Engine Runs Too Frequently?

[VGP63]

Hello All, I have been the owner of an 09 Prius T-Spirit 40k miles since June. I leave it in Eco mode all the time. I recently noticed that the ICE cuts in even under VERY light acceleration & at speeds of 22mph. This happens when moving away behind a bus that has just pulled away from a bus stop so you can judge the situation. The traction Battery indicator shows between 3 & 8 bars & seems normal. The 12v Battery also appears to be quite healthy although I am aware that at 5 years its in its old age! I would be very grateful for any help/advice.

[Grumpy Cabbie]

How do you know the 12v is ok?

Reducing fuel economy and/or engine running with no good reason is usually indicative of a failing 12v. With no starter motor to give a clue like in a traditional car, the Prius/hsd 12v will hide when it's failing.

Without guessing and speculating I'd rather you check that the 12v is good or bad. If you have a multimeter you can take a reading that way first thing on a morning BEFORE putting the car in Ready mode. Alternatively you could also test by putting the car in Acc and then wind all windows down together and then back up. If they're slow or stutter or the interior light goes out, but work find in Ready mode, then your 12v is shot and needs replacing. I'm sure it's a fixed price item at Toyota and I'm sure it's £95 fitted.

Otherwise are any rear passengers blocking the rear Battery vent, whether they admit it or not. A coat or a child or blanket could block that vent causing the HV Battery to get too warm and that can cause weird issues like you're experiencing.

Next thing would be a failing HV Battery ECU.

So check that 12v first.

[kithmo]

Do you have the heater on ?

This will cause the ICE to fire up more often, although less so in ECO mode than normal.

The higher temp you have the heater, the more the ICE will run.

[timberwolf]

Perfectly normal. Even in our moderate climate, it has got colder, the engine will fire up more often and your mpg will go down.