2006 Prius huge MPG drop. Why??

Hello all

I have been reading over quite a few of the posts in this section to try and get some ideas as to why my Prius's MPG has dropped. Right now I am seeing roughly 41 MPG and that is after changing the spark plugs with brand new NGK Iridium's, Air intake filter and did an oil change since it was due for one.
I have driven the car roughly 2,500 miles since all of this stuff was done and it just seemed to continue to drop to the 41 MPG mark.

What I have noticed though is that when the motor turns off it shudders and the when it restarts while the car is in park the car will roll back and forth slightly. I have also noticed that the car seems to struggle to get up to the posted speed while climbing an incline and also while the car idles in park or at a complete stop there seems to be a bit of a miss fire. Could that be caused by a bad coil? Because I have tried everything short of taking it to the dealer which I really do not want to do again and be told there is nothing wrong with it when I know there is.

Any help with this issue would be greatly appreciated. Thank you in advance!

 

Well the last time I had it in to Toyota for service just before the fall they apparently checked the trans axle fluid and checked for valve clearance and said everything was fine even though this car was seeing about 46 MPG at that point in time with out running the air conditioner.

 

Apparently according to my wife the fuel consumption was a lot worse than this year. She said it was down around the 30 MPG mark.. OUCH! So I guess the change in the plugs and the air filter did make some difference from last year. Yet I have seen others on here that are still getting at least 50 MPG during the winter and I sure am not seeing that. Before I changed the plugs I was seeing 46 MPG (5.1 Liters/100 kms) and that was during the summer. Either there is an underlying problem or otherwise we are doing something wrong lol. Really the thing that strikes me as bizarre is my old 95 Corolla got way better MPG than this Prius.

 

No did not change the firing order which is kind of hard to do since everything is crammed in there lol . I had thought of a defective plug but then I thought back to before the plugs were changed and it still had a miss fire. I also pulled the coil boots and the plugs one by one and reinstalled them with no change. I also used a little dielectric grease on each boot just to make sure moisture was not the culprit. The dielectric grease seemed to help tone down the miss fire for about 10-20 miles. So that got me to thinking maybe one of the coils are leaking a little bit but not to the point where the engine will not start when damp.

 

The only difference was I used NGK plugs instead of the OEM Denso plugs the dealers would install.

 

There has been a decrease in power and the engine seems to rev more than it should to climb an incline. My car is not throwing any codes either but please excuse me for my lack of Hybrid knowledge but what is the ice for? The only thing that I see is the ready light on the dash and that is it other than the display for the consumption and etc. However I assume that the 12v is the battery for the electric motor?

 

I was not sure where I was supposed to paste this so please forgive me if I have pasted this in the wrong spot

- 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) 41MPG

- What fuel economy are you expecting and why? 50 MPG. Because a lot of others are seeing that or better during the winter and the summer.

- What are the approximate outside air temps? Between -6c to +6c

- How long are your trips? long trips- 124 miles or more. Short trips 10-62 miles.

- 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? Most of the driving has been highway. Rough overall speed 50 MPH-65 MPH

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

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

- 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) Do not know the car was purchased from Toyota as previously owned vehicle. Do not know the reading of the 12v battery which I assume that this battery that is being spoken of is for the electric motor.

Have you had your alignment checked? Any pulling or abnormal tire wear? No. The car does wander on the highway but no abnormal tread ware.

- 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). Yes we are using the OEM rims with Michelin Harmony 185/65/15.

- What are your tire pressures? 42 Front 40 Rear

- 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.) 1995 Toyota Corolla 1.6 liter 5 speed manual transmission. Was getting about 50-55 MPG

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking? Trying to drive as easy as possible with out hard take offs and try to stick close to the posted speed limit. I brake very easy and not wait until the last minute to start slowing down.

- 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? I do not know. Do not know how to check what mode the car is in.

- HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to? Heater is set for auto and the temp is usually set between 22c-23c and the blower fan is usually set to medium or just under medium.

- 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 spark plugs w/ NGK Iridium, oil and filter change (Had the oil flushed on last oil change), New air intake filter. Nothing else has changed.

 

My 95 Corolla did in fact get 50-55 MPG. That is no lie my friend. Anyway I use D mode for driving. Does the mode make a difference?? Also I did read your note about the auto setting for the heater and that is the next thing I am going to change the next time I go out. As for the 12v battery I am almost sure that Toyota tested all that while it was in for service back in the summer and everything checked out fine according to Toyota. But who knows maybe something changed since then and the battery has gotten weak. I will be looking into that as well.