Attention Prius Owners, I believe I have discovered a previously unknown Mechanism of Low MPG.

Is this all just because it isn't easy to get normal fuel economy in a Prius used for rural mail delivery, presumably with climate control running and a window open?

 

Is that lawsuit for the inverter ipm module?

 

According to the tables you posted at the top of post #79, I tick all the boxes for the max mode and none for the weak mode. I think you're misunderstanding what I said by taking a small part out of context. Or I have just been too economical with the detail. But no matter.

The reason for the question is my observation after I've done a HV battery recondition the car is a lot more responsive and the acceleration from the battery's contribution is noticeably more positive. More oomph, you might say so thought it might be something.

I'll continue to observe the progress in this thread with interest.

 

Weak ev? Sorry I meant WEAK EV since it seems to be a named conspiracy . At least gen2 owners have a failure mode theory to discuss as others have with the gen3 egr hg hypothesis.

However most of us who pay attention to mpg in their gen2s and gen3s, including Mendel, have never seen a prolonged 10 mpg drop, much less one caused by disconnecting the 12v.

In my case more than 15 years and 400,000 miles of Prii combined. Of course they were bought new which may provide a special mpg mode which is disabled for used buyers through hidden telemetry triggered by title change monitoring. You know, Industrial members of the Deep State* getting into our business.

Even when my gen3 hv battery was on its last legs, I lost maybe five mpg which was instantly regained with a new oem battery.

By the way, Dr Prius is a poor hv battery health check. It will show acceptable until a blind man could see hv battery failure. It may give a fair instantaneous snapshot of capacity but is not great on expected lifetime.

There are rare reports of ridiculously bad 12v batteries forcing continuous high amp charging. This in turn has been reported to cause significantly low mpg. It must take a strange set of failure modes to allow a 12v battery to Ready the car but still draw heavy amps without smoking itself. It is also easy to check.

*Deep State: Members of the government particularly in the FBI and CIA working with financial and industrial leaders to exercise power.

 

Nice work. Very well illustrated and explained. I like the EV+engine audio.

Seems odd that it's taken so long to document properly, but I guess getting stuck in this "weak EV" state is a very rare problem. All the normal behaviour (like the Stage 1-4 stuff, and the SoC/hysteresis of the bars) has been documented because there were enough obsessive people experiencing it to do digging like you did here. A few people have hit it, but not a critical mass to actually form a community.

BTW, I always liked the fact that the 8 bars of the display did not seem to be arbitrary - the bar boundaries seemed to denote different hybrid heuristics, and I fully recognise your "weak EV" as akin to the normal 3-bar behaviour, where the electric HP becomes limited.

This "weak EV" behaviour is a mid-point between "normal operation" and "buggered battery" operation where it almost totally avoids the battery. The latter would raise a code.

This behaviour is presumably a normal calibration stage which it's not supposed to get stuck in, which is why there's no error code corresponding to it. But it would have made sense to raise an error code if it didn't leave this stage after a certain time.

And as you say, this is all perfectly rational - either some sensor problem means the ECU doesn't see what it needs to see to finish calibration, or a flaw in the software means it can fail to make the transition despite good inputs.

Do you believe it has been fixed in later generations? Is there any evidence of it in G4? It looks like a G2+G3 problem from your PriusChat trawling.

 

or maybe a weak battery

 

So glad I came across this...was averaging 49mpg mixed city/hwy Winter driving, 54mpg summer. Got a code in October, realized my air/fuel bank 1 sensor was throwing too high of voltage and replaced it. Disconnected 12v overnight to relearn the new fuel curves, and 500+ miles later I'm now averaging 33mpg. The odd thing I noticed is that my battery is now at 7 bars most of the time in city driving instead of 5-6, but I've been pulling my hair out trying to figure it out. So should I reset the ECU again and then do some long 65+mph cruise control drives for a few days? How long does the drive need to be? What about throttle %? Should it be gentle or aggressive?

 

An interesting observation in the torque pro app earlier today (now that I know what I'm looking for): sudden pedal depression up to 59% will create a brief surge of 50 to 60 amps but then immediately drop down to 15 amps or lower...at 60% and beyond amperage will dynamically fluctuate but stay above 30 amps fairly consistent. 100% pedal depression will draw a full 120 amps. So it seems like whatever the ECU is doing to reduce demand from the electric motor is tied to pedal position.

 

Another thought...I used Dr. Prius to clear any logged engine codes (after the 12v reset) in the ECU just to make sure it wasn't holding onto something that would affect the air/fuel map...I wonder if sending a "clear codes" command during the ECU learning phase screwed something up in the programing?

I wonder how many people experiencing this issue ALSO have the Dr. Prius app and have used clear code commands?

