Very initial stages of investigation: Prius v at 250k+ miles. Coolant slowly going down. No noticeable white smoke. MPG doing fine, at least lately. Car started making a brief rumble or shake, becoming more frequent (maybe once per trip). Sometimes at startup and stop-and-go traffic. Now code P0301. What are the odds this is head gasket failure?
Pretty good. Do an egr block to eliminate a sticking egr valve (however egr would not cause loss of coolant) or move straight to a high quality articulating borescope when cold with the coolant pressurized. Car Care Nut HG Borescope at 660s Egr block flow test https://youtu.be/lFVNaDJ6HOI?si=lZjJK932oimbrx8R&t=238 At about 10:15 in the video he starts talking about the egr valve letting "air" into the intake manifold. He means exhaust gasses.
