Chip / Fuel Map tuning?

[oil pan 4]

How did the original catalyst die? Did it plug up with carbon, ash or did it melt?

Lean burn is the way.
The Honda vx series used lean burn in the 1980s and got hybrid mpg without the hybrid stuff. NOx is only bad where NOx is a problem. For about 99.9% of the land area of the lower 48 crank up the NOx and up the MPGs.
Fuel injected cars are tuned to run as close as possible to 14.7:1 that's were the lowest NOx and lowest HC are.
Peak torque is achieved with about 12.6:1 air fuel ratio.
Peak Horse power is achieved around 13.2:1 air fuel ratio.
Peak fuel economy is achieved with 15.4:1 air fuel ratio, leaner mixes are used to reduce isobaric losses.

You can drive the same distance on 16 to 18:1 air fuel mix no problem and burn that much less fuel.

[Drifter]

Historically Toyota ECUs were nearly impossible to crack which meant aftermarket tuners could not rewrite fuel & ignition maps without changing to a standalone. From a cursory search, it looks like today some Toyota ECUs have been cracked, but I'm not seeing the Prius listed anywhere.

The problem with $150-$500 piggyback tuners is that they're almost universally against the law (they affect emissions in violation of the Clean Air Act). The EPA fined Bullydog $300,000 and forced them to perform millions of dollars in remediation. So there is a pretty big disincentive for capable people to make such tuners.

Of course, there are millions of people who want more horsepower and hundreds of thousands of them are willing to pay a fair bit for small gains so there is still a big carrot motivating tuners. And they can somewhat plausibly claim that their tunes are meant for off-highway use on race tracks.

There aren't a whole lot of people willing to pay a lot more for small mpg gains and tuners can't exactly claim that highway fuel economy tunes are meant for off-highway use.

Prius chat has a few DIY circuit modifications to fool the ecu into thinking it has warmed up sooner, but I don't know of any that attempt lean-burn. If I were going to attempt it myself, trying to shift the O2 sensor's output to a richer reading would probably be my first attempt. But I'd want to monitor everything else to make sure it isn't causing problems and I'd want it to turn off automatically at moderate and high loads.

[Drifter]

No sooner do I post that then I find this article from Julian Edgar about tricking ECUs out of closed loop for tuning. But this was for a gen1 prius with narrowband O2s (I presume Gen2 uses wideband):


https://www.autospeed.com/cms/a_2388/article

[oil pan 4]

Haha. The law. It's not like the prius is going to roll coal.

[Talos Woten]

Originally Posted by oil pan 4

How did the original catalyst die? Did it plug up with carbon, ash or did it melt?

The Prius line is notorious for burning oil. They burn 1 qt of oil per 10k miles when new, and it isn't officially considered a "problem" until it's burning 1 qt of oil per 2k miles. On my 240k miles car, it had eventually just burned enough oil to gunk up the cat. The mechanic actually cut it open and showed it to me... it was basically one continuous black carbon mess. You couldn't see any mesh, honeycomb, or even the side walls. It constricted airflow so badly it blew out the seal to the exhaust manifold.

So I replaced the cat, seal, etc. and installed an oil catch can. It still loses oil at ridiculous rates... but it's no longer burning all of it. I just dump it out of the catch can every oil change.

By the way, I think every car should have an oil catch can. An unexpected benefit is that it also catches a bunch of grit that makes it past the air filter, and metal shavings that comes from (gulp) places I don't even want to think about. Even if that would come out each oil change, it has to be better to catch it early so it doesn't recirculate in the engine itself.

[oil pan 4]

That sux. Yeah the only thing that's going to fix your converter problem is a hammer drill.
Luckily it blew the exhaust manifold seal and not a head gasket or burned valves.
There's likely still a concerning amount of oil going out the exhaust.

[Talos Woten]

Originally Posted by oil pan 4

Luckily it blew the exhaust manifold seal and not a head gasket or burned valves.
There's likely still a concerning amount of oil going out the exhaust.

I agree. The fundamental problem wasn't fixed, merely forestalled. We can even ballpark when it should crop up again. It took 240k miles for the first cat to fail. Integrating my rate of oil loss, if it all burned then I should expect the new one to fail in around 80k miles. But the oil catch can has significantly reduced the burnt fraction, I'd say by at least half. So I'm probably looking at another failure in 160k-ish miles.

I'm 40k miles into the new one, and looking at another 120k miles before I expect another cat problem. Realistically, the rest of the car may not survive that long. I'm already seeing the harbingers of other major problems (radiator, timing chain, transmission, etc.) So there's a good chance something else is going to catastrophically fail before the cat does. I mean... the optimal scenario is to try to get them to all fail at the same time, right?

[Drifter]

How often are you changing oil? How much are you burning between changes?

As you noted, Toyota low-tension rings sometimes get clogged with carbon and start burning oil. Additives and flushes can do a decent job of freeing up sticking rings, but if the oil return holes in the piston get clogged they pretty much have to be cleaned out mechanically (brush/pin vs chemical solvents).


Here is the piston from a 270K mile Highlander hybrid that still had pretty good cylinder crosshatching but showed signs of burning oil. This was taken after the piston & cylinder were soaked in WD40 overnight:


This next photo was taken after the piston was soaked in a jar of Kerosene overnight and then in jar of Simple Green the night after that. I also spent a few minutes brushing the ring lands with an old toothbrush before starting to use the old ring to scrape the carbon:


^Note that the oil return hole is still plugged even after I used a ring to scrape the carbon from that section of the ring land. Ultimately I had to use a needle to poke a hole through the carbon and then use a tiny pipe cleaner to brush away the remnants. Here is a picture from the underside of the piston of the oil return hole as it neared the end of the cleaning:


That engine had 4 holes per piston, 3 of which were 100% blocked. The 4th was partially blocked, but appeared to respond favorably to flushes/solvent soaks so I think there is some hope if you act before the oil burning gets too bad.

[Isaac Zachary]

Originally Posted by Drifter

How often are you changing oil? How much are you burning between changes?

As you noted, Toyota low-tension rings sometimes get clogged with carbon and start burning oil. Additives and flushes can do a decent job of freeing up sticking rings, but if the oil return holes in the piston get clogged they pretty much have to be cleaned out mechanically (brush/pin vs chemical solvents).


Here is the piston from a 270K mile Highlander hybrid that still had pretty good cylinder crosshatching but showed signs of burning oil. This was taken after the piston & cylinder were soaked in WD40 overnight:


This next photo was taken after the piston was soaked in a jar of Kerosene overnight and then in jar of Simple Green the night after that. I also spent a few minutes brushing the ring lands with an old toothbrush before starting to use the old ring to scrape the carbon:


^Note that the oil return hole is still plugged even after I used a ring to scrape the carbon from that section of the ring land. Ultimately I had to use a needle to poke a hole through the carbon and then use a tiny pipe cleaner to brush away the remnants. Here is a picture from the underside of the piston of the oil return hole as it neared the end of the cleaning:


That engine had 4 holes per piston, 3 of which were 100% blocked. The 4th was partially blocked, but appeared to respond favorably to flushes/solvent soaks so I think there is some hope if you act before the oil burning gets too bad.

Excelent post! I have the same problem with my Prius and am not looking forward to it. First I want to scope the cylinder walls to see if there's anything left to save. Then it's either pull it apart like in this post or look for another engine.

[oil pan 4]

I drilled additional oil return holes in the Pistons for my 6.5L diesel engine. The old 6.2L diesel had 8 holes and the 6.5 had 4. So I made more.