Diagnosing Coolant Burning

Vehicle History
Starting from the beginning, I bought this Prius at ~125 miles with a p0420 code.
A new Hybrid battery was installed just before I bought it, and a converter may have been replaced at some point as well.

Eventually, I got curious to the p0420, and started digging.

My scanner reports regular voltages for the converter, although relatively warm operating temperatures. Cat and cylinder cleaner were run through to clean things out. I have reason to believe the cat is fine, and the sensors are fowled or detecting strange behavior.

I noticed that on the car fax, the vehicle has oil changes only every 25k miles at a dealership. Perhaps there were others, not at dealerships, but, I believe poor oil changes were consistent in the first 125k.

So, this brings us to the ‘Coolant Burn Event’

Coolant Burn Event
This vehicle frequents Pennsylvania, climbing and descending mountains frequently. Upon a descent, preceded by a slowly worsening exhaust smell over weeks, coolant began to burn and exit the exhaust. Sluggish acceleration and other issues were noted but seemed to self remedy, and when looking at the scanner, a high knock correction value was noted and the freeze frame for the event notes lean conditions.

When this occurred, coolant was added with gasket sealant and an oil top off with cat cleaner.

Oil and coolant are not mixing, no [consistent] misfires fuel trim self corrects and is running stoichiometric.

So what’s happening here?

Simplified symptoms:

• Sluggish acceleration on steep inclines
• Power loss
• Coolant burn on steep descents
• Lowered mpg
• P0420

My running theory
Oil maintenance was neglected for far too long, too regularly, creating sludge and carbon deposits within the cylinders, crankcase, PCV valve, EGR passages, and converter. These deposits created pressure on the intake gasket via clogging the PCV, which led to a leak of coolant through the intake manifold gasket. This event occurs regularly under high vacuum, during descents, and oil is not contaminated, which seems to point towards coolant entering the intake gasket rather than the head gasket.


The car has a brand new actuator, upgraded combo meter, new hybrid battery, and I believe a somewhat new converter.


At this point, I would like to replace the pcv valve, intake gasket, spark plugs, and drain/clean oil and coolant systems.

my questions are, firstly, does this appear to be a sound assessment? I am going to investigate tomorrow to see, but, I was having a hard time finding information about replacing the intake gasket on the gen 2. I have done some repairs before, but am not intuitive enough to know how to find this part myself, nor the torque specs or bolt ordering, least of all best practices.

I was hoping for some guidance, I have access to a mechanic for labor if anything exceeds me, but wanted to know if I was on the right track and where to go from here for instructions on the intake gasket.

thanks everyone!