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Discussion Starter · #1 ·
Hi: Since purchasing a new Prius last September in Victoria, British Columbia, my city gas consumption has improved from 6.2 litres/100 to 4.5litres/100 at the present with a reasonably consistent 0.2litres/100 reduction each new tank. This was due to better driving habitats (reduced acceleration, more regen breaking, less use of AC, etc.). Last month, I took the Prius to the Toyoto dealership for the required 6 month service and checkup with no complaints other than a minor roughness to the cold morning start. The car was serviced quickly. Within several days of picking it up, I noticed that the gas motor would come on more quickly, even when backing out of the driveway, and would stay on for longer. I also noticed that gas consumption increased from 4.5 l/100 to 4.9 litres/100 over the same routes as before. I phoned the Prius service department to enquire whether they had done some adjustment or tweaking of the system that might account for the small but significant loss of gas mileage. I thought perhaps that the idle or mixture may have been altered as I had previously advised them that the motor was a little rough during cold startup. The technician assured me that there was nothing that they could have adjusted to alter the idle or anything else that might have contributed to this change. I have a learnt a few things about motors over the last 60 years and find it improbable that the Prius technicians had changed nothing. When I called up the Prius dealership and mentioned that I would be bringing the Prius back to have them undo whatever they did, they again insisted that there was nothing that they could have changed.
I would appreciate any possible insight on this issue.
Thanks, Tom
 

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Well, the engine air/fuel is controlled by a computer with the program in EPROM. The dealer -can- install a more up-to-date program that they get from Toyota, but that's about it. There are no adjustments to make re fuel or air.

Note that the temp. has been higher in the last month or two. Have you been using A/C? That alone could account for the reduced mileage. How about the tire pressure? Checked it recently? Did you check the oil level after they changed it? Sometimes they get a young untrained guy to do the oil changes, and there is then a greater chance of overfilling (though I've seen an "experienced" mechanic overfill other makes/models). That can affect mileage.

Same fuel? If you switched to Husky/Mohawk mid-grade or premium your mileage will drop a bit due to ethanol having a little less energy in it.

I would be surprised if it was anything they did to the air/fuel system, but the above things are a good starting place to check.
 

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What weight/grade of oil did they put in at service? How much?

It is quite common for a dealer to put in whatever their bulk oil is (commonly 10W30 in the US or 10W40 in the UK), rather than put in the 5W30 that is required in the Prius. You would notice a slight difference with the heavier oils.

It is also quite common to have the oil overfilled (particularly since the owner's and service manuals listed oil quantity will typically put you over the full mark on the dipstick). Too much oil can also reduce fuel economy (think of trying to run straight into a lake or ocean - ankle high water isn't too bad, but much past the knees and you aren't going anywhere fast and it takes much more effort to keep running inward...)

Have you checked your tire pressures since the service?

The regularly scheduled services are an oil change and a tire rotation. (At least in the US, and I'd assume Canada as well.) I have never had my Prius back from a tire rotation with the tire pressures set correctly (heck, they weren't correct when I bought the car(s)!). They might go with 32 all around (too low, no +2psi difference in the front), just rotate without changing the pressures (so the rear are now incorrectly higher than the front), or... Then, since you had to drive to the dealership for the service, the tires are warm, so any pressure readings that they do there are on a warm tire, so when you measure them later at the proper cold reading the readings could be all over the place even if set "correctly" while warm at the dealer...
 

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Discussion Starter · #5 ·
prius six month servce

Thanks for those suggestions. These are reasonable and one or more may be satisfactory explanations. However, I was quite careful in each of them-.i.e. same Petrocan fuel, same tire pressure, no use of AC, etc. The technicians said that they did not find any problems with respect to cold startup but given that they apparently cannot make any adjustments, I am puzzled what they would have done had they found a problem with cold startup.
Thanks, Tom
 

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They might check for codes, precodes, operating conditions, check plugs, air filter, throttle body for contamination.

The Prius is pretty sensitive to engine problems. If there is one, it would be flagged with Check Engine light.

Also, in comparison with the E85 experiment an organization did with a classic, incorrect octane or extra ethanol would give rougher warmup and less apparent fuel economy, since the energy content per liter of ethanol is different.
 
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