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Discussion Starter #1
I have a 2005 EX-L. We bought it with 70k miles on it about 2 years ago. Currently it has approx. 120k miles. Easily for the first year and a half we owned the van we had no problems with the gas mileage. Our mileage was quite consistently 18-19 MPG local and 23-25 MPG highway. For the past few months it has been getting continually worse to the point it now gets 15-16 MPG local and barely 20 MPG on 100% highway driving. There has been no change in our style driving either.


Things to note:
-NO CELs.
-Plugs replaced @105k along with TB and Water Pump.
-Replaced tires ~10k miles ago with the same exact tire it's had since 70k miles.
-Aligned when new tires were installed.
-Fluids regularly changed.
-Air filter is new.
-I just replaced both primary O2 sensors with no signs of improvement.
-I also increased the tire pressue to slightly above the recommended pressure, again with no improvement.
-I removed and cleaned/inspected the EGR and PCV valves. Neither showed any indication of any buildup.
-I also pulled the plugs and they looked to have the wear you would expect. (Neither rich nor lean)

-The only qualitative things I can note are the engine seems to feel and sound slightly rougher than it used to. Also, the exhaust has a stronger odor than what I recall previously, although it is nothing like the sulfer smell I'm used to associating with a bad cat.

If anyone has suggestions or experience with a similar issue, please respond.
 

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How is your coolant temperature? I seem to recall some posts where replacing the thermostat has brought the mpg back up - but that could be for the older gen ody.
 

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from memory.. the ody doesn't have a MAF.. and if MAF was out of specs the CEL would go off..

may be I am wrong..

IMO.. it could be an open or not fully functional fuel injectors or clogged cats..
 

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Discussion Starter #7
try putting injector cleaners in your gas tank, seafoam it?, try synthetic motor oil?
I actually have a can of seafoam in my current tank of gas right now.


After looking yesterday it appears my temperature gauge goes no higher than approximately 1/3 up from cold. I am going to buy a 180 degree thermostat today and see if that is the problem.
 

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How is your coolant temperature? I seem to recall some posts where replacing the thermostat has brought the mpg back up - but that could be for the older gen ody.
I will double check, but I do not recall any issues with the car running cold.
I actually have a can of seafoam in my current tank of gas right now.


After looking yesterday it appears my temperature gauge goes no higher than approximately 1/3 up from cold. I am going to buy a 180 degree thermostat today and see if that is the problem.
The issue of "proper" thermostat (87C vs. 89C) in our Saab forum is a popular one. Both are OEM. With most Saabs being turbocharged, a lot of people (incl. myself) prefer the lower 87C t-stat. And the debate never ends. But I don't recall hearing anyone say using a cooler t-stat would affect fuel economy, and even if so, certainly not to the extent PinheadGTO stated above. Maybe it's a YMMV thing with different cars and powertrain configurations.
 

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Discussion Starter #9
It's not that I think the 170F T-stat is causing bad gas mileage. I do believe that it may be stuck open which could significantly reduce gas mileage. Since I am going to replace it, I am going to get the 180F t-stat and see if that provides a slightly additional MPG benefit over a good 170F unit.

Long ago I had an Integra that consistently was in the 31-32-MPG range. Then it dropped to a consistent 25 MPG. A stuck open T-stat was the issue. Running cold can cause a significant drop in your gas mileage.
 

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Just to add to your comments on the T-stat.
It may not even be 'stuck-open' (like stats of old used to do), but simply not maintaining the correct temp. Todays engines and stats will modulate (open and close) as needed to maintain the temp, if it's not closing down it will let the engine run too cool, maybe cold enough to fool the computer into thinking it's still in warm up mode.
I had this problem with an '89 Caravan, also discovered that generic replacement stats were no better, only the OEM version would work right. The guage was only a needle width down from normal as well.
What would be best is if you could attach a temp guage (like a ScanGuage or similar) and get a temp reading, then you would know the before and after.
 

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I've edited your pic to indicate the following:

  • The red line is where my needle consistently sits (a couple of tiny notches higher)
  • The blue line is just the mid-point on the gauge, but has no real-world bearing as I don't know where all other temp needles average out (just guessing it's closer to the mid-point)

Having said that, the only way to know is to get a diagnostic unit that shows actual coolant temp (I'm waiting for the metric version of the ultra-gauge). Also, although my van does take a bit of time to heat the cabin, it does eventually get warm - so it's not clear to me if where my needle sits is the average or cooler than average itself.
 

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Believe mine runs midway between the blue and red lines in that modified photo.
I'll take a closer look this weekend when we make a 30 mile run.
But these guages aren't really the best, a readout from the system (which is what the computer uses) is the only way.
 

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I might swap out our thermostat. Our mileage isn't that bad but it is way worse than our pilot. We are averaging about 14.5 to 15MPG in the city (most of where my wife drives.) On the freeway from Dallas I was getting about 22 or so. I think there is room for improvement. I saw a DIY for changing the thermostat to a higher temp model. Since the parts are not that expensive I might give it a shot.

Since I have a certified used vehicle with a bumper to bumper should I try to see if the dealer could troubleshoot the mileage? I feel there is no reason my Odyssey should get 3 to 4 MPG worse than our '06 Pilot did. They both have the VCM and weigh about the same. I would think the Pilot would be worse because it sits higher and from appearance looks to have worse aerodynamics.
 

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I've edited your pic to indicate the following:

  • The red line is where my needle consistently sits (a couple of tiny notches higher)
Had a good run over last weekend, my temp also holds right where the red line on the edited pic is, just touching the bottom right edge of the symbol.
 

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Discussion Starter #17
UPDATE: I finally installed the new 180F thermostat. The temp gauge needle now sits a couple notches higher on the gauge.

So far we are about halfway through our first tank of gas. We got about 200 miles on 1/2 of a tank. I will run the actual numbers once we finally fill-up, but recently we could get no more than 130 miles on a 1/2 tank.
 

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Glad to hear of the improvement, but you're right you'll have to watch it over a full tank for a little time to get a good idea. Nonetheless, a change in the right direction!
 

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UPDATE: I finally installed the new 180F thermostat. The temp gauge needle now sits a couple notches higher on the gauge.

So far we are about halfway through our first tank of gas. We got about 200 miles on 1/2 of a tank. I will run the actual numbers once we finally fill-up, but recently we could get no more than 130 miles on a 1/2 tank.
That's a big difference, even accounting for slight discrepancies in actual fuel consumption as interpreted by your gauge. Like PVP said, a good change in the right direction ! :)
 

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There is my thread about engine issue which started with loss of 3.5qt of oil which is now believe to be engine issue, and another thread about 2007 engine issues. Could it be the main cause? I hope not, but I see more thread that is closely related together.
http://www.odyclub.com/forums/52-2005-2010-odyssey/148803-3-75qt-engine-oil-gone.html

Yes, my van had weird smell too (I looked funny smelling the tail pipe of all my 3 hondas)...didn't really notice the sound, since I didn't pay attention to that.

Check out the other posts to see if any similarities exists.
 
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