MPG Help

[Leo Brazil]

I know this thread can be annoying for most of you guys but I really need some help to find out what's happening with my XJ 98 4.0L auto.
The best MPG I can get is 12 city - 15 highway....I could see some guys here getting 16 MPG in the city, how come ?
Some things to consider:
I'm already using new plugs, wires and air filter.
I know this can make some difference for sure but it's good to mention that our gasoline is 25% of Ethanol but I'm thinking I still may have to room (somewhere) to improve this bad MPG.
I've seem in other threads some are using 195° thermo valve....I was wondering how good this would be for me, once temp average here in my city is around 90-95°F even during the winter. What's the stock temp for this valve.
Please, give me some ideas. I really need to save something for my 3" lift kit.

 

[Flash]

I know this thread can be annoying for most of you guys but I really need some help to find out what's happening with my XJ 98 4.0L auto.
The best MPG I can get is 12 city - 15 highway....I could see some guys here getting 16 MPG in the city, how come ?
Some things to consider:
I'm already using new plugs, wires and air filter.
I know this can make some difference for sure but it's good to mention that our gasoline is 25% of Ethanol but I'm thinking I still may have to room (somewhere) to improve this bad MPG.
I've seem in other threads some are using 195° thermo valve....I was wondering how good this would be for me, once temp average here in my city is around 90-95°F even during the winter. What's the stock temp for this valve.
Please, give me some ideas. I really need to save something for my 3" lift kit.

"Driving habits" have a lot to do with fuel mileage, altitude,and wind........and the ethanol is part of it.

If you were using straight Ethanol you would have to flow more then double the volume to get the same power..............All tho this isn't a tru statment.........."your are using 25% more fuel due to the Ethonal."........But
You get the point!

Flash.

 

[MaXJohnson]

I know this thread can be annoying for most of you guys but I really need some help to find out what's happening with my XJ 98 4.0L auto.
The best MPG I can get is 12 city - 15 highway....I could see some guys here getting 16 MPG in the city, how come ?
Some things to consider:
I'm already using new plugs, wires and air filter.
I know this can make some difference for sure but it's good to mention that our gasoline is 25% of Ethanol but I'm thinking I still may have to room (somewhere) to improve this bad MPG.
I've seem in other threads some are using 195° thermo valve....I was wondering how good this would be for me, once temp average here in my city is around 90-95°F even during the winter. What's the stock temp for this valve.
Please, give me some ideas. I really need to save something for my 3" lift kit.

Automatics are not as efficient as a manual. Figure a 1 mpg or more hit for the automatic
Ethanol has 35% to 40% less energy that gasoline. Running a mixture of 25% ethanol would reduce your fuel mileage by about 10%.

If you're not running a 195 degree thermostat, you should be for the best fuel mileage.

Age alone (1998 is almost 10 years old) is probably costing you 15% to 20%.

10% for ethanol, 10% for the automatic, 15% for wear and loss of efficiency due to age all adds up to a 35% hit. Poor driving habits, as already mentioned, could easily cost you 15% or more. Stay off both your brake and gas peddles as much as possible. Drive slower on the highways and coast every chance you get.

 

[Leo Brazil]

Cool comments guys....sounds fair enough to me....
It's good to hear some experiencies froms other XJ guys once I've got mine a couple of months....But I'll investigate the thermostat anyway.
Well, this MPG I've mentioned you guys have to consider I was driving like an old lady, never crossed over 2200 rpm and my XJ has 66K miles.

 

[75SV1]

The previous comments are good to follow. Some others to think about are the O2 sensor. Although I doubt at 60K yours would be bad. Another area to gain MPG is the exhaust system. The down pipe or cross over pipe has some kinks in it from the factory. I hear its about $50 to get a muffler shop to bend one up. I also think from one automatic tranny to another of the same make they vary in performance. Same with engines. I notice some MPG variance with gas. About 2 mpg. I notice a 2 mpg gain with synthetic motor oil, but others say they don't. Keep your tires inflated properly.
Tom

 

[89Daytona]

I notice some MPG variance with gas. About 2 mpg. I notice a 2 mpg gain with synthetic motor oil, but others say they don't.

