I officially hate my prius and toyota

Certainly possible. I suggested the OP have a dealer tech verify this, in post #20.

An easy way to assess engine coolant temp would be to run the car on the freeway 20 miles or so, so that the engine is fully warmed up. Use a handheld instant IR thermometer to determine the temp of the engine coolant pump housing (which should approximate the temp of the coolant itself). Operating temperature should be ~82 degrees C or so.

If much lower, it would seem likely that the engine thermostat is stuck open. Measure the temperature of the radiator hose at the thermostat housing to see how hot the coolant is there. If there is little difference in temperature then the thermostat is probably open.

Yes, this was suggested in my post #20, and is a low-cost alternative to replacing the hybrid vehicle ECU. I also suggest reflashing the engine ECU firmware since both ECUs are involved in engine control.

 

We had a stuck-open thermostat on a Honda Accord once: the symptom I noticed was coolant temperature gauge right down to the cold end after freeway driving. But with the Prius there is no in-dash coolant temp gauge, you need a ScanGauge or similar.

 

That dealer's assertion is incorrect. One bit of evidence is your personal experience with your car's recent unusual fuel consumption. It is clear that this is abnormal behavior and that something is wrong.

No engine ECU DTC will register, if engine coolant temperature is less than it should be. (There are exceptions having to do with the CHRS system, but those are not relevant to your problem.)

 

If it IS a thermostat problem, the good news is you only need a competent mechanic, not a Prius tech.

 

The problem is that it is hard for a computer to tell when something is "wrong" versus something is "out of parameters/specs".

Say you were mimicking Top Gear and driving to the North Pole in your Prius. Your coolant might never heat up fully, so the sensor may always read low temp. How can it distinguish this from a broken sensor that reports the same temperature all the time as being too low? It may seem simple to a human, but it is not an easy thing to program to make sure it doesn't flag a normal situation as a bad one. So the computers can always tell when something is out of spec (assuming the computer itself is trustworthy), but not necessarily when it is not-quite-right...

 

Beat the crap out of it until the problem shows up as a code. I think that by driving around like a lead footed teenager whatever is partially effed-up will become totally effed-up. This has worked for me in the past. Another benefit from beating the car into submission is sometimes it cures the problem. I am totally serious!

 

A stuck open thermostat would result in poor fuel efficiency but I don't think it can account for the ICE running all the time. Even with the thermostat stuck fully open the car should still make it to stage 2 and in fact the ICE would shut down even more freely in this case.

The OP is adamant that no matter how much he warms up the car that the ICE continues to run even after the car comes to rest and even after the usual 5 to 10 second delay (and even with all cabin heat demand off, AC off fan off etc). I don't think we've ever heard of even one similar case where the prius wouldn't shut off the ICE under those conditions which is what makes my think its some sort of ECU corruption.

 

Yes, perhaps a bit trickier than you realize.

When you hear (and feel) an engine running what you are mostly hearing is related to the combustion process. The compression and subsequent ignition of the gasses and the noise and pulsation that this gives rise to, particularly in the exhaust system.

When the prius spins the ICE without fuel injection it's does so with the valve timing altered and very little of the usual noise and pulsations that you would normally experience with a running engine.

Since you're in Canada then you have the same L/100k display that we have in Australia. This allows us (without any special equipment) to detect fuel consumption at a much lower rate than the US MPG guys who wont notice anything after 99.9 MPG. But 99.9 MPG US is only 2.35 L/100km and as you know we can easily detect much lower fuel consumption than that. So what you're doing by looking at the instantaneous L/100k is a good way to tell if the ICE is "running" (as in getting fuel injection) or not, I do the same thing all the time.

Trust me however that it is not so easy to tell whether or not the ICE is turning or not. It's just an extra bit of rotating mass when it's not getting fuel injection. It's whole other mode of operation compared to how a conventional ICE ever runs and it doesn't make the conventional noises of a running ICE.

If you want to test this do the following. When the car is warmed up and doing it's "B" mode trick (shutting the ICE off when coasting if you switch to "B" mode and then starting it again when you switch back to "D"). Start coasting from about 50 km/hr and (assuming the ICE keeps running as per your problem) then stick it in "B" and the ICE will "stop". Now keep it in "B" and allow the car to coast down in speed until it's almost stopped. What you'll find is that about 15 to 20 km/hr the ICE will spontaneously start back up (still in "B") so that the ICE will still be idling when you come to rest. What's actually happening is that "B" mode really just stopped the fuel injection with the ICE still spinning. As the speed decreases eventually the car has to re-start the fuel injection to prevent the ICE turning too slow.

 

Perhaps that's the reason why the Altima Hybrid has a blue low engine temperature light that's always on initially and then shuts off as the engine warms up? I previously figured it was pretty useless. As an owner of 1 Nissan now and previously another, neither had such lights.

From the 07 NAH manual:

Maybe the next gen Prius could use one of these lights, if not a temperature gauge...

If someone in the Vancouver BC area could loan the OP a ScanGauge, it could help confirm the hypothesis of the engine not getting up to the proper temp.

 

You are correct, but that along with the turtle were both dropped for 2G. Instead, 2G got the dream-catcher, aka low ambient air temp warning light. Then, the dream-catcher was dropped in model year 2006 because it was silly to have the light on all winter.

 

Hmm. It ought to be! I have a good look in the engine bays of all my vehicles at least weekly. Always best to catch small leaks and things while they're still small leaks and things (and check fluids!).

 

How about a picture showing average fuel economy.

 

20,000km between services and I only lift the bonnet between services to refill the washer bottle. But last time I did that I discovered a small leak from the water pump. 3 hours later new pump was purchased and installed by me, ready to drive away.

 

Sounds like your car is confused about what gear it is in -

If I put my '09 Prius into 'B', that would immediately bring the ICE from low to higher rpms depending on conditions.

Seems as though your engine control unit is confused. As Patrick Wong said IIRC, it would be smart to reflash the ECU(s).

I would defintely get a Prius A tech in the car with me driving and demonstrate the ICE running 90% of the time. If he blows that off, get the A tech and service manager in the car, if still no repsonse, conference call Toyo corporate on your Bluetooth, file case

 

Any correlation to this problem occuring and the 12V battery being changed?

I remember when I would first run the 09 Prius after installing seat (and having battery disconnected), the ICE would idle at higher rpm. It would take some time (30 mins?) for it to 'settle down' and be normal again. But ICE would shut off at stop lights after few seconds.