The thin or weak piston rings on the early gen 3 Prius hybrids - a theory.

So I obtained a lot of useful information in the last week here, esp. pertaining to the dreaded 12V auxiliary battery issue! I finally got there, got a new one fitted , easy as usual once you;ve done it, and got a little item on amazon to keep easy tabs on how the battery actually is…it’s called a ‘kewig car charger’ something or other. 2 usb3 charging outlets on it and an on/off, and it’s a dial so you just glance down, look for the green lights and you know it’s good.

Battery showing 14.5 V now, the old one stuck,at 75% charging forever, while gassing fit to bust, and limped up to 12.5 V max.

Anyway, my theory on the broken piston rings that I hear the early gen 3 suffers:

Apparently all the Prius cars have an ‘Atkinson cycle’ engine. This only matters this much: that it means this engine is often at low-loads and speeds of rotation.

We already have cars where we are somewhat divorced and only indirectly able to affect the ICE starting and stopping, and what rpm and how hard it works, are done at arm’s length from the driver.

There are also people issukng dire warnings about driving your Prius ‘aggressively’ and so on. I read this with a degree of nervousness, having blatted up a large UK roadway home from my purchase in the south of England, touching 100mph once. Obviously, this was a private section of this big road.

Now, I had a motorcycle, where for emissikns strictures, the manufacturer hobbled the engine something awful. It ran badly, how this helps emissions I have no idea. I bought a lot of carb cleaner too, and that stuff is toxic. No, this was silly. I found out what was up - Yamaha fitted a too-small pilot jet, which for those not into stuff like this. means the fuel was going to run very lean almost everywhere in the rev range.

The symptoms were it was knife-edge whether it would go over 50mph. In cold climate, no, so you overtook and got a nasty surprise a soon as you pulled out. Emissions measure - worth my life? No, it’s not Yamaha.

Then I fixed the issue it took a couple of years because I couldn’t believe Yamaha would sabotage their own bikes. But they did, and then told me in a letter, they had no way of helping me.

I changed the pilot jet pne or two sizes up, the bike ran sweet as a nut, and went from 45mpg to 73 mpg !!! Now, THAT is an effective reduction of emissions, it has to be, and I no longer risk life overtaking.

Why is this relevant? Because the stupid idiot who sold me the bike, refused to tell me anything about this fault, he just palmed this badly-running machine off on me, dangerous though it was. And, he made things harder, by having even changed the sprocket sizes to lower the gearing overall…because of course, this thing was weak starting off, lacking in power everywhere.

This brings us to the nub (I know, finally). This engine at 6,000 miles was therefore not even run-in properly. What I found immediately on revving it high, was it made a ‘tinkling’ sound. Now, with repeated hard caning, constantly and regularly hammering it in all the gears, the ‘tinkling’ began to diminish.

What was the tinkling sound at high rpm? It’s a thing that happens to the cylinder walls where the rings wear it. Obviously, if you only run the engine gently, because of driving style, or in our case, not being able or recommended to thrash it, the wear only reaches so far up the cylinder.

Yes, every piston ring reaches its highest point on the cylinder wall, at tdc, top dead centre of the crank position, this is as high up as the piston and rings reach, before being forced back down by the gasoline burning and expanding. But there’s ‘top’ and high-revving ‘top’ it turns out.

When you absolutely thrash an internal combustion engine, those pistons and rings, reach ever so slightly further up the cylinder than when your drive gently. On the Yamaha 600cc motorcycle, we are talking the difference between 5,000 rom tops, and 9,500rpm tops remember.

As a result of a low limit on max rpm being used at run-in of an engine, the ridge that develops way up at tdc, is a little lower by a few thou, or fractions of a millimetre, than when you thrash the engine hard. The point where, the wear has taken place, gives way to the point where, there is no piston ring that shall ever reach up that high…

And that’s my theory. I got my bike, fixed it at 6,000 miles or so, and thrashed it for about 4 months thereafter. Once I realised it was not going to break, and the tinkling sound was diminishing the more I ran it hard, I kept going, giving the bike bursts of full throttle and rpm, sending those piston rings that smidgeon further up, forcing wear past that minuscule ridge line at the top of the cylinder walls.

So, that worked out.

Now we turn to our relatively plodding but totally lovable Prius Atkinson cycle and-you-shouldn’t-drive-it-hard, 1.5 or 1.8 gasoline engine. Imagine, it’s got these slightly thinner piston rings, and they are MOT made for the giddy heights of 9,500rpm, and the engine has done say 20,000 miles, and has been, apparently, run-in.

I contest, it has not been run-in at all.

There’s a wear-ridge way up the top of those cylinders, and it’s waiting for the day, even an affable Prius owner, is late or enthusiastic, and canes the living daylights out of the plodding Atkinson-cycle 4-cylinder ICE in their Prius.

Then, as you all know, the calm whirring gives way to a rather throbbing roar, and with foot flat to the floor, on a major highway, you can make that Toyota gasoline engine, fairly buzz along. But if you have not been doing this, pretty much in short bursts at least, from quite early on in the car’s life, you are going to get these skinny lightweight piston rings, smack into that slightly-too-low wear line or ridge, way up top of the cylinder walls.

If you are lucky, you might hear a tinkling sound like on my Yamaha, but I doubt it comes through the soundproofing myself.

If you do this in short bursts like I did on my motorcycle, then you may well simply run the engine in properly, as I did on that two-wheeler once I fixed the carb jetting.

