This is on a 2012 Plug-In, with about 248,000 miles. I was getting pretty bad rattle on startup, much of the time. Removed and cleaned out EGR cooler really well, new EGR valve, new intake manifold. Started up in Maintenance Mode, and it ran really smoothly, so I thought that seemed good, but then it just started rattling in the middle of a smooth idle. Rattled for a couple of minutes, then smoothed out again, then rattled again. Some steam out the tailpipe, and a drip. Compression was 185, 184, 180, 180, but have been getting moderate coolant loss. Applied compressed air to cylinders using the hose from my compression tester with the schraeder core removed, and moderate leak from Cyl1 to coolant, smaller leak from Cyl2 to coolant, and nothing from 3 or 4. Didn't seem to be leaking past the rings or valves, or into the oil, so decided to do a head gasket, or at least pull the head and check it out. Almost done getting the head off, but thought I could use some advice on this SLUDGE! Everything in the cam train has a coating of sludge. And when I drained the oil, cold, it shot out normally when I pulled the plug, but at the end, it was slow, thick glop oozing out. Oil not milky, though, and oil level was never rising while coolant level was going down. I'll go finish getting the head off while waiting for sage advice! (Or brutal chastisement!)
The "sludge", is coolant in the oil. Normally, on the Prius you don't get that unless it's really bad. Since you kept driving it, it just got worst. You likely have a warped head. Make sure the check it. Or take it to a machine shop so they can check it. If the head is not warped, or if it is, it's not bad enough that they can't machine it, and you re-install it, you'll need to change the oil after getting it to operating temp. And the filter also. That should get most of it out. Then check the oil after few miles and see if it's clear. If it's not, change it and the filter again.
Is this the same car that you added a stop leak product into the engine cooling system in a post in December 2024? https://priuschat.com/index.php?posts/3501992 If so, it is time for a replacement car.
