3 people with camping gear from Nevada to the Arctic Circle in Alaska and back. It’s my friend’s 2012 Prius (185k miles) and it has been great. Not a single issue with the car and this is a punishing route with the frost heaves, dirt roads, and potholes of Arctic roads. 46.7 mpg average so far. I’m used to driving my truck camper on long trips so the fuel cost is amazing.
UPDATE EDIT: It is the EGR valve. I unplugged it and misfiring stopped. This is an obvious diagnostic step to take but what complicated it was this car had three separate issues (leaking head gasket, bad spark plug, and faulty EGR valve). I had tried unplugging the EGR previously with no improvement when it was misfiring badly due to bad spark plug. I had thoroughly cleaned the EGR valve, cooler, manifold, and tube when I did the head gasket but apparently the EGR issue persists. I will edit this again when the car is repaired.
My friend’s 2012 Prius 171k miles had a misfire while driving at low to medium throttle. Smoothed out at high throttle. I know what a Prius misfire sounds like, kind of a knocking sound and the car stumbles briefly. No CEL. I checked cylinder #1 with endoscope while pressurizing the coolant reservoir and a drop of coolant appeared in the cylinder. It definitely had a bad head gasket so I replaced it.
Engine was pretty dirty inside, I cleaned it up a bit but definitely not spotless. Connecting rods were not bent. I cleaned out the EGR valve and cooler, intake manifold, replaced the PCV valve and cleaned the PCV hose.
When I turned it on after the head gasket it had a much worse misfire. Took it for a short drive and threw code for cyl #1 misfire. I shifted the spark plugs one cylinder over (and coils 2 cylinders over) and the new code showed cyl #2 misfire. So I installed new OEM plugs (old plugs were OEM and only a few thousand miles old, looked good too) and it runs much better but it’s back to where it started, a mild misfire only at partial throttle but not bad enough to throw a code. I did a leak down test on cyl #1 and it was excellent, haven’t tested the others yet. It appears the head gasket leak was not bad enough yet for it to be the cause.
I could start throwing parts at it like coils and or injectors but that is getting expensive. Any ideas or suggestions?