on that EGR line in the future.

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