New Issues, cant figure out.

Hello, lately I have been running into a few issues with my 2000 Cherokee, 4d, 4wd. I am running a 6.5 in long arm, 4.11's and 33/12.5's. I used to average around 15 mpg overall, but that has changed.

Over the last year, it has steadily dropped to about 9 mpg, and thats at best. Im not sure what could cause such a drastic change. I have not changed anything on the jeep since I first bought it 10 years ago and put the lift, etc on. I recently had it smogged and it barely passed for the first time. I cant exactly remember what the NOx number was, but it was only about 25ppm less than the max here in CA. Any ideas what might cause this issue and what can be done to fix it? I replaced the lower two O2 sensors about a year ago now, I never replaced the upstream ones.

Also, I was wondering if there is a fuel filter mounted before the gas tank, or is the filter just mounted on the bottom of the fuel pump in the tank itself. I have never chnged or serviced the fuel pump or anything with the tank since I have owned it. Not sure if this is going to cause any issues or not.

I have also noticed along with the increasingly bad mileage, I also have quite a noticeable loss of power. My cherokee is all of the sudden turning into a gutless pig.

Over the past few years, I have had to replaced the TPS a few times after it kept going bad. I finally bought a factory one from the dealer and it has been fine for about a year now.

A few things that I am thinking about addressing to fix the issues are the CAT for the high NOx reading. Possibly a new exhaust header, since I have the original factory one still. Maybe having someone check the torque convertor to see if its still ok.

Any ides or suggestions would be great

I would start with basic tune up/diagnostics - look at your plugs, rotor, distributor cap, compression test engine, vacuum, O2 sensors, fuel pressure, trouble codes?

Make sure all your tune-up parts are fresh.

A drop of 30-40% in your mpg's usually signals weak or failing O2 sensors. O2 sensors ( and the cat converter ) have a normal service life of about 150,000 miles. Like tires or mufflers, O2 sensors (and cat converters) are replaced when they wear out or no longer function correctly.

The 1997-2001 fuel filter is only replaced if fuel pressure/flow testing show a problem.

XJFrankenstein said:

I would start with basic tune up/diagnostics - look at your plugs, rotor, distributor cap, compression test engine, vacuum, O2 sensors, fuel pressure, trouble codes?

Click to expand...

He doesn't have a rotor or distributor cap.

Are your sure your parking brake isn't on or the brakes seized?

I suggest that you seriously consider the upstream O2 Sensors. Those are the ones the PCM uses to control the mixture. Any Sensor that is after a Cat is there to sanity check the Cat. And, to me it sounds like at lest one of them is going away.

The fuel filter is integral to the pump (as far as I know) and it is not much of one either. I have seen one case where the pad came loose (it is just a small pad of material at the pump inlet...) from its mount and choked off the pump inlet.

The symptoms for that case was hesitation when making a hard turn in one direction only.

One of the problems we have is that Jeep does not use a wideband sensor so... Any signal within range is acceptable.

Does anyone know if the PCM is smart enough to compare front and back sensors to see if they are close?

This seems to be a very common problem lately.

If your Jeep was running fine with normal power I'd say you're where I am, screwed.
Upstream O2 sensor is easy and inexpensive. I got a Bosch to my door for $25 off e bay. Also make sure your getting up to temp. I think normal is 195-210.

Hopefully you have better luck than me. At least I'm getting better than 9mpg's.

Thanks for the replies. I will check and see if the upstream sensors are working properly. I know its getting up to temp, at least the gauge says so. Not having any hesitation issues, so Im guessing the fuel pump and filter are fine. I have around 135,000 miles on it right now, so I guess a change for the filter wont be for a little bit longer.

I never really thought about the parking brake. I will lift it up and see if the wheel are free turning with it off and in neutral.

I have been noticing alot more vehicles that I work on now are having cat problems due to the ethanol that is in the gas now... For some reason it is clogging the cats up... I had to replace all three cats on my wife's 2005 nissan quest it only has 110k on it... they should not have failed that soon....