2007 Low MPG -- Can't figure out why

Is that the hissing sound? I've heard that noise the entire time I've owned that car. I always thought it was some sort of pressure release or something.

Do you know what the buzzing noise is? I hear it, randomly it seems sometimes, but every time I open the hood to check the oil and other fluid levels, it will consistently go off also.

Honestly, I don't think I've ever owned a car that has made so many random odd noises in my life. Even the Leaf just sits silently in the garage until I plug it in then it makes clicking and beeping sounds when I insert the plug into the receptacle (and random beep sounds when it connects up with the data center) followed by the sound of the radiator fan (ostensibly to keep the inverter cool).

 

Hi Evan,

No worries. I may not enjoy the lousy gas mileage, but as an engineer, this is exciting the problem solving part of my brain. And when I do find a solution, whether I discover it on my own or through tips and advice here, I know I'll feel satisfied and confident that I can fix any future problems.

We actually use the parking break quite often. I had only driven stick shifts before the Prius and was not one of the ones who'd leave the car in first gear. My wife was taught to use the parking break even though she only ever drove cars with an automatic transmission. She helped me keep that habit (I didn't used to do that with cars that had an automatic). I'm reasonably confident that the parking break has always been fully disengaged. Do you or anyone else know if the Gen III use an electronic parking brake? Just curious.

I use a K&N Filter for my intake filter so I don't think the problem lies there (the first time I took it to EG Toyota, he had checked that and said that it looked ok after trashing K&N filters on anything other than performance cars). You are supposed to clean them every 50k miles so it is probably due for a cleaning though.

I have not looked at the MAF intake/sensor. Is that something a service department would look at? If not, it wouldn't surprise me. Is there a thread with instructions on how to inspect/clean that?

Yes! That is one of the things I tried to explain to the service people. As an engineer, I become very en-tuned to the vehicle I'm driving so I notice any odd behavior. While the engine will shut off like it's supposed to when I come to a stop (either right away or after 7-10 seconds) the engine has been running harder for mild acceleration. Also, on hill climb the engine will race as if it's at max rpm (which it very well could be). If you look at the two pics I posted in post #30(?), you'll see some snap shots I took of the energy flow screen at highway speed. It is showing what I consider unusual behavior, such as the car using the battery hardly at all for hill climbing. When I drive up the grapevine in California (a somewhat steep incline for several miles) in the past, the battery would drain down to 2 or 3 bars (sometimes one) by the time I reached more level ground. When the problem started, it would maintain the battery level at 7 bars on the freeway and only drain down to 6 bars by the time I finished climbing up the grapevine! It was a "Wth?!" moment for me.

Thanks! One of my strengths in problem solving is tenacity. I don't easily give up on a problem until it's solved. When I do find a solution, I'll be sure to post it here.

 

Forgot to mention that last night I checked the voltage on the 12 V battery before going to an appointment. The unloaded voltage (acc mode) showed 11.9 V and the loaded voltage (ig-on) showed 11.5 V. This was after sitting in the garage since Wednesday night. Don't know if the amount of time that passed makes a difference though. In any case, I plan to order the Optima today.

 

Thanks for the link. Another project for this weekend! At least it sounds a lot easier than taking apart the trunk to clean an already clean battery fan! I have nothing against uart's advice, it was a good thing to check. It just took me 2 hours to do it! And I lost one of my sockets (hopefully it will turn up when I replace the battery)!

In thinking about the whole K&N filter & MAF sensor thing, coupled by the 12 V battery, it seems to me that by themselves, they shouldn't cause a significant drop in mileage. But perhaps combined? I got a notice today that my Optima battery shipped. Don't think it'll get here before the weekend so I'll have next week to drive the Prius around and see what effect cleaning the sensor has.

Would you recommend switching back to the old paper filter rather than using the K&N?

 

Re: 2007 Low MPG -- Can't figure our why -- Update II

Hi all,

Ok, so I cleaned out the MAF sensor on Saturday. There was definitely some grime on the thing. The ambient temp bulb was black and not amber. After spraying the sensor several times (per the instructions on the can) there was definitely a ring of black on the shop towel where the sensor was sitting as I sprayed it. Once I cleaned it, the bulb was amber again and it looked almost new. I drove the Prius to work this morning but initial results are indeterminate. The battery pack stayed in the "blue zone" at 6 bars of charge on the freeway but I couldn't tell if the vehicles behavior was due to that not being the problem or if it was due to the ambient temperature (it was about 40 F this morning). I'm anticipating a warmer commute home (60+ F) so we'll see what happens (also, it wouldn't be the first time I had blue 6 bars on the trip into downtown and green 7 bars on the trip home).

The new Optima 12 V is due to arrive today and I plan to replace the existing battery with the Optima regardless of the results of the MAF Sensor cleaning since the voltage is a bit on the low side and it's about time anyway. I haven't replaced the K&N with a paper filter yet, I'll probably do that this weekend.

