Fuel Economy Dropping ???

[QuiGonJohn]

Help, my fuel economy has been dropping. I have an '09 which I got in Dec 2008. I used to always get about 40 MPG, that is it almost always displayed at least 40MPG or more. Lately I hardly get that high and if I do, it easily starts dropping into 39.X or even 38.X.

A couple of things might help determine what my problem might be and what I should do.

1. A few months ago, I found this piece of foam on the floor of my garage, under my Prius, near the front, (under the engine compartment). I cannot determine exactly where it came from, if it is even off the car. There is a small chance it is from somewhere else in my garage or my workbench, although it does not look like anything I have seen before. I have attached a picture, the quarters are there to give a size perspective.

2. The other day I got in the car and for the first time that I ever recall or noticed, I heard a loud fan noise coming from the vicinity of the Battery Vent in the back seat. My vent is not covered or blocked and I swear I never heard this before.

Any help would be appreciated, thanks!

[Rokeby]

Originally Posted by QuiGonJohn

Help, my fuel economy has been dropping. I have an '09 which I got
in Dec 2008. I used to always get about 40 MPG, that is it almost
always displayed at least 40MPG or more. Lately I hardly get that
high and if I do, it easily starts dropping into 39.X or even 38.X.

QuiGonJohn,

As a symptom of something being wrong, this slight reduction of MPG's
is not very telling, it could be due to slightly heavier air conditioning
loading. But... see below.

Have you checked your tire pressures lately...
Checked oil level after dealer service visit?

Originally Posted by QuiGonJohn

1. A few months ago, I found this piece of foam on the floor of my
garage, under my Prius, near the front, (under the engine
compartment).

I haven't a clue what that piece might be.

Originally Posted by QuiGonJohn

2. The other day I got in the car and for the first time that I ever
recall or noticed, I heard a loud fan noise coming from the vicinity of
the Battery Vent in the back seat. My vent is not covered or blocked
and I swear I never heard this before.

You didn't say when this occurred during the day. I'm going to
presume it happened in the afternoon, after the car sat in the full sun
for a lengthy period of time. In this case the car interior heats up as
high as 180 degF. In time, that heat makes it's way to the HV battery.
IIRC, this condition is often referred to as a "hot soak."

At elevated temps the battery has reduced capability to accept a
charge, and to provide power either to move the vehicle or to keep
the 12v battery charged. At any rate, the HV battery cooling fan
running at start-up in these conditions is symptomatic of the high
HV battery temps.

If the HV battery can't provide motive power, than the ICE, the
Internal Combustion Engine, has to and there would be slightly
reduced MPGs.

There are other conditions such as the DC-to-DC converter cooling
pump going on the fritz that might also be suspect here. Also, might
could be the 12v battery is going south requiring the converter to
support 12v loads, like air conditioning, more or less full time.

I'm in well over my head at this point and will defer to the dyed-in-
the-wool techno types for additional comments.

Hope this helps.

[QuiGonJohn]

The Battery Cooling fan was in the afternoon, car in the sun, but not for a real long time, I'd say 90 minutes or so.

[Rokeby]

I don't know exactly how much time is needed to overheat the HV battery.
However. I would strongly suspect "hot soaking" as the cause of the HV
battery cooling fan coming on.

If the HV battery is being overheated, there is reason to believe the 12v
battery is too. Over repeated heatings, the 12v battery service-life will
be reduced below the typical 4 years... $$

For the health of both the HV battery and the smaller 12v, it would be
worth the effort/time to find some shade to park in.

You might also consider some high UV reflectivity window tinting. A lower
cost alternative is buying the reflective window shades and plastic
window vents available at the PRIUSchat Store to help reduce the
interior temps you are apparently experiencing.

Lots of threads here on these alternatives. Use the search function to
find 'em.

[richard schumacher]

Check tires pressures at least monthly. Higher pressures, up to the maximum shown on the side of the tire, make it easier to get higher MPGs but do make the ride more harsh. Lots of people use 42/40 PSI front/rear in the original tires. Note that some service techs looooove to reduce tire pressures to those given on the door pillar...

Always check the oil level yourself after every oil change. If it's above the top mark on the dipstick have it drained down. Unfortunately the shop manual for the Prius disagrees with the dipstick; according to the dipstick three quarts are usually sufficient at an oil change.

Heavy A/C use normally reduces MPGs by a few percent. Cabin air is used to cool the battery system, so leaving the A/C off on a hot day can be counter-productive.

Account for the above and see what happens.