Hello! 2017 Gen 4 Prius 135,000km I recently had a very slow coolant leak. Eventually found evidence of coolant crust on the thermostat housing where it mounts to the timing chain cover. However, in the course of ruling out some of the other possible sources of coolant leak I pulled the plugs and borescoped for head gasket. To my horror, cylinder 4 was "steam clean" with very little carbon on the piston, but no evidence of any coolant in the cylinder. Fearing the worst I moved on to the rest of the cylinders only to find them all in identical condition - mostly clean with no evidence of coolant. All spark plugs in identical condition. Car Care Nut has a video where he eventually diagnoses a leak in the notorious EGHE, but originally suspects head gasket: Look up "Almost Misdiagnosed This Mysterious Engine Coolant Leak!" On the Car Care Nut YouTube (don't think I can post links here) At 8:15 he borescopes the cylinders and his reaction to the cleanliness says it all, but he goes on to find all cylinders in identical clean condition as well. What have other Gen 4 owners noticed in terms of cleanliness of their pistons? Is the Gen 4 simply a clean-running engine or should those of us with clean pistons be hunting for leaks in the ERG cooler or throttle body?
This is what washed pistons look like from a blown head gasket on a 2016. The head had been leaking between the 2 and 3 cylinder. The other picture is cylinder 1 with a borescope where there was no water intrusion.
Unless you have cold start misfires; I wouldn't suspect head gasket issues. That's a gen3 issue.. 2016-2018 gen4 has well documented EGHE leaks and there's a TSB on it. Don't know if it get covered in Canada. Ours is covered under the car's emissions warranty. If yours isn't covered; just loop the coolant hose to bypass the leak. Good Luck. PS; I don't know why your obsessed about the piston top cleanness; if they are all identical. What seems more reasonable? 1. You've got coolant leaks in all 4 cylinders or 2. Your engine is burning normally, without misfires? Your car barely has 84K miles on it; so I wouldn't expect a black piston top; unless your burning a lot of oil and the cylinder has been misfiring for a good long while.
Update from OP Car continued to lose coolant after the thermostat replacement and began blowing smoke on the highway at hard acceleration and occasional hard starts when ICE kicks in. Took car to Toyota dealer for a diagnostic to rule out head gasket and/or heat exchanger or EGR cooler. Specifically discussed the exhaust TSB. Toyota dealer had car for several days without conclusive diagnostic and suggested exploratory engine disassembly. Took car home and looped the exhaust heat exchanger coolant lines with the NAPA Canada version of the Gates 18777 (10777 for $27CAD) All symptoms resolved. Back to normal.
I would NEVER go back to that dealership because they obviously can't diagnose themselves out of a paper bag!! They don't know how to read an OEM TSB and don't understand ENGLISH on how to verify that???? I'd tell everyone I know to steer clear of that dealership...... The simplest way to verify this is to perform a coolant leak test. It should come out positive. Loop-out the EGHE and perform another coolant leak test. If it still leaks, the problem isn't the EGHE - If the leak stops; there's your answer. How can you get an inconclusive from that????? They are either incompetent or thieves and maybe both.... If you wanted to do a one step test, just disconnect the EGHE, plug one end and perform the leak test on the other end of the coolant hose. It really isn't that difficult, unless they're stupid or just a thief. Kinda of tough to tell them to STOP, when they have your motor apart for NO GOOD REASON; forcing you to spend more money there and probably breaking something else in the process.
