Crappy gas milage

What key factors/sensors/parts play a role in having horrible gas milage? I am getting a little over 10mpg.
Any help would be great considering that I live in the bay area of california. 87 octane gas is close to 2 bucks a gallon.
TIA

BenXJCA,

If your mileage is that bad you are money ahead to take it to the dealer or a shop with the scan tool for a Renix XJ to check it out . It can be a variety of things.

I lost MPG when:

* My EGR failed and was feeing hot exhaust into the intake all the time - the fix was replace the EGR
* My Cat converter was stopped up - the fix was replace the cat had much more power.
* When I put in Mobil 1 synthetic ATF in the transmission - the fix was to have a transfusion done on the transmission and get new fluid in it.

With the test set hooked up the tech can see if all the sensors are working and if the readings are within spec. You could also have a bad Oxygen sensor.

My guess is your are suffering from more than one problem. Also with the price of some sensors it better finiding out what wrong before you drop $70 for a dealer oxygen sensor, $85 for an EGR valve, etc.

HTH

Martin

First off, how are you calculating your gas mileage? Are you filling the tank FULL, recording the odometer reading at that time, then driving until the tank is say 1/4 full, and refilling again to the top, again recording the mileage?

IF so, then it's a simple matter to then take the difference in the two odometer readings (the mileage you drove on that tank of gas), and divide it by the number of gallons you put in the tank to fill it THE SECOND TIME you filled it.

If you are doing all of that, and you have confirmed you are getting 10MPG, then here is some stuff to check:

1. clean air filter
2. good spark plugs, distributor cap/rotor, spark plug wires
3. tires filled with air to proper pressure
4. operating temperature--does the engine run at about 200 degrees F when warmed up, or does it run much cooler?
5. how much junk are you carrying around in the vehicle. (That cord of wood you have been meaning to unload from the back for the past month probably adds 500 lbs to the gross vehicle weight! )
6. how's your driving style? slow starts, coasting, waiting to use brakes until really necessary?
7. how fast do you drive? A roof rack with KC daylighters and a big spare tire create a lot of wind resistance at 75mph!
8. what kind of gear ratios are in your vehicle (assuming they are other than stock from the factory)?

Check this stuff out, and if you are still seeing 10mpg, report back and maybe we can give some more suggestions.

Also..how 'bout your tire size? Has it changed? In other words, if you go out and put big tires on without changing out the speedo gear it'll look like you're getting crap mileage, but it's actually just teh same.

The MAP sensor can make it that bad but probably not the O2. What engine and year?

I was/am getting similar mileage in my 93 4.0/AX15 w/4.10s and 33x10.50's. I just changed my O2 sensor (140K on the stock unit), replace a crumbly cat and changed the plug. I haven't been able to do an accurate milage assesment yet, but it definately feels more powerful. My MAP sensor is stock, as far as I know, and my K&N cone is clean. I lose about a quart of oil every 3000 miles. Tire pressure is ok, as well. I suppose I should check the MAP sensor, but I am hoping that the cat and O2 were to blame. My cat was busted up, but I don't think it was per se clogged. The plug wires and cap were changed about 20k miles ago. My jeep weighs a ton with a heavy rrofrack and winch/bumpers. I am gonna lose the rack here soon, and hopefully that'll help some, too.
So, I guess I am not much help, but I'd start with the things mentioned so far in the thread.

Can a MAP sensor really have that much effect on milage? The engine *seems* to be running fine.

KY Chris

Damn Jelly. I get like 16-17. Granted my numbers are probably a bit off, but still. Your 33s and 4.10 shouldn't suck that bad, right? We have like the exact same stock jeep minus all our mods; you have only a few more miles on it than mine. How do I get 6-7 more miles to the gallon than you? I swear yours runs cleaner than mine too.

I have an 88 4.0L with 31 inch tires and no gear change. I don't know if the speedo gear has been changed, I bought it from my friend and probably should ask him. Anyway, if I drive as nice as humanly possible (no more than 2,000 rpms on acceleration) than I get about 14mpg. Does this seem right, or should I be getting more?

Mich88XJ said:

I have an 88 4.0L with 31 inch tires and no gear change. I don't know if the speedo gear has been changed, I bought it from my friend and probably should ask him. Anyway, if I drive as nice as humanly possible (no more than 2,000 rpms on acceleration) than I get about 14mpg. Does this seem right, or should I be getting more?

Click to expand...

I have two '88s. The XJ is stock and gets about 18 in mixed urban/suburban driving, 23 MPG on a highway trip. The MJ has 31s and 3.73 gears. Speedo gear has been changed and is dead on accurate. Gets about the same 18 MPG locally, and on a highway trip (tailgate up) gets between 19 and 21 MPG.

Even after correcting speedo error, 31s and larger will hurt the gas mileage if you don't re-gear.

Don't use 2000 rpms as your "magic number." If anything your magic number should be 2350rpms.

I am going to wait then to re-gear, because I am getting a set of 33s at the end of summer, and hopefully saving enough through the winter that I can get a 6" lift, and then run 35s. BTW, I was planning on the Tomken lift, and I was wondering if the 1" drop T-Case will fix driveline angles (At least until I can afford a slip yoke eliminator. Someone mentioned rotating the rear axle, and re-welding spring perches. I thought that this could be a problem with not getting enough gear lube up to the pinion. Am I right or not?

I was getting about 11.5mpg until I installed cruise control and started driving no faster than 60 to and from work. I know boring but I gained 6.5mpg on the first tank. I have a roof rack and a tire and 8 Hella 500's on the roof and it all rolls on 31's, so if there is anything I can do to help the gas situation I will try it.

Speaking of speedo gear changes, can someone link me to a chart that tells me what gear I need...

For the life of me I cant remember where I found it.

John