LOW MPG - Engine starts quiet often

It can take some trip cycles for the ECU to relearn its parameters for your Prius to operate at peak efficiency.

The mechanism that sometimes causes a weak 12v battery to lower MPG is when it continuously pulls charging current without getting fully charged. This is wasted energy. Note that you should measure the resting voltage of your battery (>12v), the voltage with some load current (still > 12v), and the voltage with your Prius in "ready" (about 14v) to determine if it is sound.

These can reduce the useful life of your HV battery, so do check them.

Check your tire pressure as a low PSI will drop MPG.

Good luck,

JeffD

 

Tyre pressures affect mpg's as do non low rolling resistance tyres. Have you changed your tyres lately?

A failing 12v really does cause a drop in mpg's for the reason given by Jeff. If yours is the original 12v then it is possibly due a change. If you can't check it with a multimeter then try the following; in a morning after the car has been sat overnight put the car in Acc mode (NOT Ready) and then wind all 4 windows down at once and then back up again. If they struggle and the interior lights go dim or out then your 12v is a likely problem. Now put the car in Ready mode and repeat. If they fly up and down quickly you know it's the 12v that is on its way out.

Where in the World are you? Helps on advice on replacement parts. If you're in the UK or Ireland then you can get a new Toyota 12v battery for the Prius for about £95/€120 fully fitted.

Keep us all posted.

 

welcome to priuschat! all the best!

 

this happens quite often, i'm not sure i've ever read a thread where there is resolution, other than the o/p being mistaken.

 

Check the obvious things like tyre pressures and 12V battery health, but other than that just drive it. Most of the loss is only in the reported fuel consumption anyway (check it at the pump). The reported MPG can take many tanks full before it calibrates to it's former accuracy.

 

The title says "engine starts quite often", so I am guessing that it means the car is not going into electric mode as expected. What is the state of charge indicator doing on the energy display? Are you seeing 6 blue bars most of the time?

For example, if the engine is not putting out its normal amount of power, the car will lean on the battery to make up the difference, but that quickly brings down the charge. That will in turn cause the engine to run more, trying to return the charge. This may rectify itself after the learning period by the ECU's mentioned above.

 

how many miles on her?

 

Not sure about a 20 mpg drop but I guess it's possible. Think of it this way, a 10% drop on a car doing 30 mpg isn't noticeable but it is on a 60 mpg car. Maybe your fuel consumption meter is now misreading (as will the speedo?) because of the difference sized tyres?

When I changed to no LRR tyres by accident my top end mpg's just couldn't be achieved. Then when I used thinner oil (gen3 Prius only) the mpg's dropped too. Amazing that these two items can make such a difference but they can. The Prius is finely tuned for economy in the same way a Ferrari is for speed.

 

original poster, that's you.

 

I would say that the tyres are not the concern here. If the engine starts even at low speed and relatively higher battery charge, then that needs to be sorted out. This happened after the 12V was removed, which resets the ECUs. This should have sorted itself out by now. As was mentioned, this kind of behavior has been posted here before, but no real understanding has come about. The only thing I can suggest is to obtain the mini VCI code reader and start looking at the various ECUs and see what temperatures and other data might be out of normal range. The car is acting as though:

*it incorrectly senses the HV battery is low
*it senses the HV system as overheated
*it senses the engine temperature as too low and still needs to warm up
* the HV ECU is out to lunch.
* ?

Only guessing here.

 

I'm not sure about you averaging 63mpg for a year and a half. That's a remarkable average that I can't say I've seen. Maybe for 1 downhill stretch for 10 miles but for a year and a half is quite remarkable. I also don't think tire pressure or a bad 12v would cause a 20mpg drop to 45mpg.

I own 3 of these Gen2 cars and my average on all of them is right around 45 mpg, one slightly higher, one slightly lower.....but I don't think any of them exceed 50mpg on combined highway/city driving.

I believe there is nothing wrong with your car.

 

The OPs based in Pakistan. I'm guessing he's using UK mpg's where 63 mpg is 52 mpg US, which is still pretty good going though.