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Huge MPG drop!

N

nrwphoto

NAXJA Forum User
Location
Appleton, WI
1998 4.0, AW4, Select-Trac, 3.55 gears, 30x9.5 Goodyear Silent Armor other wise it's stock with 120,000mi now on it. When I purchased this Jeep 2 yrs ago it had 235/75R15 street biased tires on it and I averaged 20mpg with mixed driving. High side of 24 on the highway to the mountains and low of 17 in the city. After upgrading the tires (about 1yr ago) my mileage dropped by about 1mpg city and 2mpg highway but still pretty good for a 6cyl, 4x4. Then over the last 6 months my mileage started going down. It has now stabled off at 12.5-13mpg with mixed driving, the lowest I have got was only 9mpg, the high these days is 15mpg on the highway. I have also lost some power, has to down shift into 2nd when pulling 5-6% hills at 60 mph. (I really don't like it running at those higher rpms for very long) Compression reads a very high 155-162 across the board, spark plugs look like new after 20,000mi, does not blow smoke or burn any oil, coolant looks clean and like new after replacing cooling system 2 summers ago. No check engine light, and only code that pulls out of the computer is for a low signal from sending unit. Any ideas would be very appreciated.
 
nrwphoto said:
1998 4.0, AW4, Select-Trac, 3.55 gears, 30x9.5 Goodyear Silent Armor other wise it's stock with 120,000mi now on it. When I purchased this Jeep 2 yrs ago it had 235/75R15 street biased tires on it and I averaged 20mpg with mixed driving. High side of 24 on the highway to the mountains and low of 17 in the city. After upgrading the tires (about 1yr ago) my mileage dropped by about 1mpg city and 2mpg highway but still pretty good for a 6cyl, 4x4. Then over the last 6 months my mileage started going down. It has now stabled off at 12.5-13mpg with mixed driving, the lowest I have got was only 9mpg, the high these days is 15mpg on the highway. I have also lost some power, has to down shift into 2nd when pulling 5-6% hills at 60 mph. (I really don't like it running at those higher rpms for very long) Compression reads a very high 155-162 across the board, spark plugs look like new after 20,000mi, does not blow smoke or burn any oil, coolant looks clean and like new after replacing cooling system 2 summers ago. No check engine light, and only code that pulls out of the computer is for a low signal from sending unit. Any ideas would be very appreciated.
Click to expand...

Unless you changed out the speedometer gear in the transfer case, your mileage on the odometer will be inaccurate.


First suggestion is a new air filter, followed by routine fluid changes, e.g. transmission, transfer case, differentials, brake fluid, steering fluid, engine oil. Ignition wires should be replaced every 60,000 miles, so it may help to do a complete tune-up, plugs, cap, rotor and wires.

Might be time to replace the O2 sensors as failures can cause the engine to run rich.
 
Andy Steiner said:
Unless you changed out the speedometer gear in the transfer case, your mileage on the odometer will be inaccurate.


First suggestion is a new air filter, followed by routine fluid changes, e.g. transmission, transfer case, differentials, brake fluid, steering fluid, engine oil. Ignition wires should be replaced every 60,000 miles, so it may help to do a complete tune-up, plugs, cap, rotor and wires.

Might be time to replace the O2 sensors as failures can cause the engine to run rich.
Click to expand...

I have not swapped out the speedo gear, however it reads accurate according to my GPS, it was actually a little bit off with the smaller tires.

It has all new filters and fluids in it. I swap them out on a semi regular basis depending on driving conditions... more dirt roads or water crossings equals earlier changes.

Haven't check the 02 sensors although it has been brought up before... however if that were the case wouldn't it trigger an engine light?
 
Maybe your CEL is bad. Hook up a code scanner and see what it says.
 
Test TPS, if not within spec, replace.
IT cant hurt to get a free code scan as well.
 
nrwphoto;244835699. No check engine light said:
If you could post that code number, that would be helpful. Not sure what code/description you are talking about here in reference to "low signal". If you clear this code, does it come back??

Assuming that you are calculating your gas mileage by taking miles driven divided by gallons consumed?

