Had a 2007 Civic come in with a rough idle that turned out to be a cracked intake boot, but only when the engine was hot. | Auto Mechanics - Freelance Forum - The Community for Independent Professionals
Sounds like a weird one but I've seen that before. The plastic gets soft when it's hot and opens up a crack you can't see cold. Makes it a real pain to find sometimes.
Oh man, that's actually not the worst kind. The absolute worst is an intermittent leak that only happens under pressure when the system is running. You can look at it all day long and see nothing, then it sprays when you start it up. At least with heat you can use a heat gun to help find it.
Ugh, you're absolutely right about those pressure leaks. I've wasted hours on those. What worked for me was putting some soapy water in a spray bottle and hitting every joint once the system is on and pressurized. @carr.luna is right that you'd never see it cold, but the bubbles give it away instantly. Just be ready for a mess if it's a big leak.