My 2003 Honday Odyssey has been giving really low mileage. I have 108,000 miles on it and I used to get around 21 miles per gallon. Since the past 3 months I have gotten between 12-14 miles per gallon. My driving habits have not changed, instead they have gotten better. I have tried the following:
-Check tire pressure monthly (36psi)(free)
-Replace engine oil more frequently (every 5K) ($20)
-Replace air filter ($15)
-Flush and replace transmission fluid ($200)
-Replace all spark plus with genuine Honda spark plugs for $237(they were never replaced before)
- Asked a Honda mechanic on Justanswer.com ($15)
I had all the above work done at a local mechanic, nothing helped, so I took it to the dealer. Dealer has tried the following:
- Engine valve adjustment ($850)
- Fuel injection service ($150)
- Throttle Body service ($150)
I am hoping this will help, not sure what else I can do. I am going broke trying to get the mileage back to at least 20MPG. On a full tank of gas the min-van goes less than 220 miles. It used to go above 300 miles.
Any ideas on what else I can do besides sell it? 
I forgot to mention, I usually get 87 octane at Arco/Exxon, so it’s not cheap no name brand that I use. I have tried 91 octane in vain. In addition I have tried fuel additives as well. I had gotten a new timing belt at 90,000 miles. My driving is usually 10 miles one way to work mostly sub-urban driving.
Replace engine oil more frequently (every 5K) Your owner’s manual should list oil change intervals by miles and months, which ever comes first. Modern oils and engines don’t need changes any more often than indicated in the owner’s manual. Be sure to use an oil that meets or exceeds the specifications listed in the owner’s manual.
[i] Flush and replace transmission fluid[/i] Generally we recommend not doing a transmission [b] flush[/b] Rather a fluid drain and replace along with cleaning the filter. Most of the time when a flush is recommended it is designed as nothing more than a way of increasing profit. We also generally agree that transmission service should be done at 30-40,000 miles even if it is not listed in the owner's manual.
No need to use "Honda" plugs as long as the plugs you use meet Honda specifications, but there is nothing wrong with Honda plugs other than price.
[i] Dealer has tried the following:
-
Engine valve adjustment ($850)
-
Fuel injection service ($150)
-
Throttle Body service ($150) [/i]
It sounds to me like the dealer is either just making a profit at your cost, or just does not know what is going on and is guessing at your expense.
Dealers are no better (or worse) than independent mechanics for almost anything you might need done on your car. They will almost always charge more per hour and often more for parts and supplies. They also tend to look at repairs a little different than the independent.A dealer may well recommend work that strictly may not be needed, but could be connected to the problem or maybe replace a part when a little repair would fix it ALMOST as good a new.
There is no need to bring your car to the dealer for any service other than service that is going to be paid for by a recall or original warrantee. During the warranty period be sure to have all required (as listed in the owner’s manual) maintenance done and to document all maintenance work.
I suggest that most people would be better off finding a good independent (Not working for a chain) mechanic.
Note: Never ever use a quick oil change place. They are fast cheap and very very bad.
not sure what else I can do.
I suggest you start looking around for a good local INDEPENDENT mechanic. Ask your friends neighbors ect.
High octane fuel only is useful in a car that needs it. If the owner’s manual does not list it, it is of no benefit to you.
If you don’t have one get an owner’s manual and make sure all the maintenance listed is done (miles or time which ever comes first.)
Now I have one recommendation. Have someone check for a blocked exhaust (likely the converter). Just a hunch on my part.
All your efforts so far are on the motor. Perhaps you have a brake caliper that is not fully releasing and causing drag. This is something you won’t really feel, but even a light drag from one or more brake component(s) will reduce mpg significantly.
After a fairly long drive feel each wheel, if one feels warmer, or hotter than the others check that wheel first. A bad wheel bearing can cause the same sort of drag. Also failing CV joints can do the same.
In other words start looking at the chassis and drivetrain for some reason other than the motor.
Any check engine light on? Does the engine seem to run normally and give the same amount of power?
Meehan and Turbo have good ideas, any luck following up on those? Let us know because I have a 2002 Odyssey and want to find out the conclusion.
There’s a couple of things that can cause a sudden drop in fuel mileage.
Check for a stuck open thermostat. If the thermostat is stuck partially open the engine never comes up to full operating temperature. The computer detects this and thinks the engine is still cold so the computer keeps the fuel mixture rich reducing fuel mileage.
Check for a faulty coolant temperature sensor for the computer. If this sensor has failed where it’s telling the computer that the engine never gets up to operating temperature when it actually does, again, the computer keeps the fuel mixture rich reducing fuel mileage.
Tester