Hello,
I'm the original owner of a '99 TJ Wrangler, with a 4.0 engine and 5-speed AX15 transmission. For the first few years after I bought it, it got about 18 MPG (highway or city, didn't seem to matter). Then for many years after that, it settled-in to a little less than that. 17.5, 17.6 MPG became the norm and it was very consistent that way. During those years I replaced the O2 sensors a few times, but not with OEM sensors. I used a Bosch brand. That didn't have any impact on MPG: the car continued to stay at 17.5/17.6 MPG. At one point I tried some of those 4-hole Bosch fuel injectors, to see if the vendors' claims about improved fuel economy were true. No, they weren't at all: but the MPG stayed exactly the same anyway (wasn't worse). At the 392,000 miles mark for the original engine - still going strong by-the-way, but destroyed as a result of the dealership's negligence - I put a remanufactured JASPER engine in. MPG was STILL exactly the same after that installation (17.5/17.6).
Beginning about four years ago, the MPG started gradually declining... 16... then 15, where it remained for the last two years. I know, I know, it's a JEEP, what should I expect? We don't buy them for their fantastic gas mileage of course. But I always thought it was odd. This last year, my jeep wouldn't pass smog for the first time ever (high "NOX", which I assume refers to nitrogen oxides). First, I had the mechanic install a new CA/CARB-compliant cat converter (I'm in CA as you've guessed). My car still failed the NOX test. We then installed OEM MOPAR O2 sensors. My car did pass CA smog at that point. My mechanic had a theory that the old original cat converter became clogged & dirty over time and that - along with the aftermarket O2 sensors may have been the cause of the MPG decline. A theory anyway...
Since I've been driving the car since the converter & OEM O2 sensor replacements, it's been getting even WORSE MPG. It's now only 12 MPG.
Anyone ever heard of a similar type experience? Is it possible that this is a temporary phenomenon as the car's computer adjusts to the new sensors and cat converter?
Really appreciate anyone's thoughts and ideas.
Thanks very much.
I'm the original owner of a '99 TJ Wrangler, with a 4.0 engine and 5-speed AX15 transmission. For the first few years after I bought it, it got about 18 MPG (highway or city, didn't seem to matter). Then for many years after that, it settled-in to a little less than that. 17.5, 17.6 MPG became the norm and it was very consistent that way. During those years I replaced the O2 sensors a few times, but not with OEM sensors. I used a Bosch brand. That didn't have any impact on MPG: the car continued to stay at 17.5/17.6 MPG. At one point I tried some of those 4-hole Bosch fuel injectors, to see if the vendors' claims about improved fuel economy were true. No, they weren't at all: but the MPG stayed exactly the same anyway (wasn't worse). At the 392,000 miles mark for the original engine - still going strong by-the-way, but destroyed as a result of the dealership's negligence - I put a remanufactured JASPER engine in. MPG was STILL exactly the same after that installation (17.5/17.6).
Beginning about four years ago, the MPG started gradually declining... 16... then 15, where it remained for the last two years. I know, I know, it's a JEEP, what should I expect? We don't buy them for their fantastic gas mileage of course. But I always thought it was odd. This last year, my jeep wouldn't pass smog for the first time ever (high "NOX", which I assume refers to nitrogen oxides). First, I had the mechanic install a new CA/CARB-compliant cat converter (I'm in CA as you've guessed). My car still failed the NOX test. We then installed OEM MOPAR O2 sensors. My car did pass CA smog at that point. My mechanic had a theory that the old original cat converter became clogged & dirty over time and that - along with the aftermarket O2 sensors may have been the cause of the MPG decline. A theory anyway...
Since I've been driving the car since the converter & OEM O2 sensor replacements, it's been getting even WORSE MPG. It's now only 12 MPG.
Anyone ever heard of a similar type experience? Is it possible that this is a temporary phenomenon as the car's computer adjusts to the new sensors and cat converter?
Really appreciate anyone's thoughts and ideas.
Thanks very much.
Last edited:
