less than 10 mpg

[john]

my 88 145k auto xj with 31 bfgs stock gears is getting less than 10 mpg. what are the major causes of poor gas milage?

[BrettM]

o2 sensor? plugs, wires, cap and rotor? bad gas?

[Eagle]

my 88 145k auto xj with 31 bfgs stock gears is getting less than 10 mpg. what are the major causes of poor gas milage?

31" tires and stock gears.

The result is that you cruise well below the RPM where the engine produces its best torque, and that's also the engine speed that's most economical.

Also, have you corrected the speedo for the larger tires? The injection uses data input from the vehicle speed sensor to calculate the fuel charge. Running bigger tires without correcting the speedo results in the ECU seeing low RPMs and low vacuum, which it thinks means heavy load at low speed, and it richens the mixture and retards timing to avoid detonation. This does not enhance fuel economy.

[john]

how do you tell if the 02 sensor is bad

[ILLXJ]

Look for a post called low gas mileage from 1/24/04. It tells how to test. I would post the link but not sure how. HTH, Jim

[90red]

31" tires and stock gears.

The result is that you cruise well below the RPM where the engine produces its best torque, and that's also the engine speed that's most economical.

Also, have you corrected the speedo for the larger tires? The injection uses data input from the vehicle speed sensor to calculate the fuel charge. Running bigger tires without correcting the speedo results in the ECU seeing low RPMs and low vacuum, which it thinks means heavy load at low speed, and it richens the mixture and retards timing to avoid detonation. This does not enhance fuel economy.

90 XJ 4.0 2wd 5 - Speed.

Don't mean to hijack the thread but - I installed 235/75/15 tires on my XJ, the previous tires were 215/75/15. Is that a big enough increase to cause the computer to receive bad data from the vehicle speed sensor? If it is, how can I go about correcting the problem. Thanks guys.

90red

[Kejtar]

90 XJ 4.0 2wd 5 - Speed.

Don't mean to hijack the thread but - I installed 235/75/15 tires on my XJ, the previous tires were 215/75/15. Is that a big enough increase to cause the computer to receive bad data from the vehicle speed sensor? If it is, how can I go about correcting the problem. Thanks guys.

90red

IIRC the difference is enough that you could get a ticket if you went by your speedomiter.... to correct speedo you just replace a little plastic gear. I don't have a link handy to any of the webistes that talk about it, but if you look I believe @ cherokee america you will find one there, and also search the mod tech forum here.

[90red]

IIRC the difference is enough that you could get a ticket if you went by your speedomiter.... to correct speedo you just replace a little plastic gear. I don't have a link handy to any of the webistes that talk about it, but if you look I believe @ cherokee america you will find one there, and also search the mod tech forum here.

How will the speedometer be off? Will I be going faster than it reads, or slower than it reads?

90red

[Kejtar]

How will the speedometer be off? Will I be going faster than it reads, or slower than it reads?

90red

IIRC you will go faster then it reads.... it will be a percentage change. Look at the diameter (215 75 15 is about 27.7") and calculate from it the circumference and then look at the new diameter (235 75 15 is about 28.9) and then calculate circumference from that. Then calculate the percentage change between old and new circumference, which results in about 5% speedo difference (as in you will be going 5% faster then you think you are going).

[90NHXJ]

Just to clarify, when they said you'd be off a percentage that means the faster you go, the more you'll be off. So if you were off 5% at 10mph you might be going 10.5mph. At 20 mph you'd actually be going 21. At 30- 31.5mph, at 55 you'd be going 57.25mph. You might be off more than that. My speedometer reads 55 and I'm going more like 65 to 67 according to the law. That's with 31's.

[footdale]

I drove by one of those speed detectors that they put on the side of a street and it was within +/-1mph of the speed indicated on the speedometer. (Did this a few times and assuming that thing was semi-accurate) This was at 45mph. I am running 31s and haven't done a thing to compensate for the increased tire size, so I don't know what to make of this situation really??? 10mpg is pretty low, so IMO I think there is something else in addition to the larger tires.

[Kejtar]

I drove by one of those speed detectors that they put on the side of a street and it was within +/-1mph of the speed indicated on the speedometer. (Did this a few times and assuming that thing was semi-accurate) This was at 45mph. I am running 31s and haven't done a thing to compensate for the increased tire size, so I don't know what to make of this situation really??? 10mpg is pretty low, so IMO I think there is something else in addition to the larger tires.

I never trusted those things.... :) If you have a GPS, or know someone with one, take that for a ride and see what happens.

[john]

There are probably a number of people out there running 31 tires and stock gears. whats your mpgs?

[footdale]

About 15mpg - Based on mileage from odometer and ~17 gal/tank

[woody]

on my 89 trail rig 4.0/auto (before it was a purely trail rig) I routinely got 16-18-corrected- MPG with 33" and 3.55:1, 400# of junk in the back... When I swapped in 4.56:1 the mileage didn't improve a whole lot. An O2 sensor and a set of spark plugs helped by approx 1-2 mpg.

When I bonked the vac hose off my MAP sensor in my 88 4.0, gas mileage dropped from 19-20 to approx 10 :eek: but it also went to running so badly, all of a sudden, that I knew I had messed something up under there.

In order to know what your "true" MPG is, you have to know how far off your speedometer/odometer is, and figure the MPG with your correction factor. Run ten miles of interstate and watch the mileposts. With larger dia. tires than OEM, ten miles actual will likely show as less miles on your ticker and less MPH on the speedo. IIRC the correction factor (for mine) with stock gears & 33" was like 10%

[john]

Footdals is that 15 mpg reading from the stock speedo? if so regardless of the speedo not being calibrated to the 31 tires you are getting 50% better milage than my XJ with the same set up. am I not right

[Eagle]

Actually, the correction factor is about 10% going from stock tires to 31s, and about 6% going from stock tires to 30s. However, the stock speedos almost always read faster than true speed, so when you go to 31s even though you're traveling 10% faster than before, the speedo may not be off by the full 10%.

[footdale]

Footdals is that 15 mpg reading from the stock speedo? if so regardless of the speedo not being calibrated to the 31 tires you are getting 50% better milage than my XJ with the same set up. am I not right
Sounds right. With a full tank I get ~260 miles (50% highway driving) on the odometer. Tank size is roughly 17 gallons so that works out to 15mpg. That is why I was thinking that there is something other than tire size causing you to get that low of gas mileage. Sorry I won't be able to help much on troubleshooting the problem.
good luck

[ECU88XJ]

I had an 88 just like that tuned it up replaced plugs and wires swapped in another o2 sensor(it was used though) had the dealer do ~ $600 worth of work to it and still never kicked the problem. is your jeep running at the proper operating temp, the only problem i could figure it may have been was that i had a gdi 3 core in it so it ran at like 180 all the time which would be causing the computer to run it rich. but i had that radiator in for a year before it started sucking (GAS). i gave up and sold it. good luck.

PS this isn't my jeep that was bought in raleigh nc is it?

[PaulJ]

31" tires and stock gears.

The injection uses data input from the vehicle speed sensor to calculate the fuel charge. Running bigger tires without correcting the speedo results in the ECU seeing low RPMs and low vacuum, which it thinks means heavy load at low speed, and it richens the mixture and retards timing to avoid detonation. This does not enhance fuel economy.

No, this isn't correct. The tranny speed sensor on a Renix XJ feed only the TCU. The only speed information the ECU gets is from the CPS. Correcting the speedo gear will make the speedo and MPG calculations correct, but won't have any impact on the air/fuel ratio or the timing advance/retard.