What do you mean by variance with gas? Just station to station or something else?

What brand and weight of synthetic are you using?

 

[75SV1]

What do you mean by variance with gas? Just station to station or something else?

What brand and weight of synthetic are you using?

I can't say any specific brand of gas. There was an indepent owener by me about 5 years ago. He sold CO-OP gas. I noticed a 2mpg on highway with it. There were some other stations, that I think had gas that did as well. This was in a Ford Contour. I have a 4cyl and 6cyl STV. I use 10W-30 in them. Castrol, Mobil 1 and Pennsoil Platinum. I think the Pennsoil is the best deal out there. There are others that cost more, Amsoil, Shaeffer etc. My Renix leaked a bit to much oil for me to use synthetics at the time. I plan to use synthetics in the newer motor. I can't say any of this is scientific, but I have noticed this on my trips from IN to PA several times a year for about the past 10 years.
Tom

 

[Leo Brazil]

The previous comments are good to follow. Some others to think about are the O2 sensor. Although I doubt at 60K yours would be bad. Another area to gain MPG is the exhaust system. The down pipe or cross over pipe has some kinks in it from the factory. I hear its about $50 to get a muffler shop to bend one up. I also think from one automatic tranny to another of the same make they vary in performance. Same with engines. I notice some MPG variance with gas. About 2 mpg. I notice a 2 mpg gain with synthetic motor oil, but others say they don't. Keep your tires inflated properly.
Tom

I've heard about O2 sensor could be a possibility. Q: If the 02 was bad why the ECU panel light doesn't come on ? The ECU sensor monitoring system only detects if a sensor is really dead and not out of range ?
I don't want to get out of the original post but what owuld be the correct reange for the O2 sensor in this case ?
I've noticed some good differences from a gas station to another....The MPG I post here I've got with the best one.....some of than I can even lost 1 to 2 MPG....
I'm using also 10W40 synthetic oil. Why this one and not 10W40 ? Because it's pretty hot down here and oil pressure was lower than 1.5 bar using 10W30. Using 10W40 the pressure is never lower than 2 and never higher than 3 bar during a cold start.

 

[75SV1]

Leo,
I'm not sure of the Ohms to measure on your O2 sensor. You might list your year of Jeep. I just did the ones on my Contour. The manual said 6 ohms. Mine threw a code and measure 5.4 ohms. The new one measure 3.5 ohms. It runs better though.
Does the gas your using have methanol in it? That might reduce milage. I am somewhat aware that Brazil uses gasohol.
Also, you are correct in using oil pressure as to weight of oil to use. Though I think different engines need different oil pressures. I think older Fords need much higher pressures. I wasn't thinking about the Jeep 4.0L engine. I listed the oil weight I use on my Ford Contours. Different motor and such. I ren Shell Rotella 15W-40 in my '88. Without oil analysis I would run a heavier oil. You might loose a bit of MPG with heavier oil. Not much between a 10W-30 and 10W-40. I used to loose 2 mpg running a 20W-50 in a Ford 351W. That was in Tulso OK, so it could get hot there. Maybe not like Brazil though.
I've heard since our motors are flat hydrualic lifters, it is best to run an oil with some zinc in it. You might check into that. But most of the problems seem to be with after market cams and lifters.
Tom

PS: I have heard of a recall on the catylitic converters. I forget if they give a code or not. Might check into that to. A bad cat can also reduce MPG.

 

[evil dead]

flush the tranny fluid change the diff oil oil change air fuel filters plugs(gap em) wires cap rotor tire pressure etc all of these things make a difference and if overlooked suck performance and mileage also 2200rpm you will probably have carbon build up in the chambers get some injector cleaner and run it as often as it says on the bottle i do all of this when i buy a used vehicle and you can see huge improvements if this doesnt work then check sensors

 

[Leo Brazil]

