Why Mileage Gets Worse in the Winter

I see in Boston, the Ocean came into the city!

 

oh ya, bad news on the coast, then it froze overnight. we were fortunate to just get 18" of dry snow 10 miles west.
i saw cars and dumpsters floating down the street on the news, looked like a tidal wave.

 

199.9 could also mean 10,000 mpg/ it’s a secret.

 

shhhhhh....

 

I sympathise with all the people experiencing horrible weather and hope they are smart enough to stay home.

 

unfortunately, no there are always a few who have to 'go out to see what it's like', and always a few unnecessary tragedies. do not know if alcohol or drugs play a part.

 

Many times just a youthful challenge, that’s how I was. I’m a bit smarter now.

 

survivor.

 

that’s me.

 

a

 

To respond to bisco, temp here in the Houston area not that cold, 40s to 70s, so I don't think that's it. Have checked all the easy stuff, tires inflated, changed oil, plugs, air filter, etc., etc. No change in routes, driving habits, gas stations. Owned Prius for 10 years.

I think the latest clue is that in watching the vertical bar graphs on mileage by 5 minute increments, mileage has dropped way off for the first five minutes. At or under 25 mpg for the first 5 minutes, then to around 35 for the second 5 minutes. After 10 minutes, tends to be closer to the 45-50 mpg range....where it should be. That suggests to me there's a sensor that has failed. Most of my trips are under 15-20 minutes so bad mileage in the first 5-10 minutes significantly determines overall mileage.

Used to be that I could get gas at my local gas station about a mile away and get 50+ mpg for the mile trip home after a reset on the per tank mileage monitor. Almost a game I would play to see how high I could get the mileage for that one mile trip. Sometimes in the 70-75 range. Now in the 30-35 range.

Anyone have any idea on what might be failed or what might be wrong to have the mileage drop so significantly so abruptly? Many thanks!

 

Not sure on your 2008 but my 2005 engine always started at the beginning of every trip, of course the more short trip the more the ICE runs. On the 2017 Prime, Toyota has eliminated this &\or lessened ICE run.
I don’t believe there are any individual sensors, but rather a group of hardware boxes that work in coordination depending mostly on trip length, temps outside and car settings ( Hvac, defrost, lighting etc), to:
produce the best mileage.

 

Thanks, Andy. My engine starts up at the beginning of every trip also, but did it at 50 mpg as well as the new 35 mpg. I don't think that's it as I used to get 50 mpg in the one-mile trip home after a new tank of gas.

Been doing some research and some are saying that O2 sensors, it appears Gen 2 have two, one upstream and one downstream, can have an effect on MPG. Might have gotten a warning light though if an O2 sensor fail. I don't have one. Not clear if that's it, just an idea.

All the standard things I've checked: tire pressure, new air and oil filter / change, new plugs, and new 12V battery. No problems with Hybrid battery, still working as it did on Day 1, nearly 10 years ago.

Still looking for ideas or someone who may have run across this before. Thanks all!

 

Clean the air throat ?

 

cabin filter?
what was the outcome on the tires? also, new tires have a larger circumference.

 

45ºF (10ºC) seems to be the trigger point (in my observation) where it is harder to keep the MPGs up in colder weather. So what proportion of time are the temps at the 40 end rather than the 70 end?

I don't mean to be rude, but I think you're imagining that you were getting 50 MPG on a short run. Yes, fine on the 1 mile trip home after filling up, but your engine would be fully warmed up (or largely warmed up) in that scenario and would be very easy to do a 50 MPG run over 1 mile. I would say the MPGs you indicated for the first 2 5-minute bars are quite normal starting from cold, particularly if the temp were in the 40-45ºF range.

One thing to note, your HV battery is approaching 10 years old now, so will not behave like a new one even if it is in spec and not showing many signs of ageing. The cold hard fact is it will not have the same capacity it had when it was new.

You could try cleaning the throttle body, and check the MAF, if it looks dirty clean that too, but other than that and the other things already mentioned, it is what it is until you get some DTCs thrown up.

 

Finally had a weekend to clean the throttle body and the MAF sensor per bisco, Andy, and dolj suggestion. First time I've done in the 10 years and 145K miles on my 08 Prius. Saw a couple good vids on You Tube using rag soaked in TB cleaner wrapped on toothbrush. MAF cleaner sprayed on sensor and wires. TB didn't seem that dirty as have regularly changed AF. Still pulled some gunk out of TB and sprayed the MAF sensor liberally. At first after cleaned, engine ran rough to perhaps clear some dirt that I kicked lose or for the computer to adjust to new normal. Improved mileage back to mid 40s MPG. Closer to where used to be. Might be due a little to warmer weather in 60s-70s during the day. Had pretty much tried everything else and all else suggested: oil change, all filters, new plugs, tire pressure, new 12V battery. I still believe that there is some impact of rolling friction increase with new tires as change was immediate. Hard to quantify.

Am now wondering if I should change out the PCV valve as is original? Have read that it's not likely to change mileage, but might help with running smoother although not a problem. Thanks to all!

 

A very high impact cause is wheel alignment.
This is very often overlooked.
And as cold air comes in, the effects can be clearly noted in economy.

 

You might try a does of fuel injector cleaner, the kind you add at fuel-up. Look for one that has PolyEtherAmine. That's ticked my mpg up a bit.

 

Now that you have done most, just for kicks, reboot the 12V battery. I know that you just bought a new battery,
but this isn’t for the battery, but, for all the computers that are in the car. They all take in multitudinous info, from within, from outside and all gets coordinated to form a profile of you as the driver, and to provide constant updated info also on load changes, to include OAT, wind, direction of wind, air pressure, amount of traffic around the car which changes wind vectors, Air Pressure and Altitude of course. Rebooting the battery is so incredibly simple, I don’t understand why everybody doesn’t do it. Just disconnect either the plus or the minus on the 12 V for amount 30 seconds, Reconnect and tighten well. Even tho you recently put in a new 12V, that was a few weeks ago and you have made improvements, all the computers are unaware of these changes, and one must reboot.
Next time you misassociate with some supposedly Toyota Salesperson, technician, mechanic ask him how do you reboot the car? Observe the answers and the responses.
You have rebooted your computer right?