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I chased misfires in my 240k mile 06' pilot AWD for 2 years, trying everything the forum recommended from new coils, to new plugs, to valve adjustments, to cleaning out everything known to man, to o2 sensors, everything - but could never get rid of the start up misfire I was seeing if the car sat for more than a day. What finally solved it and gave me the most direction? A leak down test. Super easy to do, tool is like $35 and all you need is the tester and an air compressor, plus a youtube video on how to do it.

In my case, the coolant was getting into the overflow bottle slowly and never coming back into the radiator, meaning the system was overpressurized from combustion gases. I never revved the car up enough with the radiator cap off to see what it was in fact combustion gases in the radiator. When I finally took the cap off and revved it up to 3k RPM quickly, the coolant shot out of the radiator like a geyser. Before trying this, a leakdown test confirmed my suspicions that I was getting air into the cooling system. While pressurizing the system, I could hear the coolant bubbling, pressure wasn't holding on the gauge, and putting the rad cap on with the leak down test connected then taking the cap off, created the same situation as when the car was running, air in the cooling system. Bingo.

The solution? I had to replace the head gasket, which is a really time consuming process, but not ultra difficult. The car now has power again and no more misfires. I never had oil in the coolant or coolant in the oil, never had any other signs a headgasket problem usually shows - just air in the cooling system. The head gasket had slowly deteriorated and lost it's coating, causing just enough pressure and air loss to create this issue. The head gasket wasn't compromised in places that would allow the coolant to enter the oil passages, the car never white smoked, may have burned just a little bit of coolant VERY slowly over 300 miles, but not a lot.

My thinking is this is a more common problem than what people realize and they are slowly losing their v6 head gaskets from wear and tear. A leakdown test will confirm this suspicion.
 

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Premium Member
2005 Pilot EXL- 227,700 Miles
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740 Posts
Great Find!!!!! Thank You for the Detailed Write Up & Sharing 😁👍👍
 
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