Keep an eye on that #5 spark plug. And listen for the "tick of doom". That ticking is a death knell for your engine.
A few years ago, with right around 90k miles on my 2008 Ridgeline, I was driving down the highway in rush-hour traffic. I juiced the accelerator to pass someone, and the engine responded with "fwup-fwup-fwup" sounds and no power. I pulled over as all of the dummy-lights came on. Once stopped, steam rolled out from under the hood. But according to the dash instruments, the engine wasn't overheated.
I popped the hood, watched the radiator leak its contents onto the pavement, and then started looking into the chaos under the hood. It took me a while to realize that the hole in the front of the engine was where the #5 spark plug used to be. The plug was dangling by its wiring, tangled up in the remains of the radiator fan. The radiator had a hole in it where the spark plug hit it. The radiator fan was missing 3 blades. The #5 plug had stripped out and ejected from the engine, and the radiator fan had cut into its wires and shorted out a bunch of stuff.
$2500 to get the engine torn down, re-machined, and put back together with a full change of plugs. Another $1500 for the radiator. The shop only had it for 3 days and prioritized the repairs, since it's my daily driver. (It's a dealer shop, but not the dealer I bought the truck from.) The "tick of doom" was gone. It hasn't returned, and my mileage is now over 148k.
All of the advice I found on these forums at the time was that you had about a 50/50 chance of this happening to the 6-cylinder Honda engines in the Ridgeline, Accord, and various other vehicles, and that the engine design has had this flaw since the late 1990's.
From what I can tell, it's a heat-related issue. The (hot) exhaust runs directly under cylinder 5, causing hot and cold regions near each other in the engine block, in turn causing deformations. The most common case seems to be that spark plug #5 wiggles loose (which is that "tick of doom" sound... the plug is jumping around in its threads and pressure is escaping past it), but it can cause all kinds of other problems in that part of the engine.
Another nasty surprise I learned from these forums all those years ago: Honda will void your warranty and/or blame you for the engine damage if you do anything to try to fix the issue beforehand (E.g. you tighten your spark plugs), and most Honda dealers will usually refuse to touch spark plugs or do tune ups before the scheduled maintenance interval (90k miles) for this exact reason.
www.ridgelineownersclub.com
