nate.184

Hey guys,
I cant figure out what's going on. I normally get 17-18 mpg on freeway but after about an hour of driving my mpg starts to get worse. After 4 hours of driving iv seen it get as low as 13mpg but it seems to level off around there. Clearing the ecu at every stop seems to help.

I replaced both o2 sensors a few months ago b/c one went bad which made me get about 9mpg.

Any ideas guys?

SuperRA

Just curious, how are you calculating MPG after only an hour of driving? Or are you going by the trip computer's MPG? (I don't even know if there is an MPG selection on the newer overhead console) BTW, we don't know the details (year, engine, etc) of your Jeep neither.

nate.184

On long trips I fill up every stop so I calculate by dividing the the miles driven by how many gallons went in on fill up.
My jeep is a 97 sport automatic with the 4.0.
4.5" lift on 33" tires and 4.56 gears.

Fred/N0AZZ

Did you replace the speedometer gear when you changed everything, if that's what your using for mpg's?

nate.184

I'm replacing it thos week but I'm already accounting for that. Odomoter tells me i do 1.15 miles for every 1 mile i drive. When I don't account for that my math tells me 20mpg but that wouldn't cause mpg to go down as I drive

andrewmp6

Driving habits can cause mpg to drop,Or most places are switching from summer gas to winter gas.The winter gas will cause you to lose a few mpg also.

sycoglitch

Please be specific. Are you saying the on board display is telling you this because that's am average per trip(s). Resetting that doesn't reset the ecu. That's not even accurate after a few mods. My jeep tells me I do 16.6mpg avg. I'm really getting 14.5-15.5 per fill up depending on trips and my foot.

nate.184

None of that should make a difference in the problem I am explaining. If I left on a long freeway only drive with cruise set at 70mph right now I would get 17 mpg. After a couple hours it is down to 15mpg. After 2 more hours it's down to 12mpg.

The way I figure the mpg is I have the trip odometer set at zero after filling the tank full. I drive on the freeway until I want to stop and when I do I refill the tank. I then take the miles driven since last stop and divide by how many gallons just went into the tank. That gives me my average fuel economy. This # goes down as I drive longer. 1st stop it's at 17. 17 miles later it's at 14. Another 75-100 miles later and I'm getting 12mpg.

If I reset the ecu through the obd2 with the torque app on my phone it brings me back up to 17mpg but I have to do this every 80 miles or so or my mpg will stay low.

When I get to my destination and let the car sit over night I go back to getting 17mpg weather I clear the ecu or not.

nate.184

Hopefully my above post answers your questions. Also, the fact that I travel 1 mile and the odometer tells me I traveled 1.15 miles is gps verified by driving to reno which I know is 80 miles from home then figuring the difference of what I drove and what the odometer says I drove

t.mcginley.jr

What brand O2 sensors did you use? Mileage on the engine? Did you use heat/air during this trip?

I'm kinda surprised you even get 17 mpg with those mods lol

nate.184

Yeah I hear cherokees vary alot on mpg even if they have the same mods.
I think they were Bosch o2 sensors. They have a 2 year warranty from oriley. And my jeep has 165k miles.

The same thing has happened on the last 4 trips I took in the past 2 months.
Last week I had to drive about 115 miles to see a doctor. I gassed up right when I got to town and it took 6.9 gallons to fill after driving 112 miles so that's 16.2 mpg.

On the way back I drove 127 miles and it took 9.9 gallons to fill up so that's 12.8 mpg.

That's about 3.5mpg difference making almost the same drive. Like I said this isn't a fluke it has happened in the same way on multiple trips taking different routes and in different weather.

XJwonders

Ever checked your coolant temp sensor?

nate.184

No I havent. Where is that? Lol

mschi772

Resetting the ECU puts the whole thing back to default values which may not be ideal for you. As you drive, your ECU gathers data and adapts to your conditions and driving.

Also, if you're getting this MPG data from fill-up calculations alone, I don't trust at all that you're actually getting 17 MPG in the first few hours of driving. Why? Because if you fill your tank and drive for only a few hours, that isn't a whole lot of gas used. What is that, like 4 or 5 gallons at most? Such a small amount of driving/distance traveled/gas burnt has such a large room for error and is likely to err on the "optimistic" side especially if you keep giving your ECU amnesia thinking that you're fixing some sort of problem by doing so.

Leave your ECU alone. Drive through whole tanks at a time and collect MPG data after multiple tanks and average those values. Then, if the resulting MPG isn't to your liking, stop blaming your ECU and start searching for a cause.


P.S. Bosch O2 sensors tend to be trouble for XJ's. Buy NGK O2 sensors only.


P.P.S. Is the doctor "downhill" from you? It's highly unlikely that your home and your doctor are at the same elevation, and if the doc is lower than home, you SHOULD experience some additional ease getting there vs getting back.

nate.184

I was having this problem for awhile (at least 6 weeks and 2 long tripsb) before I started reseting the ecu. First 2 hrs of driving I get 17mpg then next hrs I'm getting 13-15. And the doc was at a significantly lower elevation but i have had the almost exact same results when driving a long stretch of I-5 where the elevation hardly changes. I wouldn't be here if I didn't have a real issue. Like I said its been doing this for months.