Blowing coolant

Any evidence of stop leak product, white goop in the drained coolant?

 

p0420 can be cat, upstream sensor or downstream o2 sensor.

 

Couple of thoughts. P0420 is more than likely a bad CAT or bad O2 sensor. When you replaced the thermostat did you make sure the bleed hole was on top? Are your coolant fans turning on? When I replaced my coolant I had to massage the radiator hoses for quite a while to bleed all of the air out of the radiator cap and bleed port over a few days. I have a tech stream scanner and I freaked out watching the live data temperature climb after I thought I had bled my coolant. It ran right up to 250 initially quickly. That being said with my experience on other vehicles throughout the years usually when your blowing coolant its a head gasket issue. Sometimes you luck out and it's just a pocket of air. Likewise the head itself could be cracked. Don't want to jump to conclusions but I've gone down this road a few times. You can do an oil analysis for about $20 and that should tell you if any coolant is making its way into the oil system. There is a also test for that as well where you can take a sample of the coolant to check for exhaust gasses. It will pick up even the slightest amount if there's a leak. But you could have a leak in the head gasket that is just pushing combustion gasses into the water jacket and not contaminating the oil. Also on many vehicles a dead giveaway of a bad head gasket is when the upper radiator hose gets rock hard soon after starting the car. For this test you'd want to force the engine to run continuously in maintenance mode. There is a very good video on YouTube on changing a prius head gasket. You sound quite competent and it's not the nightmare of a job that you would think. Just takes some patience. But for a few bucks I'd look into the oil and coolant analysis. Hope that helps a little. Frustrating I know.

 

Good one Desert on the top rad bleed hole and the hoses.

It's such a pain to do a total drain on the ICE coolant loop. Some many valves in the system that work against you.

To lurkers I highly recommend to not do a total drain just dump the rad a few times over a few months time it works great no bleeding necessary and no airlock drama. It has a real good positive impact on the coolant.

The rad drain petcock in the on the drivers side bottom of the rad kind of difficult to get to as its behind some hoses in the way. Open the rad cap when dumping it. Take it eassy on the rad petcock its plastic. Not necessary to tighten it super tight.

To get to the rad cap take off the plastic shield on top of the rad and take the cap off the rad and fill it back up. Dont bother with the over flow tank it does nothing on a G2.
You can leave the screws out of the plastic cover just fit it back on and the hood when closed traps it. Screws have been off on mine for about 10 years. You want to check rad level by taking the cap off the overflow tank level is meaningless the rad could be empty and the tank full.

Its a good thing to check that rad level after a few miles after dumping it as it settles in. Always open cap when is cold.

I check the rad level and the oil level on all my cars every Saturday before starting them. That routine has worked really well for me for many years never any drama with my cars.

 

Since you are in California, you may want to check if warranty is still good for P0420. CA emmissions warranty is good for 180 month(15 years) and 150,000 Miles which you are within.

Otherwise since it is just 135K miles, you may want to try with oxygen sensors. There is a good chance that the cat is good.

 

You may want to try installing an after-market oil catch can between the PCV valve and the vacuum port on the intake. Oil catch can stopped the P0420 codes on my Gen2. Oil catch can functions by catching the the oil blowby, unburnt fuel and water condensation (AKA "Spoils"). Absent the oil catch can, the Spoils are run through the motor and ultimately the exhaust and catalyst. Catalysts do NOT function well with excessive Spoils run through them.

I have a thread on the install of my oil catch can. If you are interested, then see: Oil Catch Can from Ebay Installed | PriusChat

PS My analysis of my 1.5l and the cause of the blowby was: gummed/carboned oil ring at a minimum.

 

Not sure what I'm looking at. Is that some sort of coolant stop-leak product in some of the coolant passages? I've flagged a couple: