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Leak can not be found even in the head gasket

2007 Pilot leaks coolant but, mechanic can not find the leak anywhere including the head gasket. He was a certified Honda mechanic.

Find another mechanic. It’s been my experience that a bad radiator cap can cause the coolant to leak out through the reservoir. That’s the most common problem that I can think of at the moment and it can be hard to detect. The engine may be rock solid but the cooling system can fine numerous ways for the coolant to escape.

I assume you mean the coolant level in the radiator/overflow tank is dropping, but there’s no evidence of an external leak. How did the mechanic test the head gasket? There are several ways.

Most likely the head gasket is, in fact, leaking, but for some reason your mechanic couldn’t verify it. One easy test to do is wait til the engine is cold, then remove radiator cap. Start the engine and look for bubbles inside the radiator. Bubbles indicate exhaust gas blowing into the coolant through the head gasket. Another test is to take a portable emissions tester probe and use it to sniff the coolant. Hydrocarbons from exhaust gas in the coolant will set off the detector.

Or you can buy a chemical test kit that will indicate acidification of the coolant by exhaust gases.

One of these methods will likely indicate a leaking head gasket.

And if your mechanic can’t positively diagnose this, you need a different mechanic. The coolant is going somewhere, and it shouldn’t be difficult to determine where.

Besides radiator caps not holding pressure I’ve seen water pumps that leak a bit when cold then stop when it warms up.
Start the engine cold then take a close look at things under the hood as it idles.
The top radiator hose should get firm before it gets hot (when thermostat opens).

I suppose the first place the mechanic checked is the water pump weep hole. But maybe not.

jesmed1-both radiator and expansion tank levels have to be checked. The radiator coolant level will not follow the coolant tank level with a leaking head gasket.

kirkwood-was your mechanic a Honda mechanic prior to this diagnosis? X2 missleman, find a new mechanic.

I’ve seen a pressure applied to each cylinder in turn with the radiator cap removed used with success on my '04 Civic.

Oliver, Yes, that’s why I mentioned both the radiator and the overflow tank in the first sentence of my reply. Sorry if I was not clear.

Check the transmission fluid for the proper level/color.

If the transmission fluid doesn’t look right, there may be a leak in the transmission fluid cooler in the radiator.


What’s a radiator cap cost? Not much. How long to replace it? 5 minutes. Just do it.

Don’t forget a possible leak at the heater core. Of course this can lead to wet carpets and a funky smell from the vents. Even steam on the windshield.

Thank you all for your great advice. I will start with the simplest, which is to replace the radiator cap, then move on with the other suggestions if this is not the problem. I will post results. I really am hoping it is just the cap. Thank you all again.

Honda has had problems with cracked engine blocks. See Nov. 2014 Consumer Reports, p. 57, re: secret warranty on Civics 2006-early 2009, campaign 10-048. Could be that not only Civics had that problem. In any case, Honda dealers would be aware of this possibility.

Don’t wait to do what Tester said. If his guess is right, your transmission is being damaged by the coolant.

Your car like mine probably has a pinhole leak in the head-gasket. So small that it can’t be detected by said mechanic but big enough to cause the anti-freeze to deplete and thus cause overheating. Once you have ruled out the radiator cap, which in my experience it doesn’t sound like that’s what it is, although it can be. It sounds more indicative of a head gasket or possible water pump. My suggestion would be once you rule out the others if they aren’t the case get a head-gasket sealant. $60 a cheap fix for a head-gasket, but you must follow the directions very carefully.

The only thing I will add is that a head gasket diagnosis should be done carefully. They are often erroneously blamed for any low coolant or overheating problems and sometimes it is true; sometimes it is not.

I replaced the radiator cap and it has stopped leaking. Unbelievable that it was that simple. Thank you all.

The leak was probably in the form of steam or water vapor escaping through the cap. In that case there would be no sign of any external coolant dripping. Coolant has a distinctive odor, and using your nose is one way to tell if coolant is escaping in the form of steam or vapor.