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!

 

Just a few updates before I go looking any deeper.

the cat I trust because of voltage readings, but, I ordered a Magnaflow speced for Priuses, just in case the cat was bad - I believe it’s reporting p0420 from the strange air fuel mixtures caused from contaminated combustion.

I know the gen 2 doesn’t have an egr cooler, but, its intake manifold has egr passages - could these foreseeably clog?

sealant and cleaner - I know these are suspicious, but I had to drive 500 miles back home to investigate this, so, I had to make it far enough before then. I intend to fully flush, clean, and refill all fluids today.

the car has 175k miles now, oil consumption is very minor at maybe .2qts per 1k miles

Combo meter just needed a capacitor, so, I did that myself, mileage is maybe off by less than 5 miles.
I mention the possible new converter because when I bought it it had an exhaust leak, I had that welded and the tech offhandedly mentioned they thought it looked like a poor weld from a replacement, and that the presence of a cat shield pointed at it being new. I haven’t looked inside with a camera yet, but, voltage readings seem correct from the sensors in the cat.

Coolant temps never reach beyond ~190f, I believe the engine is maintaining temps, but coolant burning is certainly happening on descents.


From here, I will get photos of each cylinder, and the cat too if I can. I intend to gather photos of the intake manifold too. Can someone provide some documentation for the intake gasket please?

Additionally, where else should I be looking for problems in this car?
Thank you!

 

I was mistaken thinking the intake gasket had coolant lines; learning a lot of information fast, overlooking things.

I’m convinced it’s a head gasket. I was under the impression that it was only failing under pressure, so was looking for places that could be building pressure, but in the end, I believe poor coolant maintenance had led to the degradation of the now 20 year old part, and that the coolant leaking is triggering p0420 codes despite a healthy looking cat - I snaked a camera into the o2 port and the thing shines like brand new, save for a single small carbon scar.

notably I couldn’t find my way into the a/f sensor to look pre-cat, and I do suspect some damage on that area (temps report at 1k at b1s1) though voltages from both sensors appear normal under regular loads.

I’ve flushed coolant and oil replaced both and installed a new pcv valve for good measure. Head gasket will be installed in some months, hopefully the car can be saved - I want another 150k out of it, 50k I put on was expensive and hasn’t made me enough money in return.

thank you all for the replies, this forum is such a great resource for this car and I’ve learned a lot just reading through the posts i find.

While I’m at it, what could cause a knock correction value or 21 and the other value being -3? Does this indicate anything to be aware of? A/F lambda is nearly 1 but p0420 freeze frames have both low and high short and long fuel trims - trims are level under normal load.

I can post scanner screenshots if anything sounds suspicious.


cheers for the info and thoughts!

 

that was welded shut when I bought it, just was putting out the condition I bought it in in case it helped indicate anything in its history