2007 Low MPG -- Can't figure out why

Hi all,

New to the forum, but not to the Prius (I got my 2007 Prius brand new nearly 5 years ago). Until recently, it has been a great car. I would typically average about 45 MPG in summer, 43-44 MPG in winter and 48 MPG the rest of the year. However, starting in November (2011) the mileage in the car took a huge nose dive. When we drove down to SoCal, it got horrible mileage. We typically do not get the best mileage on these trips to SoCal, averaging usually around 43 MPG with our worst ever having been about 40. That is, until November. We got our worst ever mileage on this trip of 36 MPG (at the time). At first I thought it was just a fluke, especially when we did a little better on the trip back. I needed to have its 50k service done so I mentioned it to the service folks. After giving every stinking excuse in the book, blaming everything from tire pressure, to gas to anything else you could think of, they performed a health check. They didn't find any problems but as it turned out, the tires were low (~28 psi) but I had filled them up before the SoCal trip. I had chalked it up to a faulty pressure gauge (I had problems with it before) and didn't think anything of it until our Christmas trip. I double checked the tire pressure before leaving and the tires showed about 35 psi on all 4 tires when I left (using a different gauge). However, I once again got horrible gas mileage, worse than the last at 33 MPG!!! I had to stop half way to LA for gas for the first time EVER!!! (Most of the time I am able to make it on a single tank and only 2 or 3 times I had to stop over the grapevine to fill up). When I got to LA, I again checked the tires just to make sure and they STILL showed 35 psi. Elk Grove Toyota had neglected to reset the maintenance light after the service so I reset it myself while in LA. When that happened, the car went back to normal (more on that later) and we averaged 44 MPG on the trip back.

So after resetting the maintenance light, I thought, "ok, maybe the car went into 'limp home mode' when the maintenance light was on and that was why it behaved the way it did." Wrong! On a trip to Stockton, again, low gas mileage of 33 MPG. I checked the tires with a digital pressure gauge and it showed 31 PSI on all 4 tires. I refilled them to about 37 PSI only to get an average of 36 MPG on the trip back! Since then, it has flip flopped between ok gas mileage of around 43 MPG (I once got 44 MPG coming from Folsom which is mostly downhill) and 36 MPG (which has been mostly typical lately). I have noticed the car doing some odd things which I'll explain after the questionnaire.

I am filling out the MPG questionnaire since it is required and may help you guys rule out the more common problems (answers in blue):

- Have you read This Thead Yet?

Yes

- What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations)

Approximately 36 MPG, sometimes closer to 40 or 41 if I'm having a good day. I'm using both.

- What fuel economy are you expecting and why?

This time of year, I am expecting around 43 MPG on cold days (~30F in the mornings and ~50F in the afternoon) and closer to 45 MPG on warmer days (afternoons ~60F, mornings same)

- What are the approximate outside air temps?

See above

- How long are your trips?

The trips to SoCal are around 370 miles. Most other trips are 15 miles one way. Ever since we got the Nissan Leaf, we have rarely used the Prius for short trips (in fact, the problems started after we got the Leaf).

- How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving?

It is mostly highway. We generally go around 70 in the Sac area but 75 on our trips to LA.

- What region/state are you in? (if you haven't set your location in your profile)

Sacramento, CA

- What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.)

Mostly flat with some gentle hills. On LA trips, there is the grapevine with its steep hills but I usually make up the loss going up on the downhill trip into the LA basin.

- Is your oil overfilled? (i.e. above the full mark on the dipstick)

No

How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night? (Method Here)

As old as the car. Voltage reading shows 12.1 Volts after sitting overnight (more on the battery after the questionnaire).

Have you had your alignment checked? Any pulling or abnormal tire wear?

No checks on my own (don't know if Elk Grove Toyota checks it). No unusual wear on my last set of tires and no pulling.

- Are you using the factory tires and wheels? If not, please indicate tire make, model and size (e.g. Goodyear Assurance Fuel Max 185/65R15).

Kumho Eco Solus HM KR22 P185/65R15. Replaced factory tires in October.

- What are your tire pressures?

Last check was on 1/15 and read ~35 psi on 3 tires and ~33 PSI on one. I topped off to 37 PSI on all 4.

- Make, model, year, engine and transmission of previous car? (e.g. 08 Honda Civic Si 2.0L 4 cylinder, manual transmission) What did you actually get on the same trips/commute? (Please give us actual numbers, not EPA ratings.)

2005 Mazda 3 2.3L 4 cylinder, manual transmission. Lived in LA at the time but trips between Stockton and LA, also about 360 miles, I averaged about 28 - 29 MPG (my calc).

