Fresh P0301

IMO, Very high probability.

 

Pretty good.

Do an egr block to eliminate a sticking egr valve (however egr would not cause loss of coolant) or move straight to a high quality articulating borescope when cold with the coolant pressurized.

Car Care Nut HG Borescope at 660s

Egr block flow test
https://youtu.be/lFVNaDJ6HOI?si=lZjJK932oimbrx8R&t=238

At about 10:15 in the video he starts talking about the egr valve letting "air" into the intake manifold. He means exhaust gasses.

 

You ever monitor the ECT while traveling? Hopefully the previous owner didn't put 'stop-leak' into it to cover-up that issue.Has that electronic water pump ever been changed?
If it turns out to be the HG; the engine may have been 'running hot' and popped that HG.

 

How long have you had it?

 

Normally early stages are very occasional severe rattles without coolant loss. I would do something asap because of possible overheating, warped heads, bent connecting rods or worse.

The famous "Hey Walt" video

 

From post #4

 

Prius gen3 engines do have an overheating light but it is often momentary with low coolant. Monitoring the engine temperature with an aftermarket reader is useful before problems exist to possibly see normal 185-205f coolant temperatures move up. Sometimes they don't move up to the light's 248f setpoint, which again, can be momentary and missed with low coolant.

No overheating code usually appears but a misfire code is common when you are past the early stages.

Water pump codes can be stored but there are some "impeller dragging" situations where water pump codes do not occur.

 

See that was previously disclosed, should read more carefully. Any 3rd gen or v, for sale with 220k miles, there's a decent chance it's being dumped with a stop-leak product in the engine coolant.

 

The Prius doesn't save or log over-temperature.
The water pump will only throw a code if it there's a dead short or open in the current pathway. So if it's intermittently stalling, you won't know unless you notice the temp lamp flashing. PS; there's no accompanying alarm or beep sounds when this happens - you just need to notice it.
A 3nd party HUD that plugs into your OBD2 port will remedy this.

 

There's a P261B code for when "Water pump speed is less than 900 rpm while the water pump assembly is operating (1 trip detection logic)".

In 2011 and later, there's also a P148F code for when the pump goes too fast. Seems like in 2010 the engineers didn't think "pump going too fast" merited detection logic and a code, but then in hindsight realized it's probably not going to go too fast unless, say, the impeller has broken off or the coolant is gone, and those would be things worth knowing.

Still, when they added the P148F code, they didn't make it light a warning light. So you don't get any heads-up when that happens, but you might see the code if you connect a scan tool.

 

Sometimes you'll get a false negative.
Do the bore-scope test. The best way to test is when the car is stone cold and the cooling system has been pressurized.
I know some people can't find the leak or say there wasn't a leak, BUT the engine was warm and the aluminum had expanded, so the leak disappeared. Not following basic instructions isn't the test fault, it's the tester's fault.

Good luck....

 

If you are talking about a combustion gas in the coolant test it can give you a valid positive or negative. However on these engines it has been known to give a false negative.

So if it's positive you have a leaking head gasket. If it's negative you really don't know.

 

IIRC it tests only for carbon monoxide in coolant, which is NOT the the way the 3rd gen head gaskets fail, at least not in the early stages.

It's typically coolant leaking into cylinders, and borescope inspection of the combustion chambers (preferably with cooling system pressurized to accelerate leakage), is the usual method.

 

The easiest swap will be a late model (2017-2021) low mile (50k) JDM gen3 Prius v/+/alpha engine as these come with accessories already installed and will fit properly - no wiring or coolant loop changes needed as typical when swapping in a gen4 engine. No reusing some gen3 parts on a gen4 engine to make it fit.

A primer on jdm engine-replacement.
A Beginner's Guide to JDM Engine Swaps – JDM Westcoast

In this case you are not looking for added performance in a Prius. You want an engine that does not burn oil and obviously has the latest head gasket, intake and egr valve you get with a recent model year.

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