Apparent "lugging" under slight load

Have you done any maintenance on it? First thing I would check is if there's coolant in your oil. Then I would pull the spark plugs and see what they look like. And after that, maybe remove the intake manifold, clean EGR circuit. While you're doing all that, see if there's evidence of a bad head gasket. That's probably the cause of all engine issues with a 300K+ miles Prius Turd generation car.

 

Well there was another mechanic just like you with 40 years experience, bought a 300k miles plug in Prius with the same issue you're describing. Needed a new engine, discovered after removing the intake manifold and seeing lots of coolant.

You unfortunately in your 40 years haven't seen a turd gen Prius or encountered it's problems.

 

You can't say it's strictly a drivetrain train problem and have no idea how the drive train actually works...

You say it has nothing to do with the EV side, yet the ICE is fine and the problem is the drivetrain? There is only one interface between the ICE and the rest of the drivetrain and EV side of things.

I'd start by learning how the whole drivetrain works and then start making educated comments on what has been ruled out or not. You can watch the Weber auto videos to learn more.

Having 40 years of mechanical experience means little. Some people have 40 years of 1 years worth of experience 40X over. It doesn't necessarily mean ne continually grew and learned over those 40 years. The Prius is new to a lot of people. Maybe not dismiss what those who know more about them are saying, especially when you don't know how they work yourself...

 

replace the ecvt

 

Has the Exhaust Gas Recirculation System, and intake manifold, ever been cleaned. I'd suspect it has, with those miles, but maybe too little, too late. Who knows. Try a leak-down test?

More info about EGR cleaning in my signature.

Just editorial:

 

Youtube, nice. Why did you come here if you don't want to take the advice of people that actually know this car? You haven't even looked into the engine with 360k miles, haven't pulled the spark plugs, No leak down test, haven't determine if the super common head gasket failure is there. There are 100 cases of head gasket failure for every 1 transaxle failure on this Prius generation.

 

no, but at 360k, it doesn't surprise me

 

Pretty much one in the same

IMS bearing is more of a big deal then the EGR issues on a Prius.

Also, at 50mph, the engine is gonna be 1200-1600 or so and running on an Atkinson cycle. That feels and sounds totally different then when you are hard on the throttle and it's running a more conventional otto cycle. The car is going to try and go to the lowest possible RPM for that road speed if you aren't asking for a lot of power and the rpm constantly fluctuates. That's also a possibility that's what you're noticing if you are new to driving a Prius. I found that very disconcerting when I first started driving one. To me, a change in rpm at a constant speed either meant a slipping clutch or the torque converter just unlocked for some reason. In a Prius it's just another day.

 

" it only happens under a slight load under about 50 mph." You mean like around 46 mph?

When the ICE (engine) stops, the RPM really does go to zero. With the Classic & Iconic models, that threshold is 42 mph. With the 2010+ it's 46. With the plug-in (while charge is available) it's 62. The carriers in the PSD (power-split device) simply rotate around the stopped component. Above the speed threshold, the engine pumps without consuming any gas.
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See number 2 from:
About that clutch | PriusChat

 

Don’t understand; how many miles?

Lot of interesting info btw. That damper twixt engine and transaxle does take a beating; there’s been a few failures. There’s a plastic piece on the underside, at the junction; believe it can be removed for inspection.

 

You might have stumbled on one of the very few cases where that "brilliant tip" actually tells you something useful.

A lot of the mentions of it on PriusChat fall into the "you didn't really think that out, did you?" category:

1. People who think they have a too-little EGR problem because their cooler is plugged, and try to find out by unplugging the valve control, which gives them a too-little EGR problem ...
2. People who think they have a too-much EGR problem because the valve is sticking open, and try to find out by unplugging the valve control, which doesn't magically close a stuck valve, giving them a too-much EGR problem ...

Your decision to pull the intake is probably sensible, because there are four small EGR passages in the intake manifold, one to each cylinder. If those four passages are clogged in different amounts, then you could see symptoms like you've reported, where things get rough just under moderate load (which is when the ECM will be calling for EGR). The existence of the problem tells you the valve does open and close and the cooler and piping are allowing EGR to flow, but the engine roughens because the EGR flow comes to the cylinders in different amounts. If you didn't get the roughening when you prevented EGR from happening, the intake is where I would look next.

 

Why a lot of head gasket failures commence at cylinder 1:


(A little hard to tell, but looks near-totally blocked.)

Still wondering about that. And the miiles on the odometer, but can kinda judge by the intake condition.

I'd turn to the EGR now. Judging by your intake condition, for the cooler I'd try protracted soaks in hot, concentrated Oxi-Clean solution. The simplest way to accompish this is to cork one end. This info will help size the cork:

Intake opening diameter (at exhaust): 20.9 mm
exit opening diameter (at EGR valve): 25.7 mm

Don't use Oxi on the EGR valve, or the intake btw. Brake cleaner is sufficient, and mechanical cleaning (brushing, scrubbing) is more doable, and the Oxi can react with various metals. The cooler however, is stainless steel.

More info in my signature.

 

Points for new gasket color matching the hood emblem!