But the catastrophic possibility, is that say you are the first person to thrash this engine, in let’s say 60,000 miles or something, that ridge or wear line, is actually kinda deep and not very high up at all ( we are still talking thou though).

Skinny piston rings, we might call them fragile even if we’d handled them (I have snapped a few in my time), meeting a ridge of cast-iron, which we might call a sudden reduced diameter of bore, at say 4,500rpm, is sometimes not going to work out.

The engine instead of wearing-in, has too much of a ridge line to do that, and the piston ring smacks intl that slight reduction in bore way up top. Fragile meets cast-iron ridge at speed. No surprises really. It can snap a ring easy, even non-skinny rings.

So that’s my theory. That if the engines are properly thrashed a little, from early on, the engines do wear-in properly, and it all works out, the wear-line or ridge high up on the cylinder walls, is kept small, and we imagine perhaps, even graduated in a ramp, because we’ve thrashed the engine to differing amounts, over the course of say 50k miles.

I think there have been some cars with the slightly skinny rings, where they have never really been thrashed until they get their second owner. Or the first owner, doesn’t know about this technical phenomenon, or why running-in is still a relevant thing in the 21st century, and drives kind of normally, except this one day, in a hurry, unusually, they put the foot down only after already covering 40k miles or so, at sub-3,000rpm.

PING!

That’s my theory. Other theories may well have merit too, but this is based on what I know of ICE, and what happens if a low rpm ceiling is held to, for whatever reason, meaning there has not been a wear-in of the engine, across the full possible speed range.

So my advice contravenes a lot that’s said. I am saying, thrash it sometimes, from early on in ownership. But if you never did run it to full rpm much at all, and it’s an early skinny-ring model, maybe stick to that or very slowly, raise the rpm of short-burst high-revving, a little at a time, for short few seconds, graduating to higher rpm.

It’s a problem, but it can be addressed, up until your engine has created such a ridge inside the cylinders, that it really is too much to expect any careful further running-in to be possible. I have to put my money where my mouth is here, same as with my Yamaha.

Short of taking it apart and grinding the ridge off up top, the only method is to coax the engine by short bursts of induced high-rpm running, to move that wear further and further up the cylinder, a few thou and a few seconds, a few hundred rotations or reciprocations, at a time. Keep wearing into that virgin cast-iron territory until there’s no way the piston rings will ever go any further up.

Cannot advise this is a good idea, because I don’t want someone who is perfectly happy plodding, to smack their accelerator/gas pedal to the floor and bust their piston rings, trying to fix an issue that for them, never was an issue.

That would be very unfair of me, to make someone else wear the consequences of my so-called ‘solution’ to an engine not being ‘properly’ run-in!

Anyway, you do you people, don’t do what your gut screams ‘no!’ at. I am just explaining a theory, and my theory definitely includes a health warning: trying to resolve the possible issue, could result in an expensive repair and your much-loved Prius sat out of commission too for a bit.

Which would make me stupid, not smart for having a theory worked-out but dumb for encouraging damage to someone’s pride and joy. So don’t feel pressured into something, FGS. I merely put the theory or explanation forward, and do underline, it’s got a 50/50 outcome of ‘broken’ which wouldn’t be your fault because frankly, I could have broken my Yamaha.

6,000 miles, is probably quite a wear ridge. Maybe, a few years older and wiser, I’d have settled for running below 5,000 rpm forevermore. Instead of bouncing the valves and hearing that slowly-diminishing pinging/tinkling sound.

Take care all and don’t follow someone else’s slightly risky ideas, when it’s at YOUR cost. Anyway, despite the dire warnings, back to the positive, I understand a bit better after a fortnight of ownership, how people can get quite fond of their middle-of-the-road hybrid. I’ve often been in the middle of the road myself!

irm.

 

Italian tune-up! Quite.

On the voltage, I did wonder if it was the charging circuit…ok, I will measure with the car powered down, and at the battery terminals, for my own info.

Cleaned the EGR circuit - heck no. I cleaned my Passat diesel EGR once, and my Volvo 5-pot diesel once. This Prius only has 43k miles, seems genuine by all measures I apply (stone chips, almost zero I can find, and pedal soft rubbery covers, barely worn, seat edges, not wrinkled or frayed, steering wheel surface intact, etc etc). Lowest mileage car I ever have bought. But, it has been neglected for a year I would say, since the guy selling me it, had it. I think I have deduced that the brake disks are prone to rusting to death. It seems many owners are able to almost entirely brake with the Regen system alone. So, I suspect a degree of seizing-up and not-bothering is in play with my ‘new’ car.

I only knew of the EGR on this car because of this forum I think. I thought only diesels had them. I knew hot exhaust gases got fed back in but not that it was now done this way on gasoline/petrol engines. I am expecting stuttery behaviour if EGR is clogging I believe? So, I will be aware in future but I won’t look otherwise.

Thanks though, for the voltage point. I do know this, but been spinning a bit since buying the car last week when it all went horribly wrong. Now ok, seems past the worst.

 

Yes, apparently I should make sure not to buy the exact car I’ve bought, for these reasons. Situation normal for me.

 

Thank you so much for the welcome - and well, that’s a guideline in there as to what’s considered best practice. Completely agree. Always have agreed with the need for brevity. The minimum sufficient words consistent with fully describing the situation is how one lecturer I heard put it.

Everything Prius is all new to me and I felt quite enthusiastic to compile a brand-new thread.