Leo,
I'm not sure of the Ohms to measure on your O2 sensor. You might list your year of Jeep. I just did the ones on my Contour. The manual said 6 ohms. Mine threw a code and measure 5.4 ohms. The new one measure 3.5 ohms. It runs better though.
Does the gas your using have methanol in it? That might reduce milage. I am somewhat aware that Brazil uses gasohol.
Also, you are correct in using oil pressure as to weight of oil to use. Though I think different engines need different oil pressures. I think older Fords need much higher pressures. I wasn't thinking about the Jeep 4.0L engine. I listed the oil weight I use on my Ford Contours. Different motor and such. I ren Shell Rotella 15W-40 in my '88. Without oil analysis I would run a heavier oil. You might loose a bit of MPG with heavier oil. Not much between a 10W-30 and 10W-40. I used to loose 2 mpg running a 20W-50 in a Ford 351W. That was in Tulso OK, so it could get hot there. Maybe not like Brazil though.
I've heard since our motors are flat hydrualic lifters, it is best to run an oil with some zinc in it. You might check into that. But most of the problems seem to be with after market cams and lifters.
Tom

PS: I have heard of a recall on the catylitic converters. I forget if they give a code or not. Might check into that to. A bad cat can also reduce MPG.

Our gasoline in fact is a mix that takes 25% of Ethanol.....I'm aware about loosing MPG with this but normally 10-15%....and not as much as I'm getting. Many guys here use 20W50 mineral with this engine because it's pretty cheap. But I don't want to take any chances using a poor quality stuff. Average temp here in my city is 90-95°F even during the "winter"....but the best places to go trail with my Jeep get 100°F pretty easy...
Anyway, I'll check the O2 and also check all the injector just to be sure they're working OK. It might give me a MPG gain.

 

[75SV1]

Also, check for exhaust leaks. The Factory header is known to crack.
Tom

 

[rustysxj]

would a TBS help?

 

[j99xj]

Just make yourself a "make believe" rev limit when you drive. (yes this is possible even with an auto, just let off the gas pedal slightly until you get to your make believe rev limit, when you let off enough the tranny will short shift itself)

When I really want the best mileage I keep my rpms around 2000.

 

[75SV1]

would a TBS help?

It could. You should be able to check it out with an Ohm meter. Also, cleaning out one should help. I here Ford has a cleaner the mechanics call 'foamy' they use to ceal theirs out with.
Tom

 

[MaXJohnson]

would a TBS help?

no

 

[75SV1]

It could. You should be able to check it out with an Ohm meter. Also, cleaning out one should help. I here Ford has a cleaner the mechanics call 'foamy' they use to ceal theirs out with.
Tom

For some reason, I was thinking MAF sensor or Throttle body on this one. You also might look for a vacum leak.
Tom

 

[Leo Brazil]

Just make yourself a "make believe" rev limit when you drive. (yes this is possible even with an auto, just let off the gas pedal slightly until you get to your make believe rev limit, when you let off enough the tranny will short shift itself)

When I really want the best mileage I keep my rpms around 2000.

I liked this expression you used, "make believe"...that's cool....
If fact, I'm doing exactly what you said but I know every engine has a different curve RPM x HP so....doing this "make believe" thing what should be better ? Increase the RPM slowly before let oof the pedal gas or just get strenght to the point before it ?
I mean, it's two different situatons and depending on this curve it will give me better MPG, right ?

 

[j99xj]

If you are driving a 4.0 automatic, 2000 rpms is about the sweet spot for the best gas mileage. (at least with my jeep)

2000 rpms or so is when the torque converter is nice and happily stalled and locked up, and 2000 rpms is when the 4.0 not being taxed or lugged.

But you can see dramatic mileage gains from simply slowing down and accelerating smoothly.

IF you use those 190 horses a lot, your mileage will suck.

 

[Leo Brazil]

If you are driving a 4.0 automatic, 2000 rpms is about the sweet spot for the best gas mileage. (at least with my jeep)

2000 rpms or so is when the torque converter is nice and happily stalled and locked up, and 2000 rpms is when the 4.0 not being taxed or lugged.

But you can see dramatic mileage gains from simply slowing down and accelerating smoothly.

IF you use those 190 horses a lot, your mileage will suck.

Thanks for the tip.