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?

I mostly just drive normally, try to accelerate briskly but not aggressively and try to allow myself enough time to brake

- Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on?

No

- Are you driving using D or B mode?

Almost always in D mode. I rarely use B mode and its usually on a steep downhill with a relatively full battery

- HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to?

When the problem started, we had already stopped using AC. We usually have the temp set to low or around 65 to 68. Low fan speed. AC button is deliberately set to off to keep the compressor from running.

- If reporting a mileage drop, did anything significant change on your car (e.g. accident, hit a curb or big pothole throwing off alignment, oil change/other maintenance/repairs, changed tires or wheels, etc.) or your commute?

Changed tires. Also driving car less since we got the Nissan Leaf. But that is not unusual since before our son was born, we used to ride the bus a lot and the car would still sit for long periods of time. It is still driven at least once a week.

Change in behavior:

Since the problems started, I've been watching the power flow screen more to observe its behavior and I've noticed that it relies on the gas engine far more than it used to. For example, when going uphill, the engine races and the battery hardly contributes to the overall power. When going up the grapevine, I would typically have the battery drain down to about two bars of charge by the time I reach the top. Since the problem started, I would still have 6 bars of charge at the top! I had noticed that the engine was trying to charge the battery while going uphill even though it was in the green at 7 bars! It was as if the car thought it was on flat land! Also, the Prius normally maintains a charge of 6 bars at highway speed on flat land. Lately, it has been maintaining it at 7 bars in the green zone (see pics below).

Highway speed pic:
View attachment 34341

Hill climbing pic (not a gentle hill but not too steep either):
View attachment 34342

I have been checking on the 12 volt battery quite frequently lately using the "hold info flash headlights 4 times" technique after at least 24 hours of the car sitting. It has been at least 12.1 Volts consistently. When putting the car from accessory mode to "on" the voltage drops to about 11.6 - 11.7 volts but then stabilizes at 11.8 - 11.9 volts. When I turn the car on (engine running) the voltage goes up to about 14.1 volts. I had hoped that the problem would be the 12 volt battery but based on what I've read in the forums, these readings are definitely within normal parameters.

I have pounded the forums relentlessly and googled extensively and I have not found anything that sounds like the problem I am having. If anyone has any idea what the problem might be and how to fix, please tell me!

PS - Earlier this month, I took the car back to Elk Grove Toyota to have them check it out again. After another "health check" they again found no problems like with the 50k checkup (I had them do a "health check then as well") and the tire pressure was ok (according to their printout). At that time, I had them reset the onboard computer. It doesn't seem to have fixed the problem at all. Please help!

Thanks,
Jeff

 

Me again. Let me know if the pics didn't work and I'll try to repost.

Thanks,
Jeff

 

Hi cwerdna,

Yes, that is essentially what I got from their explanation of a health check. Yes, I am checking the cold pressure. I usually try and check tire pressure on a day when I haven't driven the car yet so it has had 12 - 24 hours to reach ambient temp. Also, in my crazy engineer's head, I try and fill the tires so that they are at manufacturer spec (35/33) during the coldest part of the day. For example, if low of the day is 30F and it is 60F in my garage, I will fill the tires to 38/36, although lately I have been filling all four tires the same at Elk Grove Toyota's suggestion so in this case it would be 38 psi for all four tires. (No worries about it being a "dumb question," I believe that their is no such thing as a dumb question).

As for the mileage in winter and on trips to LA, I never expect really good mileage. Generally in winter because of the cold and weather, I get around 42 to 43 MPG and in previous winters have not gotten worse than that. This is the first time I've ever been consistently below 40 MPG. Same with my trips to LA. I think the mileage on LA trips using I5 has to do with a combination of speed, hills (especially the grapevine), and the fact that it is frequently very windy (I call that stretch of I5 the wind tunnel, especially the stretch between 205 and 580). I haven't heard of gallonage, I'll have to look into that.

What has boggled my mind is the cars behavior. The Prius has always coddled the battery but lately it won't even use it on hill climbing and acceleration. I don't know if my pictures came through, but in it, you can see the car is maintaining charge in the battery while I'm climbing a hill, even though it is already in the green zone! (7 bars of charge). On my recent trip to Monterey when I climbed over Pacheco Pass on 152, it barely used the battery. The charge dropped from 7 bars of charge (green) to 6 bars (blue). At one point, the electric motor was powering the wheels AND charging the battery! (NOT powering the motor!) I've only ever seen it do that when the charge level is low, like 2 or 3 bars (pink or low blue). It's also very random. This morning when I drove into work, it was mostly back to normal behavior (seems like it fixes itself after I do a battery check, go figure). Has anyone heard of a Prius doing that? Any ideas why? This type of behavior started before EG Toyota reset the onboard computer in case anyone was wondering.

