We've been very happy with our 2005 HAH, averaging 34.5 mpg so far for almost 50,000 miles until recently. As we approached the 50,000 mile mark, the car has started to declined in MPG. We used to average 33-34.5 mpg, and we even had a 400 mile trip where we got 41.5 mpg. The worst that we have ever gotten in the past in the cold winters here driving mostly around town is 24 mpg. The gas mileage has seemed to steadily decline to where we are only getting 25-27 mpg on the highway, and more like 17 mpg around town. The dealer had been warning me that the regular OEM battery is dying, and after reading threads here about the gas mileage being worse as the battery declines I just recently replaced it with an Interstate Megatron MPT-35.

The IMA system seemed to reset itself after I installed the new battery, and all seemed well. Until my wife drove it to work the next day. When she started the car to drive home from the hospital around midnight, she said that the Engine Malfunction Light came on, and the car drove as if it was only running on 3 cylinder mode all the way home (~10 miles). So she stopped in the driveway and shut it off, and when I restarted it and drove a couple of miles it seemed to run fine, except that the Engine Malfunction Light remained on.

So after reading the manual I was hoping that it was just the gas cap, except that I am aware of the gas cap issue and I'm pretty good about making sure that I install it properly and turn it for several 'clicks' when I put it back on. So we drive ~450 miles to Phoenix the next day and we're only getting 25-27 mpg, the Engine Malfunction Light stays on, and the car never goes into ECONO mode! I got some Chevron Techtron and dumped it into the gas tank and the next day the Engine Malfunction Light went off and ECONO mode appears to be working properly again.

So all is well I'm thinking, except that on the 450 mile drive home we still only get 25 mpg! Any ideas what may be happening? I really miss the 30+ mpg even thought the car seems to run great - I'm feeling like that we are only getting the same mileage as a regular V6 Accord. Any ideas?? I know that I probably need to take it to the dealer to see if there are any codes in the computer from the Engine Malfunction Light being on, but I would appreciate any thoughts that you might have. Thanks for your help.

 

There are definitely codes if the MIL (engine malfunction light) is on. And even if the MIL extinguished, the codes should still be there. Either take it to a dealer or take it to an auto parts store that does code readings for free. Could be something simple, hopefully.

What I've found, if the MIL is on, the engine won't go into ECO, so it'll stay in V6 mode and never into VCM (3-cyl) mode. That'll definitely kill your highway mileage.

Keep us posted!

 

That's what I was thinking, so I was very surprised after the Engine Malfuction Light went off and ECONO mode reappeared, that my gas mileage wasn't any better on the return trip back from Phoenix. I was figuring that the gas mileage would have retuned to normal.

Sure wish that I had taken it to the Honda dealer in Scottsdale while I was there, but since the Engine Malfunction Light went off I just figured that it must have been the gas cap. Do you think that the battery replacement and resulting resetting of the IMA might have anything to do with my problem?

 

You should bring your car to a dealer after you saw MIL to read stored DTCs.
Resetting your MIL will only make hard a further diagnostic process.
My bet you have HV battery premature deterioration.
The HV battery managment in Honda hybrids is not good at all.
Did you bring your car to a dealer for maintenance and they ran software update regularly?

 

Drozzin:

Durango Dave didn't reset his MIL, it reset on it's own. Depending on the DTC code set, after a predetermined period (I don't know if it's time, distance, or both), if the vehicle doesn't sense whatever set the code again, it'll extinguish the MIL. However, the codes will still be in storage and are retrievable.

Durango Dave:

Another thing that would cause your return trip MPG to be low is the climb back up to your Colorado altitude. I know, especially the climb from 1,100 ft Phoenix, up I-17 to 7,700 ft Flagstaff really kills my MPG. I'm sure this isn't the sole cause of your lowered FE, but it's definitely contributory.

The ECO light isn't directly related to V6 and VCM mode. However, I've read an article that said it's a good indicator that VCM was activated. The article went on to say what actually triggers the ECO light is if your FE is 25 MPG or better. When using the Scangauge, I've noticed ECO is lit up when the car is registering 24 MPG or better. Perhaps whatever set off your MIL to begin with is not letting the car go into VCM?

 

Sorry, I thought he replaced 12V battery after MIL light up but in fact he did before that.
Replacing 12V battery = resetting MIL. If the car does not use IMA as much as before and dealer did not update software immediately before that then it is a good chance of HV battery problem.

 

I am not aware that there are any software updates available for my car.

I know what you are saying, but we got 27 mpg on the trip down, and 25 mpg on the trip back thru Payson to Holbrook, and we even had a slight tailwind on the freeway around Gallup. And we have taken the HAH to Phoenix many times and never had this poor gas mileage.

I was not aware that the ECO light did not indicate that VCM was activated, but I think that you are correct. And the car does run as if VCM is now disabled. It runs great, strong accelleration, and the IMA seems to be working properly, but the gas mileage sucks! And I don't notice the slightly rough stumbling of VCM mode anymore.

Now I'm beginning to wonder if I got a bad tank of gas that has caused the VCM to be disabled. After I replaced the battery I decided to treat the car to a tank of premium from a gas station that I don't normally go to, because the gas was cheaper. And the very next day was when my wife tried to drive home and the way she described it sounded as if the car were stuck in VCM mode with the MIL on.

I think that I will make an appointment to get the car checked next week at the dealer (50 miles away). I would pull the wire on the battery to reset the computer to see if that helps but I don't want to erase any codes before I get it checked out first. My fear is that this is the only HAH that this dealer has ever worked on.

 

It is easy to check with a dealer when they hook up their scan tool.

 

Well, good news to report! I've fueled the car with better gasoline (at the station where I used to go) and it appears that all is well. I drove the car 50 miles to the Honda dealer today and got 31.8 mpg, and the familiar slight stumble/rumble of VCM was evident. When the dealer checked the computer they found error code P0301 - misfire on cylinder #1. I'm fairly certain that bad gas from the bargain gas station was my problem, maybe made worse by the installation of the new battery and resetting of the IMA.

Also, there was an update for the ECM so they installed that too. The car seems to run great, and it's good too see 30+ mpg again, even with the cold weather here, so life is good!

 

Awesome!