MPG Trouble

homebrew

New member
I hate to be the guy who post up about having sucky MPG's but Im running out of Ideas.
97, 4.0, AW4, 4.56 gears, 33's

So im struggling to get 14 on the highway The following has been completed in the last 6 months resulting in little change.

Replaced cracked manifold, Installed high flow cat and muffler, Throttle body spacer, full tune up including coil, synthetic oil, k&n drop in, Down stream O2 sensor, New calipers and rotors in the front(suspected drag) replaced 165 t-stat for 195, IAC sensor.

Jeep runs like a champ, has great power but I can damn near watch the needle drop as I drive. I know these things get crappy mileage, but I am below the average. No I dont need to buy a honda

Appreciate any ideas
 

Alex E

New member
I agree, driving style. How fast are you going at what RPM when you get 14mpg? Is your speedo accurate? I'm just asking the obvious, also I have 32's on my junk BUT really...even though they say 32 on the sidewall I think really they were 31s new and now they are barely 30. Measure your 33's to see what size they really are, maybe your calculations are off. Have you tried running different tire pressures? Don't feel bad, my 87 XJ thats basically all bran new, engine, tranny, axles, sensors, etc...with a manual tranny and 4.56s gets like 15 or so...

Side note, you still in Bremerton? Thats like 30 min south of Bainbridge isn't it? Or close to Silverdale? I have family out there, nice area.
 

homebrew

New member
I use my GPS to track my speed. rpms are around 2250 at 65. Around town I know I am sucking because I have a heavy foot, but force myself to keep it under 65 on the freeway.

Still in bremerto, just a few miles from silverdale.
 

Alex E

New member
I use my GPS also, some times I find its inaccurate, other times its not. I find my engine likes 2700ish rpms. Some where above 2500 and below 3000 seems to be the best cruising spot for engine load. Last time I was up there I went wheeling in, Tahuya? There was one gnarly short trail I remember but I was the only Jeep there on a week day so I wheeled conservatively, looks like a fun spot. Its kind of small though isn't it? Sorry not trying to hijack your thread.
 

GRANDLICH

New member
I agree, driving style. How fast are you going at what RPM when you get 14mpg? Is your speedo accurate? I'm just asking the obvious, also I have 32's on my junk BUT really...even though they say 32 on the sidewall I think really they were 31s new and now they are barely 30. Measure your 33's to see what size they really are, maybe your calculations are off. Have you tried running different tire pressures? Don't feel bad, my 87 XJ thats basically all bran new, engine, tranny, axles, sensors, etc...with a manual tranny and 4.56s gets like 15 or so...

Side note, you still in Bremerton? Thats like 30 min south of Bainbridge isn't it? Or close to Silverdale? I have family out there, nice area.
First post on here and hate to sound like a noob but never know till i ask. what are the calculations you have to do with having bigger tires?
 

birchlakeXJ

New member
The 165 degree stat isn't going to help you any. Any specific reason you went with that?

The downstream oxygen sensor really just measures the efficiency of the catalytic converter, it is the upstream one that contributes to fuel/air ratio.

The coolant temp sensor and the map sensor also contribute to your mileage. They both can be tested and I would recommend doing so.

If there is no codes and you have addressed many of the usual suspects, I like to get the engine on a scantool, where you can verify that all of the different components of the engine management system are working properly.

Good luck!
 

jdougn

NAXJA Member
NAXJA Member
Should I even bring up the issue of what speedo gear is being used to compensate for regearing and tires, or how actual mileage is being tracked to determine mpg? Another suggestion, my XJ gained about 3mpg when I took the spare tire & rack off the top. Still gets crappy (expected) mileage of around 13-14 city & around 16 highway.
 

the_weirdo

Ugh
NAXJA Member
First post on here and hate to sound like a noob but never know till i ask. what are the calculations you have to do with having bigger tires?
Background:
Assume stock tires are 28" diameter. Let's say you put on 31" tires - now your speedo is not going to be correct. If you put on bigger tires your speedo will read slower than you are really going. It will read slower by the ratio [(31-28)/28] which is over 10%. So when your speedo says 65MPH, you are actually going just over 73MPH. When you change gearing ratios or tire diameters you need to order a new speedo gear to correct the speedo.

For the post you were quoting:
Assuming that the 33" tires actually measure closer to 31" or 32" in diameter (which is very common with some manufacturers) - the speedo would be off by about 3-6% and give an incorrect MPG calculation.
 

homebrew

New member
I use my GPS for my calculations I know they are not exact but its a lot closer than my speedo which is not correct.

I replaced the 165 Tstat for the correct 195

Im running Trxus MT's according to their site they appear to be a true 33.

