2010 Gen 3 Prius Fuel Computer - False readings

Odd. The number is calculated by the on-board engine computer. It knows how much gas is going into the engine since the gas in the fuel injection pipeline is at at constant pressure, and the computer controls how long the valve(s) stay open on the injectors, and what the flow is when the injectors are working properly. This number is then divided into the distance traveled, thus yielding Miles per Gallon. (There are other places in Europe where they flip the divisor, and give Liters/100 Km, but, seeing as you're in Scotland, your car isn't doing that.)

There are a couple of possibilities.

1. Your car really is doing that. There have been cases where somebody in the U.S. somehow, typically by accident, got a tankful of gas without ethanol. (We have 5% Ethanol in the gas in the U.S... Don't ask.) When that happens, the gas mileage really perks right up. Second surprise trick: Warm weather definitely improves gas mileage by up to 5 mpg. Scotland's north, so I guess if you've been having a summer of average temperatures in the 60's, and it suddenly bumped up into the 80's, that might have done something.
2. Somehow, you're running much leaner in the gas mixture than usual, and the car computer knows this. Not sure how it would know, but we'll give the Toyota engineers credit for the moment. If you really were doing this, I'd expect pinging on the lean mixture.. But maybe not, since engines these days have both ping detectors that detect the condition and have the ability to rapidly advance the spark, thus preventing the ping. If this is really happening it's not good: Combustion temperatures are much higher with a lean mixture and can, in the medium to long term, damage the engine. There's a fairly straightfoward way to check to see if the mixture is roughly right: Look at the tailpipe. Cars that run rich have black-colored tail pipes near the exhaust from all the soot; those that run lean are a white to light gray. One that's tuned correctly should be a gray-tan color, best checked after the car has been running for a half-hour or so. (A newly started car runs rich, by design, which can throw one off.). You can look at practically any other car as a reference, then check yours. If this is happening (no guarantees), there might be something stuck or clogged in the fuel injectors, the fuel pump, or the fuel filter. By the by, gas stations sometimes have a little sensor they can stick in the tailpipe which tells the user %CO, %O2, and the like, and will directly tell one if one is running lean or not. If this is your problem, you should get it fixed soon. And I'd be surprised that you wouldn't have warning lights all over.
3. There's a real, live math error going on. Speaking as a EE who works on and troubleshoots distributed computer systems for a living, there are a few possibilities here: If things like distance traveled is being point A to point B, and then point B is doing the calculating, there may be problems with the actual sending of the data at point A; something wrong with the wires between A and B; and the reading of the data at point B. If, for example, your odometer is also saying things like you've gone 10 miles when you've only gone 8, then there's something wrong with the magnetic sensor that picks up wheel rotation and its associated circuitry. (That stuff is exposed to salt on the roads and such: My old Chief Petty Officer used to say that 90% of the problems in electronics are with the wires. He was right.) If your odometer/speedometer are reading correctly, well, I guess that the distance sensor goes to the computer that displays that information on the dash, first, then gets sent to the engine computer. Then you've got the wires between the two computers, however the data is read out of A, how it's read into B, and so forth. I would take a flying guess that the dealership and their computer tools could make pretty short work of something like this.. But only if the engineers at Toyota put the software support to read values back and forth so they can be tested.

So: Check your speedometer and odometer. They reading correctly? Next, check your tailpipe. Signs of funny colors? If nothing appears wrong, then you've either got some really good gas or a (likely) wiring fault under the dash somewhere.

Good luck!

KBeck

 

Mike,
I was on the opposite side of the ship: Ship's company on an aircraft carrier, but assigned to fix aircraft electronics. (Airdale or tweet, by the jargon of the day.)

Been in the engine rooms for visits, though. Fun: 3-story high boilers, 2-story high feed pumps, and 2 1/2-foot diameter rotating shafts that one has to duck under. Seems that the XO got the idea that during the bi-weekly inspections the the officers who worked up on the flight deck would check the boiler rooms and vice versa. I was dragged along to take notes. After a couple of months of this it was discovered the the engine guys, who could actually see paint (rather than oil-covered grit) were giving 100% ratings to upstairs corridors no matter how dirty they were, and the flight guys were kind of having apoplexy, no matter how much scrubbing the engine gang did. We won't talk about views of the bilge.

KBeck

 

It's the weather we're having. 20-25c is a sweet spot for quick engine warm up and thus improved economy.

My experience of summer mpg's for the Prius on a steady flat A road is as follows;

40 mph (stuck behind a slow truck) = 75-80+ mpg (yes you read right)
50 mph steady drive = 65-75 mpg
60 mph stead A road = 60-65 mpg

In winter these drop to about 50-55 mpg at 60, 55-60 at 50 mph and about 65 at a steady 40 mph.

In those 50 mph average speed camera sections on the motorway you'll get 80+ mpg. Almost unbelievable but true. I have many photos of 30 minutes of average fuel economy at over 80 mpg UK.




ps Just noticed your mileage. My Prius was just about run in between 10 - 15 k miles.