Inexplicable Gas Mileage Drop

Hey TXPriusOwner. Check your tire pressures! :evil:

Arrh just kidding. That's a pretty serious drop in MPG you've got there. I'd wait until you re-fill with a fresh tank of gas and see if the problem persists and take it from there.

 

There's not enough info to guess at the reason.

As long as you mention it, what were the tire pressures? Were they measured cold, or hot? If any of them were, say, 32 PSI immediately after a three hour drive, that's waaayyy too low.

 

Agreed on both of the above, TXPriusOwner hasn't given enough info. "My tire pressure on this trip has been the same from beginning to end" is leaves us the ambiguity of hot vs. cold pressures.

Also, we don't know how he's measuring his mileage vs. before? MFD? Manual calculations? Both? There can be a huge amount of variance w/manual calculations due to the bladder and a huge divergence on some tanks from MFD values. One needs to average them out, over time.

Answering many of the questions at http://priuschat.com/forums/gen-ii-...-answer-these-questions-esp-if-youre-new.html would help.

 

You folks with 2007 and earlier prius that are having dramatic gas mileage drops need to consider your 12v battery. It's quite well documented that the 12v battery is only good for 3-4 years and when it begins to die (not hold full charge) it causes the ICE to run more than usual and reduce gas mileage considerably.
Symptoms of a failing 12v battery are dim interior lights when you first get in, car not starting (ready mode), needing to get a jump start, and unusual MFD messages about "parking lock malfunction".
You can get the battery from the dealer, or a better alternative is the Optima prius replacement battery kit from elearnaid.com.

I had all these symptoms (and a mileage drop from consistent high 50's to low 40's)and when I replacaed the 12v battery my Prius was perfect again.

PS. all you folks need to increase your tire pressure to at least 40 psi.

 

I have the same problem with my 2005. Overall average since purchase is 48 mpg. Now getting high 30's. I know it's winter but I've gone through 5 previous winters WITHOUT this problem. I think my 12vdc battery is ok, but I'll check. The oil level may be a little high too, I'll check that too. I'll also check the air filter. But, my biggest concern is ethanol. Ohio now let's stations add up to 10% without any notice. A phone call to a UDF manager should know said all brands were now 10%. There are different state rules regarding posting, e.g Florida requires posting ANY ethanol on the pump. Michigan is same as Ohio. Google topic to find your state. This could very well be a new "TAX" courtesy of the liberals. Regards, John

 

E10 has 3% less energy per gallon than gasoline. It maybe could reduce MPGs more than 3% during the first few tanks because it is removing water from the fuel tank, but if your fuel economy has dropped by 25% there is some other contributer.

 

Hi. I have almost the same problem with my 2007 prius.I got 33 mpg before changing tires to pirelli low rolling resistance and with 36lbs. pressure, I get exactly the same mileage! A lousy 32 mpg. I have had the injectors and throttle body cleaned, spark plugs checked, new filters, air & oil & still have the problem Gosch Toyota is scratching their heads along with me! Any feedback?????
H.Ray

 

As you were directed the first time you posted w/mileage complaints, please answer the questions at http://priuschat.com/forums/gen-ii-...-answer-these-questions-esp-if-youre-new.html.

 

I just checked the oil and filter. The oil is up to the upper dot and the filter needs replaced. I'll be making a trip soon so I'll see if fixing these two items will get it back to "normal." Thanks Richard

 

Oil at the FULL dot shouldn't by itself be the cause either. Check the 12V battery; sometimes while dying they can reduce MPGs because the car uses energy trying to keep them charged. If you don't have a voltmeter use the built-in test procedure (originally written up by jdenenberg):
http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html

 

I'm having a very similar issue with my 2007 Driftwood Pearl Prius!

I put on Michelin Energy tires about 6 months ago and was running in the low-mid 50s for mileage. After my 90,000 mile service about 8 weeks ago (you read that correctly, I drive A LOT), my car now lurches, very labored acceleration with the motor winding out and:

"While looking at the Energy Monitor screen I found that the car seems to be charging the battery CONSTANTLY. Usually when you accelerate, it uses the battery power and the ICE, however it seemed to be using ONLY the ICE to move the car while simultaneously charging the battery, hence less than ideal gas mileage."

I hit the lowest mileage in my car's life a few weeks ago- 36mpgs. Since then, I've gotten it back up to the mid 40s and taken it to the dealer and there are no diagnostic codes popping up. Nothing explains the drop in mileage and performance. It has gotten colder recently but at this time last year, I was bogging through 16 inches of snow in bonafide SNOW tires and still bringing in 46mpgs. Those conditions do not exist currently.

I have to miss a few hours of work tomorrow to take the master tech out on a test drive to prove my point. My Prius used to glide down the road, now I have to hold the gas pedal down to go downhill - no regeneration anymore. But, nothing is wrong, right? :/

P.S. I'm fanatical about my tire pressure - 40/38.

