ok so recently I noticed something bizzare in my jeep, 1989 Cherokee xj. the fuel gauge drops so quick. I filled it up, 50 miles later im at half a tank. put in a new fuel pump and bam SAME thing is happening, could the gauge be bad or what could it be. I cant believe I would get 50 miles to half a tank

 

Check for fuel leaks. 5-6 MPGs is a little low...

 

I thought so to, I don't see any fuel leaks, nor do I smell any gas.

 

It really can be a number of things. Have you tried to calculate how much fuel you are really using? When you refill the tank is it taking as much gas that you think you are using? If so when was the last time you did a complete tune up? Check for any fuel leaks. Make sure that you have a locking gas cap and that no one is stealing you gas. Also are you stock or lifted, what size tires. Need a little more info before we can really give you any good advise. But what I have mentioned will give you somewhere to start.

 

lifted ABOUT 3 inches. 29 inch tires. I was getting about 250 miles to the tank before

 

im on 33's 4.5 lift and stock gears and still manage to get 200-250 a tank depending on how I drive. it would be a little better if I didn't have a huge crack in my header

 

Same question as Robert, when you're filling up when the gauge says it's at half, are you putting in ~8-10 gallons? Either your gauge is lying, you have a fuel leak, or your sensors, etc. are so extremely fouled up that they are causing it. If it's option 3 though, I would imagine it would be running like absolute dog****. How does it run/idle? Smoothly or is it rough?

Even with huge tires, lots of armor, roofrack, stock gears, you'll do better than 5. More in the 9 range. So something else is going on here.

 

it seams to idle fine. nothing wrong there. i have good throttle response

 

cruiser54 should weigh in here since he's the Renix guru but you should start with refreshing all the grounds that you can get your hands on. And really look at the fuel lines from the pump to the fuel rail to make sure you aren't leaking anywhere. That's some seriously bad mileage.

 

yeah and I'm pretty much stock, only upgrade to the mtor was an air intake

 

First off, you should fill it, drive, and fill it again. Divide the number of miles driven by the number of gallons used.

And no matter what, you should click on the pink link in my signature below and do #1,3,4, and 5.

 

If it runs ok (I don't mean just idle) when in open loop (cold) im guessing the TPS and MAP sensors are somewhat good

If the 02 sensor is "stuck" and think its running to lean, The PCM will dump fuel trying to compensate.

Also, if their is an exhaust leak before the 02 sensor, it may fool the sensor to think its lean also.

You can do a quick voltage check on the output of the o2 sensor and see if it varies (when hot) with throttle. if the output voltage is steady or varies slowly change your sensor.

If the sensor is over 5 ish years old, I would replace it anyway.

An other thing. If your temp sensor (Not the one on the head) is not working fine, the PCM may think the engine is not warming up and stay in open loop.

 

It's a Jeep, just enjoy it. If it really bothers you check the sending unit or the resistor in the gauge.

 

Correct. The MPG hasn't been confirmed by the correct method.

 

will fill it up and post back here