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My 2007 V6 Tacoma is overheating, I have done just about any test imaginable, replaced the thermostat, water pump, did many other checks, incl. chemical test for gas residue in the coolant system etc. All tests come back negative. It looks like the head gaskets are not leaking.

The only other thing that comes to mind is that the heater core might be a problem. How do I test that?

  • What have you done with your radiator itself? Have you had it cleaned? Also, you may just check to see how much a new replacement costs. They are a lot cheaper than you'd think. – Pᴀᴜʟsᴛᴇʀ2 Jun 18 '15 at 22:54
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The heater core will not cause an issue with cooling. In fact, some vehicles use a shutoff valve to regulate the amount of coolant flow, which increases/decreases the amount of heat going to the core due to no heat to exchange. This is not an area to check for a cooling issue.

One way to test your heater core (without removing it) is to see if it produces heat when you put the selector to high. If it is plugged no heat will come out. If it is partially plugged, only lukewarm heat will come out. You could possibly check it by removing both coolant hoses (inlet and outlet) and blowing air through one side to see how it comes out the other side. That is a little more involved, as you need to drain the coolant down far enough to drain the core.

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