My brother came to the Willamette Valley in Oregon 2 years ago from S.Calif. Since then his 2000 Odyssey’s in town mpg has gone from about 18 to about 9 and 28 to 25 on the freeway. The engine light does not come on. There are no starting, idling, driving, exhaust issues, or excess heat from under the car that he can detect. He has taken it to several Honda dealers who just say try this and that, and 3 other experienced mechanics poured over it with no luck. One year ago it passed California and Oregon smog checks. It is parked outside summer and winter and I believe he lets it idle too long before driving and the trips are almost always just 4-6 miles one way. Any help with this will be greatly appreciated. Oh, it has 150,000 miles on it.
Thank you for any help/advice received,
Crater
“I believe he lets it idle too long before driving and the trips are almost always just 4-6 miles one way.”
I think that you have already identified the source of the problem, based on that statement.
Modern engines do not normally need to be idled for more than…maybe…30 seconds before driving.
Yes, one should avoid strong acceleration until the engine has warmed-up fully, but idling for more than 30 seconds only results in a waste of gasoline. That being said, yesterday when the temp was 5 degrees, I did allow the engine to idle for ~2 minutes before I drove, but I did that for my comfort, and with the full knowledge that my gas mileage would take a whack as a result.
And, because those short trips are notorious for producing poor gas mileage, I think that the best bet is to try to get your brother to modify his driving habits. Besides not idling the engine for more than a few seconds, he should try to combine his errands so that he is not doing just very short-trip driving.
All idiling does is waste gas. Everyone’s gas mileage at an idle is 0. Starting your engine and driving gently warms upyour engine much faster than idling.
I’d check for dragging brakes and replace the thermostat (OEM only!) if it’s over 10 years old.
+1 to VDC’s post. Emphatically.
I cannot totally agree with oldtimer… it was -11F (eleven below zero) at my house at 6:00 AM this morning, +6F in my car in my unheated attached garage at 11:00 AM when I started the car. There’s more than the oil to think about on mornings like this. There’s my buns. The fleshy ones. Also driving with a cold engine in subzero weather renders the defroster useless. There’s also a safety issue (functional defroster) to consider.
Good ideas above. My first suspicion when lower mpg than the same car had before is reported, the engine coolant temperature isn’t reaching spec; i.e. the engine is running too cool. If the vehicle has an engine coolant temp gauge on the dash, does it read the same now after the engine is warmed up as it did when the mpg was normal?
I guess some people have less tolerance for cold than others. People were driving cars before they had defrosters, The people who drove air cooled VWs in cold weather didn’t have functioning defrosters either, they dressed warm, drove with one hand and scraped with the other.
“The people who drove air cooled VWs in cold weather didn’t have functioning defrosters either.”
Well, it was possible to improve that situation, as I did with my Karmann Ghia.
I temporarily removed the luggage deck in back of the drivers seat, hacked into the steel heater duct on the floor, and installed a heater blower from a '55 Chevy, along with the wiring to tie it into the wiring harness, and a dashboard-mounted switch, and–voila!–I had a fairly strong flow of air from the heater and defroster ducts on the driver’s side. That was the good news.
The bad news was that I would have had to repeat the process on the passenger side if I wanted to have a really decent heating/defrosting system, but I concluded that the electrical system was too weak to power two blowers operating simultaneously, so I had to make do with just the boost on the driver’s side.
It was an imperfect solution, but it did help to a great extent.
Have the spark plugs ever been replaced? If not, replace them and see if mileage improves. The car doesn’t have to run rough or have difficulty starting to need new plugs. I also agree with the others here that everyone should drive shortly after starting the car. The car will warm up as you drive gently down the road. Given the short drives, it would make sens for your brother to drive the Honda 20 minutes to a half hour each week to thoroughly warm up the drive train.
@MickTaylor
Hey Mick, Crater already knows there exists a mileage problem.
You don’t happen to be affiliated with the company that makes whatever it is you’re advocating, eh? 
There are rules here against SPAM.
CSA
Of course he is. He gets paid for every post he makes in different forums in the internet. I’ve never seen a legitimate company advertise this way. They are pretty much all sleaze companies. See advertising like this…RUN AWAY.
Gone! Thanks.
I want to thank everyone for commenting on my 2000 Honda low mpg issue. I got sidetracked with some personal issues and just got back to this site. I will pass the info onto my brother.