Testing of individual sensors makes some sense if you're up to it. Throttle position sensor, map sensor, coolant temp sensor come to mind on the "to-do" list. Oxygen sensors can get "lazy" and become slow to optimize fuel/air ratio before they actually trigger the check engine light/code. They can be tested too but you gotta know what you're doing.

Or get an actual scan tool on it which is very valuable at seeing how efficient everything is running on your XJ. I've seen guys beat themselves up and throw lots of parts at a vehicle only to put a scan tool on it and determine root cause of their problem in about 2 minutes.
Click to expand...
 
Last edited:
Alright after heading up to the mountains the other day my check engine light came on after coasting down a hill for a few miles. The light went out later that day. Autozone pulled this code, P0107 for the MAP sensor. Which would mean either a bad sensor, loose or cracked vacuum hose, sensor dirty or throttle body intake tube loose. This code came up about 9 months ago, shortly after I had a mechanic install a new Banks header. I pulled the intake hoses apart cleaned everything including the MAP sensor, a bit dirty but otherwise everything looked ok. After this it ran like crap for about a day and a half, with no power, then started running fine. This wasn't the start of the decline in fuel economy, but it didn't help either. The check engine light hasn't turned on since the other day.
 
A lazy o2 sensor won't necessarily throw a code but will rob you of MPG.

You have 120k, when was the last time the o2 sensor was changed?

JP Magazine, www.jpmagazine.com, did a three part article on XJ mileage--interesting reading. Search "mileage master" on their website.

I don't know what kind of fuel you get in AZ, but 10% ethanol will cost you about 10% of your MPG, and winter blend fuel will take away another 10% of your MPG.
 
nrwphoto said:
Alright after heading up to the mountains the other day my check engine light came on after coasting down a hill for a few miles. The light went out later that day. Autozone pulled this code, P0107 for the MAP sensor. Which would mean either a bad sensor, loose or cracked vacuum hose, sensor dirty or throttle body intake tube loose.
Click to expand...

The MAP sensor can be tested with a meter and a manual. It is in play for determining fuel/air ratio so I wouldn't ignore an intermittent code for this, especially when you suspect a gas mileage drop.
 
So a little update, yesterday the TPS (throttle position sensor) suddenly died on me, giving me erratic shifts and varying power. Switched it out this morning and it now runs good again. Not sure if this will have any effect on my mileage however.
 
one thing i havnt read here is changing the fuel filter. start from the basics before going to electronics that normally have a much longer life span. a clogged fuel filter will give you poor mileage, low power, poor fuel preasure which can also cause sensors to go crazy. this can lead to so many issues. even your timing will start going wacky to compensate, which can get a lean burn, ruin spark plugs, ect. all from a stupid fuel filter that hasnt been changed. try it, you may be surprised of the outcome.
 
Good god yall have some horrible running jeeps.

I pull 17-18mpg all day long with 35s and stock 3:07s with an auto. And i drive 100 miles a day!
 
Bobby@Bullydog said:
Good god yall have some horrible running jeeps.

I pull 17-18mpg all day long with 35s and stock 3:07s with an auto. And i drive 100 miles a day!
Click to expand...
If you have an auto you probably have 3.55s not 3.07s, but I am quite jealous either way.
 
just a heads up, if you are calculating your mpg solely from your gps readings you will always be inaccurate. GPS is only accurate to 3 meters, i'm not going to convert that into yank units for you but approx 3 feet in a meter.

farmers who plot gps coordinates for combine purposed often approach their desired waypoint etc ( corner of property) from multiple directions (3-5) using gps they average the coordinates to get an accurate reading. this is very common when determining property lines.
even DARPA would agree with the general inaccuracies of gps.

If your speedo gear is stock, your odometer will be out, and if you are taking gps readings gospel, you have no chance in being accurate.

This being said i have no doubts that you have lost some mpg, but i would be willing to bet it isnt to the the degree you have stated . More like your value +/- 3 meters x distance traveled standard deviation
 
I know that GPS is off by 3 meters.. but does that really mean it will improperly measure distance? Won't it be off those same three meters all the time? I wouldn't think that would make the speed/odo of the GPS wrong..
 
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