 

@TheLastMojojomo

I found this when searching for ECU reset procedures, this guy had a problem with his gen 3 storing brake ecu info DESPITE a 12v reset...turns out there is flash memory. This is the procedure he had to do to clear calibration data and get it to recalibrate, not sure if it would be the same procedure for gen 2, OR if it only applies to brake calibration, but it appears there is an actual process to do this.

 

The best advice for anybody in that situation is to start by saying what actual brake codes are being stored. Just saying there's some brake codes being stored doesn't cut it. One procedure no matter what the problem is? It's like your doc giving you one prescription without asking why you came in.

Not all of the codes are stored in nonvolatile memory; some of them will go away if power is lost (not to say they might not come right back, if the problem's still there!). Others are nonvolatile, and won't go away at all until the specific right thing is done. For example, a Gen 3 showing C1451 won't give that code up until you do a brake bleed (and also won't let you do the calibration steps, either).

For the win, find out what the brake system is telling you about, then proceed accordingly.

 

This isn't about the ABS system, its about the fact that the ECU DOES store certain things in flash despite disconnecting the 12V.

It's possible some of the calibration code on how to engage the electric motor (or potentially a limp mode of some sort) at specific pedal positions (see my post above) is stored in flash as well, which may be the cause of the low electric motor utilization after a 12V reset...most armchair experts on here will say "it's because your battery is failing DUUUUH" not realizing I've already tested each of my banks, and that at 100% pedal position it will maintain a full 120A draw.

This is more about the fact that shorting out pins in OBDII port zero's out memory and can execute calibration procedure, so it MAY resolve the issue...which I plan on testing today.

 

There's quite a lot more about that, in fact, in the repair manual and New Car Features manual, which will tell you what calibrations are kept there, what procedures clear them or relearn them, and when those actions are called for.

The brake ECU (there are around a dozen ECUs in a Gen 2 Prius) doesn't have a very direct role in how much regen current will be used. The brake ECU uses your pedal pressure and pedal speed to determine how much overall braking you want, and it asks the HV control ECU "I want this much braking, how much can you do with regen?". When it gets that answer from the HV control ECU, it subtracts that amount and applies the brakes to make up the rest.

So it's the HV control ECU (based on information it gets from the battery ECU) that makes the decisions about how much regen it can offer at a given time. The brake ECU's role is to ask for what it wants and make do with whatever the answer is.

The primary signal for how hard you want to brake isn't the pedal stroke sensor, but the master cylinder pressure sensors; those read the pressure developed in the brake fluid from the force of your foot on the pedal.

The stroke sensor on the pedal pivot is there for a secondary purpose; its rate of change is used to judge how fast you've gone on the pedal. When you press extra fast, that's considered more urgent braking (which, in a Gen 3 anyway, bypasses the regen negotiation entirely and goes straight to friction braking; I don't know for sure if that's the same for Gen 2.)

 

I feel compelled to say something relevant and on-topic, just to not leave @ChaseE totally despairing about PriusChat users.

Not that I've got anything particularly useful to add, not having experienced the issue personally. There's nothing much to suggest a factor in the problem is non-volatile ECU storage, but it could be, so any ECU resets you can find are worth a try, if you seem to have a car that's firmly stuck in the weak EV state. Each ECU will have its own magic sequence, like that brake ECU. Of course, resets may just make other things worse - this problem was ltriggered by a reset...

My alternative theory is that maybe the software is just overly picky about looking for some "good" condition to complete its transition to full EV state, and in some cars that "good" condition is never reached, due to some sensor(s) being slightly out of nominal range.

 

Thanks for the insight, but again the post was primarily to highlight that non-volitile memory exists for calibration settings, so I was speculating it may not be solely for the ABS. Do you have a link to the ECU clear and relearn procedures for the other gen2 systems? I think starting from a baseline of everything zeroed would be the best way to analyze different system behavior.

Last Friday I did a standard ECU relearn (short obd port pins 4 and 13, ignition on to accessory for 30 minutes) because I had an event to go to that was 95% highway travel at 60mph 120mi round trip for 3 days in a row...53.7mpg before refilling Sunday evening. So far this week with city only driving display shows 36.9mpg at 68mi, not a lot of data but it appears the issue persists. Granted it's colder right now (high 30s to 40s) and there's higher ethanol in winter gas mixes, but in previous years with similar conditions city driving would be around 48mpg, with summer around 52-56mpg.

The only code previously was P0420 which hasn't come back since replacing the bank 1 air/fuel sensor. The only other thing I can come up with regarding the low city MPG is that in torque pro my air fuel ratio always hovers around 14.1 and never goes above that regardless of condition, but based on other posts I have searched it appears that's normal and that the 1.5L doesn't operate at 14.6-14.7 nominally like some other engines for ideal stoichiometric.