Thanks,
Jeff

 

Hi uart,

Love the pic!

Last time I checked, the ethanol content has been 10% which I believe has been that way in California for 10 years at least.

Do you think that even though the battery is on the low end of the normal range that it might still be causing the cars weird behavior? And why it seems to fix itself each time I do a battery check?

Losing mileage due to the new tires is something that had crossed my mind. It didn't escape my notice that this started around the same time I got the tires but I've since ruled that out. I'm well past the break in period for them and I would think that the new tires would only cause a small drop, like maybe 0.5 MPG. But maybe, as you pointed out, it's a combination of things.

That is a definite possibility, we had become more aware of the Prius's engine after getting the Leaf. However, we live in a suburban area with high speed limits. We usually get better mileage on the freeway because in the suburban area, we are usually doing high speed stop and go due to the higher speed limits (around 45 mph) and frequent traffic lights. Also, this is not the first time the Prius has seen limited use. When we used to ride the bus a lot, the Prius would spend much of its time sitting in the garage as well. On the other hand, we'd use it for errands on the weekend which is what we primarily use the Leaf for now. So, while we've always used it for mostly highway driving, you could say that it is driven on the highway even more so now. We still tend to meet or exceed the EPA highway mileage figure of 45 MPG though, especially on a clear day in winter since we barely use the heater and I go out of my way to avoid using the defroster.

Thanks for your feedback!
Jeff

 

Hi Bisco,

Yeah, it may come to that. I've read a few threads here that said their Prius was getting bad mileage and determined it to be related to the 12 V battery. I was really hopeful that the 12 V was the source of my problem but it had been testing consistently at about 12.1 Volts which folks here say is normal. The only thing that seemed abnormal was the drop to about 11.5 V when switching to ig-on but then it stabilizes to about 11.8 or 11.9 V which I also read was normal (though on the lower side of normal). I'm not really wanting to spend the $200 to replace the battery (but I am seriously considering the optima when I do) but if I can't figure out any other cause, I may give that a shot.

Thanks for the feedback!

 

Hmmm, hadn't considered the battery fan. It could explain why the Prius is behaving the way it is, protecting the battery more than usual. I'll try that on my drive home this evening and see what happens.

Yes, once warmed up, the engine does cut out after its normal 7 - 10 second run time.

 

Yeah, I hope it is a simple thing like the battery fan. Is there a way to test the HV battery itself? EG Toyota claims they tested it but I really have no way of knowing if they did or did not.

 

Hi Tom,

It is currently at about 53,000 miles. I have ruled out most things but I'm going to take a look at the HV battery fan as uart suggested. Failing that, I'm going to try replacing the 12 V before I'm ready to face the possibility of a bad cell in the HV battery (and trying to convince EG Toyota that the problem is with the HV battery who will no doubt vehemently deny that there is a problem at all). I'm hopeful that the HV battery is fine and that the problem lies somewhere else.

Thanks for your feedback!

 

Hi 6lark5,

I was hoping to put this off until late spring early summer. I am hoping that the aux battery is not the problem especially since it's reading at about 12.1 V in acc mode. However, as others have suggested, that is on the low end of spec and may be contributing.

No, it doesn't, but this is the worst I've gotten by far (at least it was until the December trip). As an example, when I drove to LA back in October before the problem started, I averaged about 42 - 43 MPG driving around 78 - 80 MPH (I always look at it as a compromise in milage to minimize the rather long mundane trip). When I went down in November, I was far easier on the gas pedal than I normally am because of traffic (night before Thanksgiving) and so cruised at 70 until about Santa Nella and then kicked it up to about 75 when traffic eased up, yet I averaged 36 MPG!! I looked at my past mileage records and I usually average at least 5 MPG more than that at the same time of year, and I'm pretty sure the weather was milder this past November than November 2010.

I prefer to keep it around 37 - 38 psi so the tires will wear a bit more evenly. I also don't want to take the risk that they could get overinflated in summer when we get our triple digit temps (since our garage is usually about 30F cooler than outside air).

We're cold weather people. The heater is rarely, if ever, higher than that.

No, I just recently learned about that, but I figure California winters were not cold enough to warrant that technique. If I ever live in Truckee, I'll probably give that a go. But until then, I probably won't practice that.

Shell/87 Octane

If EG Toyota is not using 5W30, I'll have words with them.