Decreased MPG Post Spark Plug and Air Filter Change

How many miles on the car? First spark plug change?
Have you checked 12v battery via the batt check on the MFD?

 

Reporting this courtesy CWERDNA

To test the 12 volt, go through Weird stuff happening? MPGs dropping? Test The Battery after the car has sat overnight.

 

It would help if you could be more specific. For example, which particular model of NGK plugs did you use? And "It appears the battery is ok" is the sort of thing that really sets us off, because the numbers are important. People think, "it must be okay if it is 12.0V" when the reality is that a 12.0V reading with no load indicates a deeply discharged battery.

Is that air filter paper or oiled? How about engine oil, did you install 3.5 quarts of 5w30 on the last change?

Next time you go out to start the car if it has been sitting for a while, see how long it takes to reach idle shutoff.

 

Please state the actual battery voltages. The car is not going to just tell you that the battery is bad. If the voltage was below 12.4v with no accessories on and after the car has sat for 6+ hours then your battery is weak and should be replaced.

You stated that the plugs were pregapped but did you check them to ensure they were within spec? Improperly gapped plugs could reduce mpg but they would have to be way off. Have you tried pulling codes to see if you have any misfires?

 

You left out a critical part of the check sequence. Add a load while NOT in "READY" mode (e.g. accessories, headlights) and see how far the voltage drops. If noticeably below 12v your battery is suspect. If it continues to drop at a noticeable rate your battery is toast.

JeffD

 

There will not be any codes if you do not have any of the check engine or other warning lamps lit. Well, okay, there might be a pending code, but you'd have to have something serious like a misfire to take the MPG hit.

I think your 12V battery is weak and should be replaced. Did you have the radio on while you were working on the plugs?

Idle shutoff: This is the hallmark of most hybrids, where, if the car is not moving, the engine will turn itself off to save fuel and reduce emissions. If you start the car, it should turn itself off within a few minutes, given the warm weather that you bastard Californians are currently enjoying.

 

There are three choices:
- another identical Toyota battery
- Optima DS46B24R YellowTop
- Optima D51 with adapter kit from eLearnaid Toyota Prius 12 Volt Auxiliary Battery with install kit for 2004 - 11

The two Optimas are identical except for post size, which is why eLearnaid sells an adapter kit. There are no other direct drop-in replacements.

 

Why not the Toyota OEM filter?

 

Yes that's pretty low, but I wonder why it would go bad (as something is now different) just after you changed the plugs? Can you think of anything that may have drained the battery, like seilerts suggested about the radio for example. You could try giving the battery a 12 hour slow charge and see if it improves.

 

Nquinto,
I want to know what happened. Did the new battery fix your FE problem?

 

If the 12v battery has not been replaced, it needs to be. That will help alot. Get the Optima with proper post size.
Remove and clean your MAF sensor. Very easy, buy some MAF sensor cleaner, unplug it, remove 2 small phillips head screws. Pull it out. Spray the temp sensor bulb and "hot wires" of the sensor. Reinstall and plug in.
While you have it out I'd clean the throttle body throat with TB cleaner too, while its easy to get to.
Your Prius will run like new and be back to great mileage.
You did get the spark plugs tight, right
Don't worry about the gap.