Does Prius have engine thermostat and what if it is stuck?

The #1 cause for a mileage decrease (according to this forum) seems to be the 12volt battery, if it has an internal short, the engine is constantly (well almost) trying to charge it, hence the drop in mileage. The cooling system must have a thermostat, but I would suggest testing the battery first.

 

it does have a thermostat, but if the temp is off, it should give a dash light. start with munpots advice above, and if the 12v is good, go throughout the 'why has my mileage decreased' sticky.

 

As popular as it is on this forum, I would still love to see somebody, anybody, just back-of-the-envelope that idea, even once. It would be so easy to do: in a Prius, the charge voltage is held constant right around 13.8 to 14 at all times when the car is READY; doesn't matter if it's moving, doesn't matter if it's in gear, doesn't matter if it's doing anything. If even one person who thought he or she had that problem would just make the car READY, open the hatch, and stick a clamp meter over the battery wire to measure the amps flowing, and multiply by 14, that would be the total number of watts being lost into the battery under any driving condition at any time. That would tell us something.

There is a conventional wax-driven engine thermostat (at least in the earlier gens) and the car specs say what the opening temperature should be. For my Gen 1 it's 82 celsius, fully open by 85. If you have the Torque smartphone app, or a ScanGauge, or any old OBD-II scanner at all (engine temp is a standard parameter, not a Prius private one), you can easily watch the temperature. If the temp goes up reasonably quickly, hits 82 by the time you're cruising and then slows way down and hangs right around 82-85 most of the time, your thermostat's just fine. Mine still is, and that's after 226,000 miles....

The temp can go above 85 in poor-cooling situations like a traffic jam; the electric fans should come on around 90 or 95.

-Chap

 

The one time I noticed excess engine cooling was on freeway, early morning, no slowdowns, fwiw. Thermostat was leaky and opening too soon.

 

did your mpg's drop?

 

Mpg was already in the toilet, I wouldn't have noticed, lol. This was with a long-in-the-tooth Accord.

 

With a digital multimeter you can do a rudimentary battery check. It's not a " load test", but checking the at-rest voltage will tell you if there's a significant degrade, or not. If it's 12.6~12.8 volt it's in good shape. 12.4~12.5 volt is so-so. Lower, its ailing, at the least could use a charge session, or might need replacing.

 

I don't think a thermostat failure would send a code. They are mechanical items, and when they fail, they are designed to lock in the OPEN position. End result is a long(er) warm-up period because you're heating the whole coolant system and not just what's in the engine block. You MAY best notice this in wintertime if you either can't seem to get much heat (real cold out) or it takes longer than it should to start blowing "warm" air.

I'm unsure with the Prius because I don't know (off hand) if the heater core runs off the ICE coolant/inverter coolant/or both.

 

I kept saying this for a while but gave up .... try to explain that the way the 12V battery is charged it less then the headlights (headlights 110W) !!! If there were a 'short' bigger load/higher amp than that, the heat would dissipate on the over that "short" in the battery that would be noticable !! By the way when the headlights is ON the chargint circuit increases the voltage to the same as it were charging the battery about 14.2V

I think this is a myth which now a self-evidence by the number it was repeated.

 

and yet, some people report higher mileage after changing out the battery. can we agree that a weak battery will need more charging, causing the hybrid battery to discharge faster and the engine to run more?

 

I would agree with above statement but no way that causes 8 mpg decrease by itself the battery charging even constantly by 3-4 amp current ... while the two headlights take 7-8amp while operating ... so running the headlights would result comparable or even larger loss of mpg ... in similar operating mode (this loss is depend on velocity of course but that is true to the "battery loss") ... we can agree that is not case right ??? Even A/C which is much more then headlights would NOT cause this level of mpg loss

 

agreed. i wonder if we could be missing some other factor, but i'm no ee.

 

It's about time the strange popular misconception of the lead-acid battery consuming vast quantities of gasoline was put to rest!

Back to the original topic ... Does the 3rd generation have a conventional coolant thermostat, or some sort of sensor-computer-actuator loop to manage engine temperature? Incidentally ScanGauge usually says mine hangs around 195-198°F (about 90.5-92.2°C), a little higher than expected.

 

same thing here/
guess that's the regular temperature it's supposed to run//

 

Sure, put that way with no numbers or magnitudes, that's the art of the statement that can't be disagreed with. So is "smashed mayflies stuck to the windshield increase air drag and make the engine run more."

In either case, the next step called "making a back-of-the-envelope estimate" is where you (without even bothering to strive too hard at precision yet) put together the numbers you know or can easily get, the most basic forms of the formulas that apply, and for the stuff you don't know and can't get, throw in some numbers in the ballpark and even favorable to the hypothesis, and see whether even under those rough conditions the size of the effect possibly comes anywhere near what you're trying to explain.

It's surprising to see so little sign of anyone doing that with the aux-battery-mileage idea, because it would be so easy to do. The fact that the car regulates charge voltage and charge rate the same under all READY conditions (unlike conventional cars with alternators) means you don't even have to try to catch some peak rate under elusive driving conditions or whatever. You can literally make the car READY, in Park, walk around back to the trunk/hatch and measure the charging current, and your data collection is complete.

I know that the normal charging current for the aux battery is intended to be under 5 amps (so, max 70 watts or so). But let's grant that the aux-battery-mileage notion is about a battery that could be compromised (shorted cell?) and drawing more than the normal current. How much more? That's what I'd love to see someone take the necessary moment to measure, when they think that's the problem they're seeing. From that, we'd learn something. I'd really much rather have the data, than just write back-and-forth forum posts about it. I'd measure it myself, but I haven't ever thought I was having that problem. I only see current draws in the normal range.

People self-report remarkable results after, fer instance, adding aftermarket ground wires to the throttle body before they've even looked at the wiring diagram to notice that no part of it is electrically connected to the bolt they stuck the wires on.

The kinds of companies that prey on would-be tuners/modders by selling that kind of stuff are themselves masters of the kind of statement that sounds plausible and handwaves over everything measurable or testable.

-Chap

 

I was quoting the temp specs in the manual for my Gen 1. Quite possible they're changed for Gen 3. You'd find what they're supposed to be in the Specifications section of your manual on techinfo.toyota.com.

-Chap

 

sounds like a job for superman. err, i mean bob wilson.

 

A good habit of remembering to back-of-the-envelope is more of an important life skill for those of us who aren't superman. Superman has many more options to resist being taken advantage of. And x-ray vision. X-ray vision can help a lot.

-Chap