Don't Blow a Gasket About Leaking Antifreeze

Dear Car Talk:

I have a 2015 Honda Fit that has been losing antifreeze for about two months. It started with a very small amount, then got much worse.

My mechanic couldn't find a leak. But he did find that cylinder no. 3 is misfiring and making the car sputter sometimes. My mechanic thought it might have a bad head gasket, so I had it towed to the Honda dealer.

The dealer says they don't see any issue with the head gasket. They say it might be a bad fuel injector or burned valve, which, of course, cost $1,000 more.

What would you look for if it was brought into your shop? Thanks! -- Jaimee

First, I'd look at your credit limit, Jaimee. Actually, I'd be thinking along the same lines as your mechanic.

If you're losing coolant and it's not leaking out, it has to be burning up and coming out the tailpipe. That means it's getting into the cylinders somehow, where it doesn't belong.

What's the most common cause of coolant in the cylinders? A blown head gasket. The fact that one of your cylinders is misfiring also lends credence to the head gasket theory. If enough coolant is getting into cylinder no. 3, it could interfere with combustion and create a misfire. And the problem with the "fuel injector or burnt valve" theory is it doesn't explain your coolant loss. The head gasket theory explains everything.

So, what I'd do is look harder for a head-gasket problem. We'd put a pressure tester on your cooling system and then get the engine up to operating temperature. Then we'd shut off the car and pump the pressure up as high as we could get it, maybe double its normal operating pressure -- and hope the hoses don't blow. Then we'd let the car sit overnight.

If there's a puddle on the floor in the morning, then you do have an external leak, and you can locate it, fix it, then go back to looking at other causes for the misfire. But more likely, during the night, coolant, under pressure, is going to seep into one or more of your cylinders. And that's a sure sign of a head-gasket failure -- at best. At worst, it's a cracked cylinder head.

But let's not catastrophize, Jaimee. A head gasket is expensive enough. So, start by having a mechanic of your choice do a really thorough, overnight pressure test. I suspect the answer is going to come up "head gasket." Condolences.

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