Any thoughts on what to check for low MPG?

I’ve got a 2004 Prius with 195,800 miles on it, and I’m taking it on a cross country highway trip (cruise control at 75mph, no hills) and my MPG is in the 34-36 range. This seems low considering it used to get 42-47 in the same conditions, and I’m at a little bit of a loss as far as what I should check. Here’s what’s been done so far:

HV battery replaced with one from NewPriusBatteries.com 400 miles ago

12v battery replaced about 800 miles (about a month ago). Voltage at 11.9V with car in accessory mode with headlights on.

Struts and control arm bushings at 190,000, along with an alignment done at that time

Tires were replaced at 190,000 with Michelin Defenders, running at 42PSI all the way around. (I knew I’d take an MPG hit without low rolling resistance tires, but 10+mpg seems excessive)

Oil change at 194,800. Level is exactly at the top mark on the dipstick still

Spark Plugs were done at around 175k, replaced with Bosch Iridium plugs.


The only thing that’s odd is that batt temp sensor 1 seems to be displaying a value about 10-15 degrees less than the other sensors, so I’m wondering if I didn’t clip it properly when I replaced the battery pack. However no warning lights are on. Does anyone happen to know which sensor sensor 1 is, and would this cause a dramatic decrease in MPG? I’m probably going to end up pulling the battery pack again to check the sensors.

I’d appreciate any ideas.

Thanks!

 

So it finally threw a P0A9B code. I think that might be what’s causing it. I was able to get 45+mpg in February before the battery issues started, so I don’t think that the wear on the engine and transmission has increased all that crazy much in the 5k miles I’ve put on it since then. I might look into the fuel/air sensors after I fix the battery sensor. I’ll take care of that and then report back.

 

I didn’t take the actual ECU apart, but I did look at the plug and socket, and didn’t see any corrosion. The bus bars are all new, so no corrosion on them. Would corrosion on the ECU cause an erroneous reading on just one sensor? Because it’s only that one that’s reading low. I’m kinda thinking it didn’t get clipped properly and fell to the bottom of the case (I didn’t move them to the top unfortunately) while the battery was getting installed. If I can’t see a sensor lying on the bottom of the case, I’ll pull apart the computer and look at it.

 

If it does end up happening to be the ECU, where do you suggest I source a new one? If I just buy one used off eBay, what’s the likelihood hood that it will just have the same issue as my current one?


Also, I realized I misread your earlier question a bit. Corrosion on the factory bus bars didn’t look excessively bad to me, although there was some discoloration.

 

I understood what you meant regardless.

Moving off the topic of grammar, the more I drive it the more it wouldn’t surprise me if the ECU is bad. It seems to be not managing the SOC of the battery in a normal manner. (For example it will keep charging the battery at a low current even if it is at the top of the blue bars and I accelerate hard.) I don’t think this would be caused by the temp sensor, even if it isn’t clipped properly (although I will check it).

if I find corrosion, I need to replace the ECU, correct? And what’s the likelihood that if I get a used one it has failed in the exact same way as mine? Is there a way to use circuit board cleaner or something to “fix” my existing computer?

 

The “new” battery is not a rebuild. It is new with cylindrical cells. Look at the post by 2k1Toaster for more info on the packs he sells.

I am confident that the pack is not the issue. I think at this point it is either a.) The battery ECU, or b.) An improperly installed or damaged temperature sensor. See my earlier post for the code that tripped.

I understand that the car is not as efficient as it was when it was new. That is a given. However with the new battery pack, I think it’s reasonable to expect similar fuel mileage to early last year. I’ve only put about 7,000 miles on it since December of last year. I have a hard time believing that a car with a new battery pack and 195,000 miles on it will be so much less efficient than a car with a factory battery and 188,000 miles on it that it loses 10+ mpg.

 

And that’s fine. I’m not expecting it to be a new car. However the all of the new modules were supposedly tested at 6.5ah with a calibrated tester, which is why I installed the kit that I did, but that’s neither here nor there.

The low mpg was likely the early symptom of the P0A9B code that eventually tripped. I just made this thread before the code tripped, so I couldn’t put it in the thread title. Perhaps I should make a new thread that’s more representative, but I didn’t really want to spam the forum with threads.

To be honest, I don’t really care all that much about what gas mileage I get. I just noticed a sudden rapid drop, which told me that something was wrong before the code tripped. The code is now helping to narrow down what is wrong. I want the car to be as reliable as a 200k+mile car can be. If it isn’t as efficient as it was from the factory, I’m okay with that.