 

Simple stuff: Check the 12V battery. Clean the MAF sensor.

What exactly did they do at the 80K service?

 

They replaced the PCV valve, performed a regular oil change (5w30), rotated tires, performed an intake service, etc. They also found that the transmission fluid was dirty so they flushed and refilled the transmission.

My first inclination was that they did not refill the fluid to the proper level or that they used the wrong fluid. I have asked them repeatedly to examine the transmission fluid and my service advisor assures me that it checks out.

My days of pulse and glide are over (that's what she said?). My car sits like a brick. I have to keep my foot on the gas to maintain speed going down a hill - if I take my foot off the gas while traveling downhill, I rapidly lose acceleration as if stepping on the brake. Previously, I could coast down the hill, foot off the gas, and maintain speed. I'm using twice as much fuel as I did before this issue surfaced after the 90k service and it seems like the engine is always running to charge the battery. But that's "normal" according to the dealership.

My car is with these people now - I asked them to check the 12V battery but I will likely do the test previously cited here simply because I don't trust the jerk who's handling my case.

 

Hi ETh...,

This is why so many of us perfer to do standard maintenance tasks ourselves. Its difficult to know what they screwed up. If their "intake service" included cleaning the MAF sensor, and they greased it up with dirty fingers, that could cause the issues you are having. If they changed the oil to 10W40 and/or filled above the upper dipstick mark, that could cause the issues you have. If they dropped the tire pressure to the door panel sticker value, and it was 80 f when they did it, and now its 30 F, well, now your tires are down around 30 PSI.

I am not sure how they can be sure the transmission fluid checks out. I have done the fluid change on the 2nd Gen Prius. Its just not that simple to get at. And, how did they know it was dirty in the first place? Although, its wise to get it changed at 80 K miles (I did mine at 40 K miles), there is no transmission dip stick. You basically have to change it, to know its dirty.

I vote for the engine oil being at fault. In a service department like at a dealarship, the oil comes down through a spigot that hangs from the ceiling. There is a selector on the spigot to select the oil type. If the guy just was in a hurry, and did not select the right oil, and then held the spigot gun for just a second too long, he has done the damage. And to fix it, he has to pull the drain, replace the drain seal (go back to parts for a new seal, and admidt he screwed up , waste the oil he did put in). Its just so much simpler for the incompetent oil mechanic to send you on your way. When you come back, its so much simpler for the dealarship to say "we did not do anything wrong". Rather than to do the job completely over from scratch, and waste all their profit on the job.

Do do the 12 V battery voltage test outlined here on PC. Its possible you had a simultaneous battery failure around the time of your sevice visit.

I would call Toyota Customer Care and report what happens. Hopefully, they will be on your side and lean on this stealership to get there act together. You had the fantastic car you paid for, and now you got so so, and your out the extra money you paid for that fanstastic car. Its just not right....

 

Are you 100% certain that you're driving in "D" and not in "B". That's a really weird symptom and I cant really think of any other explanation. I cant see how the wrong grade engine oil could possibly cause that particular problem.

The only other thing I could think of is they did something seriously wrong with the transmission during the fluid change.

 

I'm definitely not driving in B - I only use that function when I need help slowing down in a hurry. Admittedly, I'm a tad OCD so I always check and double check things like that.

When I initially got the car back from the dealer after the 90k service and realized something was wonky, I parked it and checked the oil level before firing it up the next morning. Level was fine. I also checked the tires to see if that had under-inflated from my usual setting, the tires were fine and continue to be so.

These are great ideas and I'll mention them to the dealer, who at this point, STILL has my car. I went on a test drive with their Master Technician this morning who didn't seem to think that there was anything off. The car did seem to be performing better although nothing has really changed, including the weather and my driving habits.

A hybrid-savvy-but-big-SUV-driving friend of mine suggested that the electric motor is not disengaging properly and therefore creating a braking resistance on the ICE.

Has anyone heard of such a thing happening?

 

By chance is your emergency brake engaged or partially engaged?

 

Where are you in VA? If you're anywhere close to Richmond, I don't mind taking a ride with you to see what guidance, if any, I might be able to offer. I'm nothing close to a technician, but we can hook up my ScanGauge to see if the numbers look right. And after five years I have a pretty good idea of normal operation in a variety of conditions.

Your friend may be hybrid-savvy, but hybrid designs vary. Prius electric motors (there are two of them) don't "disengage." They are always engaged through the power split device within the transaxle. That doesn't rule out a problem with them, but problems are rare. I would look elsewhere.

 

I use my parking brake all of the time, and each time I release it, I always put my toe behind the pedal and push it toward me, just to make sure it isn't still hanging on a little bit. Also, I had them check to see if they had overtightened it the last time it was serviced - to no avail. :/

 

I'm about an hour away from Chesterfield but I may take you up on your offer if the problem persists - it seems to have evaporated for the moment. All of the checks came back clear today so I'll just have to closely monitor the performance - nothing new there.