I plan on swapping out the upstream 02 and i'll look into getting the map sensor checked.
 

joe_peters

New member
Ok, XJ MPG 101:

1. It is a box, so you are always pushing the air.

2. You are most likely purchasing E10 fuel--that will cost you 10 percent of your mileage.

3. You are purchasing Winter blend fuel--that will cost you 10 percent of your mileage.

4. Your right foot.

5. Slushbox transmission.

6. Things that can make the XJ run rich: CTS reading wrong--you can pull it, clean the scale from it, clean the connector, and test it; upstream o2; thermostat too cold or not opening at the correct temperature; MAP; MAT; exhaust leak upstream of the o2 sensor; intake leaks--manifold gasket, injector o-rings, vacuum lines.

7. Tire inflation.

8. Use all synthetic lubricants.

JP Magazine did a three part series on XJ mileage. A failing PS pump was costing them quite a bit in mileage. Go to jpmagazine.com and search "mileage master", read the articles. Also, while at JP Mag, search "4.0 myths", it is a good read. There is also a short write up on increasing mileage by using common sense practices: http://www.jpmagazine.com/techarticles/engine/154_1008_increase_your_mileage_for_free/index.html

Good luck.
 

homebrew

New member
Checked the MAT and CTS both were fairly clean. Forgot to mention in my first post that I also replaced the ac compressor that was beginning to seize up.

I know that my PS pump isn't at 100% I sometimes have to give it a little gas at lower rpm in order to turn the wheel ie pulling into a parking spot. However it doesn't make any odd noises maybe I'll try popping off the belt and seeing how free the pulley is.

I plan on changing out the upstream 02 so this I know for sure that it is good.

I'll post any notable changes.
 

the_weirdo

Ugh
NAXJA Member
Your mileage doesn't sound too far off to me. With the winter(oxygenated) gas, I am lucky to get 13MPG mixed driving. With 33's and 4.56 gears, I am lucky to get 15-16MPG on the freeway with no hills. My average is about 12-13MPG combined. That's the nature of driving a boxy lifted vehicle.
 

homebrew

New member
I'd believe the 10% hype, but I was getting the same mileage in san diego 2 months ago that I am now. There's gotta be something else going on.
 

GRANDLICH

New member
Background:
Assume stock tires are 28" diameter. Let's say you put on 31" tires - now your speedo is not going to be correct. If you put on bigger tires your speedo will read slower than you are really going. It will read slower by the ratio [(31-28)/28] which is over 10%. So when your speedo says 65MPH, you are actually going just over 73MPH. When you change gearing ratios or tire diameters you need to order a new speedo gear to correct the speedo.

For the post you were quoting:
Assuming that the 33" tires actually measure closer to 31" or 32" in diameter (which is very common with some manufacturers) - the speedo would be off by about 3-6% and give an incorrect MPG calculation.
I see what your sayin. well i get right about 11 or 12 MPG and i have 31s and stock gears.
 

uRabbit

New member
I'm getting 18-20 in my dying '90 4.0. A rebuild shop said it has sludge build-up and not a single piston is on time.

What I do, tho, is accelerate very slowly, and never use the heavy foot that I use in cars. Using cruise control is your best bet, also. Mine has it but it doesn't work. Don't use cruise, however, if you're in a hilly area. Instead, use a steady foot. Don't try to maintain your speed, but the depth at which you're depressing the pedal.

It's taken a lot of self discipline to get this good of mileage from my 21 year old Jeep nearing 200k miles. That, and plenty of additives. I use Seafoam in my tank (you could use it every fill up or whenever you think about it) and Mystery Oil at every oil change.
 

CharlesS

New member
Since doing the fuel injector replacement and changing the sparkplugs I have been monitoring the highway gas mileage over the past 6 weeks.
Vehicle info: 5.5" lift, 32" GY MTRs, 115K 4.0L, 5 spd, and 4.56 gears.
On a trip back from an off road event in Lucerne Valley I decided to retest the gas mileage while driving back to San Diego.
I filled up the gas tank in Barstow until the pump clicked off and aired up the tires to 36 psi before leaving.
Driving 55-65 mph on the way back I checed the speedometer against a Garmin Oregon 300 to ensure the speed was accurate; I arrived in Escondido with more than 1/2 a tank remaining. Refilled the gas tank and calculated the gas mileage at 16 mpg and that is with the Winter gas we have here in So Cal.
This is an increase of 2-3 mpg from before replacing the parts. Keeping the speed down to less than 65 mph probably helped too.
 

O-Gauge Steamer

New member
Calling it a box is charitable... More like a brick wall. Stock gearing, 30's, Auto, CAI, 99+ manifold, header. 16mpg combined. Gets slightly better on Hiway. Have not changed speedo drive as bigger is coming this spring. Checked accuracy of odometer by the mile markers and it is 4% slow.

7,000' altitude.
 
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