Drop out of S4 for no apparent reason?

Occasionally my car would drop out of S4 for no apparent reason and it happened for many times during my drive home today. The weather was nice, temperature about 70F. The SOC wasn't low. The engine was warmed up. The car had gone through the S4 "dance". It would drop out of S4 (engine not shut down during glide or regen). Then it came back to S4. Then later it would drop out again, and then came back to S4 again. This happened well into the drive when everything should have all warmed up. In the past I had assumed the cat temps may not be high enough. But today I watched them and noticed that there were occasions when the car dropped out of S4 at cat temps higher than those at which the car came back to S4 later. Any idea why the car would drop out of S4 well into a long drive?

Here are some data collected during the trip. First the S4 dance:


The 1st drop out:


End of 1st drop out and recovering of S4:


A brief drop out:


2nd drop out:


S4 OK with low cat temps:


S4 OK even after a long cool down (stop and go traffic jam):

i think my car does the same thing, and it's particularly notieable when i go to work. i do 4 miles on back roads, 2 miles on the highway and then hit the back of a shopping plaza. it's a windy slow (10-15mph) flat access road that i should be able to do in all electric. but many times, even tho the battery is full green, just a slight touch of the accellerator turns the engine on.

I'll be watching this thread, hoping to learn, as I have noticed the same thing. I've told myself there are probably more variables than we are aware of that go into those software decisions.

Could be as simple as low coolant temp. If it drops back below 70C the car will run according to Stage 2 rules until it warms back up.

right, but unlikely on a 70 degree day after 15 minutes of driving?

OK, what is "S4".

Coolant temp was monitored with three separate CANIDs. It's well above the threshold.

Look up "Prius warm up stages". "S4" is short for "stage 4", the final stage of warm up. In this stage the engine will shut down during glide.

This is a pain in the ass when it happens. The engine will run when coolant is being transferred to the thermos, but the odd thing I don't understand is that when this happens (if that's the cause, even, I don't actually know...), it happens throughout an entire ride, not just once.

The times it's happened there's been a huge range of SOC, sometimes low, sometimes high. Coolant always plenty warm, but no one specific temperature. In other words, I don't see any "cause" from the numbers.

The other day I was so annoyed that I pulled over and turned off the car. Upon restart it was fine, that's the only way I've found to get around it. I can't remember if I had to get back into stage 4 again or if it was already there somehow when I did this.

http://priuschat.com/forums/knowled...12919-five-stages-prius-hybrid-operation.html
http://priuschat.com/forums/knowled...perating-mode-availability-reference-aid.html

Wanted to update on something.

It happened to me tonight, so after grumbling down the backside of a bridge with the engine running, I pulled over, turned off the car, and started it right up again. I didn't wait even a couple seconds, cause I wanted to make sure ScanGauge kept thinking it was the "current" trip. I started up again, and the problem was gone. Even better, it was in stage 4 right away! This was in a 2007 Prius.

interesting. what do you think happened? glitch?

P.S. Forgot I took this pic, too. You can see after I turned it back on the first five minutes were in stage 4.

It seems to be, doesn't it? I have no technical expertise to comment on it, but at least we know a somewhat easy way to fix it if the car is stopped.

Just added some data in the original post.

Very interesting data, 2009. I've read before that the coolant can take a minute or so to do its thing. On some of those it looks like a full 100 seconds, so a minute and forty seconds(!!) that the engine wouldn't turn off.

whattayou got there, francais?

nice! wish bob wilson was still around.

Just to make sure I understand, you started the car again and continue the drive right away without the S4 dance (stopping the car to let the ICE shut down) and the car was back into S4?

Yuppers, exactly. Stopped completely, with foot on brake turned off car. Without removing foot from brake started it up probably within one second or so. Started driving and the car was in S4 right away. I pulsed up to 20 mph, let off the pedal, and I was gliding at 0 RPM. No more S4 dance for the remainder of the half hour trip, and no idle ceremony to get into it, was just there.

My 08 seems to work fine. I will attempt to log RPM today to confirm.

Is there an EVAP canister that is being purged?

What is the HV bat Temp?