Sudden & Consistent Drop in MPG

I suggest that you raise the rear of the car, release the parking brake, and try spinning the rear wheels to see if they freely spin. If they do not freely spin, then the parking brake is dragging and causing the mpg drop that you noticed.

 

It's warmed up here on the East Coast, enough so that the war department insists on turning on the A/C! I swear she doesn't think the windows go up and down!
It's caused a small drop in the MPG.
Could you be running the A/C a bit more than normal?

 

I would have your spark plugs checked.. Reduction in Power and MPG ? Ummm.....

 

bad gas? water in the tank? You said it came on all at once. junk in the line or injectors?

 

Hi John,

When the traction battery fails, you will see the SOC indicator move up and down much more quickly as compared to when the battery is functioning. This is due to increased internal resistance which causes the measured battery voltage to fluctuate up and down more than normal.

If your SOC gauge is staying in the low green to high blue area, then this seems pretty normal.

It wouldn't hurt to check the spark igniters and spark plugs just to make sure that there are no problems (for example, moisture in the spark plug tubes that cause misfiring.)

I assume that you have no warning lights on, so if any engine problem exists, it must be subtle.

What gasoline brand and octane level are you using? Should be using 87 octane regular, ideally a top-tier brand such as Chevron, Shell, Union 76 etc.

 

Did you bump a curb? Check the wheel alignment. Other suggestions have been covered by others.

Oh by the way there is no fuel flow measuring device. Fuel consumption calculations are made from injector dwell time, multiplied by frequency or RPM then multiplied by a known injector flow rate divided by speed over the road.

 

Is there any chance that your gasoline now has ethanol added????

 

Check your engine air filter? Perhaps something wrong with your oil filter?

 

I'd also try another gas station. Shell for example. Perhaps that station has had a different set of cost-cutting additives added or has switched suppliers?

 

We need to know what is happening when your vehicle is getting 39 MPG but we don't have any detailed data. We need to take an empirical approach to find out what is going on and that means running instrumented tests. This may require some technical skills and instrumentation that you'll have to setup and run.

My first question is to find out what your vehicle performance is above and below 42 mph, the boundary between two control laws. What I would like to propose is to look around Orange County for a test route you can drive without stopping, or a minimum number, in light traffic conditions:

• 45-60 mph cruise control speed - some place were you can drive for ~20 minutes at ONE constant, cruise control speed on a level road. Check your local freeways to find a loop that you can safely drive at this given speed. Do one practice loop and then reset the MFD display and listen to the radio or tunes for 20-30 minutes. When you get back to the starting point after 20-30 minutes, report the MPG and miles and speed (or start/stop times.) Temperature is also needed.
• 25-35 mph cruise control speed - a route where you can drive for ~20 minutes at one constant, cruise control speed on a level road. Check your local streets but large shopping center parking lots are prime candidates. Repeat the same test at one speed for 20 minutes and record the MPG and miles and speed (or start/stop times.) Temperature is also needed.

Since these areas are away from your home and work, report their location so we can look at the route using Google Earth. This will let us understand how much altitude change was involved. We will take your numbers and compare them with this chart to see if we can see a pattern that indicates where the problem might lie:


This will let us isolate auto-stop operation apart from ICE powered operation. It will also put the ICE in two power ranges to help fault-isolate the problem(s).

Do you have a laptop with a stereo microphone input? This would allow using audio capture software to measure some vehicle sounds during operation. In some cases, we can actually record some energy flows that are ordinarily invisible. What operating system does it use?

Do you have or would you consider getting an OBD scanner that runs from a laptop? This would allow us to look at some of the metrics that can be captured from the bus. The problem is this is an advanced area and we'll have to ask some of the folks at the leading edge to help with the data analysis. The capture is simple, just record everything from the OBD. The analysis will be the problem.

I do not remember seeing your existing 39 MPG route being posted with start and stop addresses and route. Using Google Earth, we can look at alternate routes that might for the expense of a little more time, give some immediate mileage relief. If you are concerned about privacy, give starting and ending locations offset a block or so from the real addresses. Just try to make sure the 'mock' addresses are at the same altitude.

There are a few problems that are very difficult to diagnose. Folks have already shared what we call "low hanging fruit" and it looks like you've make a good effort. But the hard problems often take a bit longer to figure out.

One thing not clear is how you measure your MPG. Is this on a daily basis or just per tank? It would be useful to reset the MPG display in the morning and start tracking the MPG and distance at the end of each day. In my case, I monitor each trip by resetting the MPG each time I start and check it when I will be parking for more than an hour. This allows me to see patterns that have and continues to teach lessons about how our Prius runs.

Bob Wilson

 

1. Did the mpg improve at all after the rear brakes were adjusted?
2. Did the mechanic check the engine coolant temp? It should be around 85 degrees C. I am wondering whether your engine thermostat might be stuck open, which would cause the engine to run inefficiently.
3. What tire pressure are you using?
4. Although you had previously dismissed this as an issue, I suggest that you have your wheel alignment checked. Front toe-in should be as close to zero as possible. Rear toe-in is normally not adjustable by the dealer, but see how close to spec it is.

 

I've noticed exactly the same problem with my 2006 for the past month or more, and I'm willing to bet money it has something to do with all the gas pumps having a "10% Ethanol Added" sticker on them here in South Florida. Does anyone know a station NOT adding ethanol?