2010 Prius – Misfire (Cyl 1 & 3) / P0403 – Head Gasket or EGR?

How many miles or km on the car?

 

EGR, including intake manifold, ever been cleaned?

 

Unless the EGR cooler has failed and allows coolant to enter the intake system there is no explanation for the coolant loss other than a head gasket. Your white smoke on start up is an indication of coolant in the cylinders, most likely from a head gasket failure, but given that you don't mention having rough idle on start up it could indicate the problem is in the EGR cooler and it takes time to get to the combustion chambers.

Probably worth removing, cleaning, and testing or just replacing the EGR cooler (follow Mendel Leisk's advice on removval, etc.) to see if that is the problem before investing in a head gasket replacement.

 

Probably head gasket has failed though, and for some time, considering multiple cylinder misfire codes.

 

Sometimes different people use the term "leak-down test" to mean different things.

When I use it, I mean a way of measuring compression that involves a spark-plug-threaded adapter hose, an air compressor, and two pressure gauges separated by a calibrated orifice.

Is that the kind of test that "came back normal" here?

 

The engine coolant reservoir? Maybe he was pressurizing the coolant system, for a borescope inspection of cylinders??

Just from my flakey memory of videos: a leak-down test involves setting a particular cylinder's piston to Top Dead Center on the compression stroke, replacing the spark plug with a hose similar to compression tester (but without a Schrader valve?), hooking it up to a special gauge set (has 2 dials, and a knob or two), applying pressure (typically with a typical shop compressor), then watching for pressure drops (with the 2 gauges), and a fair bit of sleuthing, listening/feeling at the dipstick, oil filler cap, coolant reservoir, and exhaust pipe.

 

A 3rd gen with those miles, head gasket failure is very common, virtually inevitable. My hunch is it's tied to clogged EGR, but I'm supposedly jumping to conclusions.

 

Yeah, that's the other kind of test people often call a leak-down test, though that tests how well the cooling system holds pressure. The other kind, with the air compressor and two gauges, is a test for how well the combustion chamber holds pressure.

Neither one is an unreasonable test to do if there's a suspected head-gasket issue. They're just different tests, so it's worth clearly saying which one was done.