I need your help 2001 prius engine revs but no DTC codes

No, if you have no CEL or Hybrid warning indicators, you have nothing wrong with your car and the service advisor is likely correct. I drove my 01 for years and never had a scanguage.

 

The bad news is the NHW11 has weakness that can lead to poor mileage. The good news is many of these weaknesses have been mapped out:

• bumper air-inlet block - I use a split, water noodle held in by thin bungie cords. Use it without a concern up to about 70F, it reduces the cold-air, drag and helps engine warm-up.
• block heater - if you can, the Canadians sell a bunch. Just 30-45 minutes when the temperature is below 40F can save about 1-2 minutes of warm-up.
• thermistor hack - strictly do-it-yourself, as simple as a diode, switch, and wires or more complex, micro controller operated, they spoof the engine into thinking it is at 70C and can use the hybrid control laws when the coolant is at 40C.
• fully inflate the tires - use the max sidewall pressure although some suggest +2 psi on the front over the rear.
• avoid 39-44 mph - either slow down to 38 mph so you can run in hybrid mode or faster, 45 mph+ to avoid excessive transitions
• top speed 65 mph - on a standard day, 52 MPG, at 70 mph, 48-49 MPG, at 75 mph, 39 MPG
• park in sunny place - in the winter, let the sun heat the interior and avoid using the heater until the "blue" coolant light goes out

A Scangauge II can give you more insights and instrumentation but these basics should help a little.

Bob Wilson

 

Both! The majority of your heat (about five thousand watts worth, max) comes from the heat of the coolant, while to get you comfortable a tiny bit faster after a cold start, about three hundred watts worth of electric heat elements are buried inside the heater, that come on if (1) you're asking for heat, (2) the coolant isn't warm yet, (3) the DC/DC converter hasn't signaled it's near capacity.

If the coolant temperature drops to 60 C the engine will start and run, even for no other reason, until it is back to at least 70 C.

If you're in an area where there's a chance you could have to stay warm in the car for a spell, some experimenting I did a couple years ago could be of interest.

-Chap

 

Differences in mileage summer <-> winter depend on what sort of driving you're doing. Fully warmed up at highways speeds the difference is small. But if you're doing short trips around town there can be a big difference in overall mileage. Your reported 25-35% decrease in mileage between summer and winter is consistent with what I've seen for local short trips. There's a lot of extra fuel consumed until the car is up to operating temperature, and sometimes after that, depending on the rate of cooling and whether you're driving slowly enough to otherwise take advantage of EV mode. Pool noodles blocking the air inlet help a bit as Bob suggests.

I take some comfort in knowing that this isn't unique to the Prius. The non-hybrid cars see a drop as well with colder weather, though it's not as pronounced.