Hopefully, I'll have good news tomorrow. If not, the next step will be the 12 V battery!

 

Re: 2007 Low MPG -- Can't figure our why -- Update II

No, not yet. I was on foot when I went to the auto parts store so I didn't want to carry anymore than just the can of cleaner (although I went back later and forgot all about getting an air filter).

In any case, the results of my drive home yesterday are that the Prius stopped trying to keep the battery at 7 bars of charge and started using the battery more while driving on the freeway. I still think the engine is racing a little too much and the battery is not helping enough though. Perhaps that will be fixed with replacing the 12 V? If that's the case, then that might mean my recent premise (that it is a combination of the battery and sensor) might turn out to be true.

I'll be installing the 12 V this weekend (as well as a standard air filter) and we'll see what that does. I suspect, however, the true test will be the next trip to SoCal (whenever that will be).

 

Hi LS5099:

Thanks for your info. I've addressed them individually below.

I am running Kumyo's (forgot which type but they're supposed to be the fuel efficient all season tires). I had Kumyo's on my '96 Sentra before we gave that car to my brother-in-law and didn't have any problems. Until recently, I had been running them at around 36 to 38 psi depending on how cold mornings are, but starting yesterday, I'm filling them up to 40 along with my Leaf's Ecopia's (which is what I really wanted on the Prius but America's Tire didn't have them in stock). I rotate my tires around every 10,000 miles (every other oil change). Funny your stock tires only lasted 20k. Mine made it to about 45k before I had to replace them. We'll see how long the Kumyo's last (supposed to be warranted up to 100k).

Ah yes, the injector cleaner thing. I could actually go into a lengthy explanation as to why those really don't do much. But suffice it to say, it has been proven to me by more than one person from California's Air Resources Board (where I used to work) and California's Air Districts that they do not do much. But that is the subject for another post (if you're interested).

I actually did find the problem which is following in the next post.

Thanks for the feedback!

 

Re: 2007 Low MPG -- Can't figure our why -- Update III

Hi all,

This should be one of my last updates. Yesterday I installed the Optima 12 V battery I ordered (and found my 10 mm socket, woo hoo!!) and drove the car around for a little bit. After some odd startup behavior, everything ran smoothly. My first impression is that she is back to her old self. MG2 is helping as much as it should be, MG1 is charging the battery as much as it should be, it looks like everything is back to normal! There is a good chance we will be driving to SoCal this weekend so the real test will be to see how our mileage does on the trip and how the car behaves on the grapevine. If we're in the neighborhood of about 43 MPG on the trip and if the HV battery is drained at the top of the grapevine, I will call it problem solved!

I am still convinced that it was probably both the 12 V and MAF sensor though. The car didn't stop trying to keep the HV battery charged at 7 bars until after I cleaned it. I think I may make cleaning that thing a regular maintenance item (especially since its really easy!).

@efusco -- I went ahead and replaced the K&N Filter with a paper filter. We'll see how that goes. I may decide to clean the K&N Filter and reinstall it at some point. I'm thinking that cleaning the MAF sensor and K&N Filter at the same time (roughly every 50k miles) is probably a good maintenance practice. Unless you think they should both be cleaned sooner?

Thanks everyone for all of your help! It was greatly appreciated!

 

Re: 2007 Low MPG -- Can't figure our why -- Update III

No worries, Evan picked up on it.

Yeah, the tech at EG Toyota wasn't too excited about my having a K&N filter either. He said that they were primarily for performance cars and didn't think much of them in your typical street car. Did he say why he advised against them?

For my part, as an engineer who understands how cars work but hasn't really spent much time working on them, I always thought that more air and less restriction improved combustion efficiency and thus fuel efficiency and emissions (NOx being the exception, but that's what TWCs are for). But maybe there is so little difference for a car like the Prius between the K&N and standard paper filter that it's pointless to get the K&N? Unless you don't want to be throwing away filters. I don't know, but every car I've ever owned had a K&N, the '94 Saturn, the '05 Mazda and the Prius (I didn't put one in the Sentra but it was so old by the time I got it, I didn't see the point).

 

Ah ha! You just cleared up some of my confusion! I always thought that was compression ratio which didn't make sense to me since the Otto Cycle compression ratio should be around 8:1 or 9:1. Incidentally, when I first wrote that, I was thinking of the stoichiometric ratio for ideal air of 14.7 but somehow equated that with most power rather than least emissions (although, since you can never hit 14.7 exactly, you can have excess air and still achieve complete combustion, but I digress). I think what I was trying to say is that the more "restrictive" paper filter would cause the engine to "choke" a little more (ie less air flow not less air) vs the K&N filter which is less restrictive allowing better air flow and less "choking." But maybe my understanding is still off.

Oh well, combustion was never my strong point, too much chemistry which I suck at. Now power cycles, especially the Brayton and Rankine cycles...!