Prius Prime cylinder wash-down potential issue

I don't know if this is related to the topic of this thread: I use my Phev 90% of the time in electric, so the ICE engine may stay off for as long as two-three months. When it starts again it runs very rough for 4-5 seconds. I told the mechanic at the dealer and he suggested that a very small amount of oil remains at the cylinders walls....so he suggested to run it every now and then.
My question is: if we assume that the start is the critical phase, wouldn't "starting it every now and then" just make things worse?

 

Possibly, in the sense of increasing total amount of wear. But it might reduce the number of rough starts, which is what you were complaining about.

I think you're probably onto a loser here of trying to guess what the best thing to do is. Toyota will have actually done real-life analysis, which will beat our intuition and guesses. I think you just have to trust that their engine management will be as reasonably conservative as everything they do, and that it starts the engine enough.

No personal experience here - I doubt I've ever gone more than 3 weeks without starting the engine - there's a fairly regular bi-weekly trip beyond EV range. Never noticed any particular issues.

 

So far ICE has never automatically started without the traction battery being depleted, so, limited to my experience, no strategy seems to be implemented to prevent any wear.

 

My recollection was that it did have a function to run the engine periodically, but it was set out at the 6 month range. But I can't find a reference for that now.

Only thing I can find in the manual is a note on refuelling sufficiently:

 

That recommendation on refuelling would imply to score at least 20 l/year * 15 km/l = 300 km/year, which I manage to do regularly, the only difference is that I do it concentrated in my holidays (1 month in the summer). So I don't think I'm within specs.

I hope you are right and I'm over worried, but the that noise and vibration really scared me.

 

How so? The fuel-use-over-12-month spec (user instructions) is separate from how-often-to-start-the-engine (managed by the car, I believe).

BTW, found this older thread discussing the same issues in G4:

Can gas go stale in the 2020 Prius Prime hybrid plugin? | PriusChat

 

Well striclty speaking you are right, but i suspect that that recommendation is to be intended as a "distributed" minimum consumption.

 

Why? It's unrelated to how often the engine starts. It's just about fuel going stale.

They are so specific about "total over a 12 month period" that they make a special effort to restate it like that - rather than something more evened out like "5 L every 3 months", which they could have said if they wanted something distributed.

I can only infer that they don't want fuel to go older than 12 months - so make sure you've put in enough new fuel in a 12-month period that you don't have just 12-month-old fuel.

I guess if you do do it all in summer, you are pushing towards that limit, with 11-month old fuel, but you're still under it.

And if you refuel after 11 months before you start using fuel again, so you don't actually use that 11-month-old fuel, that would avoid most of the potential issue.

 

Must say though, it's a bit different here.

I can go a whole week without the engine ever stopping in EV mode, with the temperature below -10°C. The ability to not run the engine for 11 months is a bit fantastical

 

We don't know exactly why it is running rough. Maybe using a higher viscosity oil will result if thicker layer of oil left by the time you use the engine, if that is the cause. Or perhaps there are deposits in the injection system left by evaporated fuel, or even just air bubbles from that.

 

My 213k mile Prius C that was used in LA traffic as a commuter for the first 138 k miles doesn't use a drop of oil between 5k mile changes and the oil is barely discolored. at over 12 years age, almost clean enough to sell it for new (not that I would). The symptoms described by the OP are going to oil contaminate almost any engine going back to my 1976 280Z. If you started it cold repeatedly and did not actually drive any distance it would foul out the plugs and there was nothing wrong with the engine.
In extreme cases I would smell the customer's dipstick and if it smelled of gas, I would light it on fire with a butane lighter and show the customer why they needed to not drive it that way or they would wreck a perfectly good engine. That's a 50 year old fuel injected car, doing exactly what is described by the OP.
I also had a 1937 Ford that you could crank with the ignition off. With the cracking speed at 100 RPM (6 volt system) it would get 60 PSI oil pressure within 2 seconds of cranking time. I pre primed the oil doing just that to keep it from starting without pressure and it had NO oil filter, a slinger for a real seal, and a tube that blew crankcase fumes out into the atmosphere.
I think that the issue is non existent unless you drive the car PERFECTLY WRONG and never allow it to warm up.
Modern hybrids are trading high operating engine temperatures for efficiency,
When I was driving my Echo and hypermiling it coming back home on a 55 MPH road and P&Ging the oil would not even be hot enough to bother me at all when I changed it when I got home.
It would take 5 miles for the thermostat to open on that Echo and in extreme cases like when I got 167 MPG on another non hybrid car I owned the coolant temperature would fall off over the 3.3 miles on that drive since the engine was only running 36 seconds out of the 210 seconds it took to drive the 3.3 mile test road.