New 4.0 mileage

I have a new crate 4.0. 3k miles. I have a new K&N filter, Flow master muffler, free flow cat. I am running 31" tires with 3:55s and I'm only getting 12 miles to the gallon. The 31" tires were calibrated into the mileage figures.The EFI rail has no leaks and I don't smell fuel any where. I don't think my O2 sensor was replaced when I got the new motor. Does this sound like the culprit ? Some one please respond. I haven't had much luck with responses in the past.
Does any one have the same set up and what kind of mileage are you getting if so.

Thanks a Bunch for any help

1988 XJ Pioneer

boise49ers said:

I have a new crate 4.0. 3k miles. I have a new K&N filter, Flow master muffler, free flow cat. I am running 31" tires with 3:55s and I'm only getting 12 miles to the gallon. The 31" tires were calibrated into the mileage figures.The EFI rail has no leaks and I don't smell fuel any where. I don't think my O2 sensor was replaced when I got the new motor. Does this sound like the culprit ? Some one please respond. I haven't had much luck with responses in the past.
Does any one have the same set up and what kind of mileage are you getting if so.

Thanks a Bunch for any help

1988 XJ Pioneer

Click to expand...

new engine and you reused the old o2 sensor? of course it could be part of your problem. next would be the stock gearing with 31" tires. with only 3k, the engine still needs to break in.

i would change to o2 sensors and reset the computer and see what happens.

also, when you increase air in and air out with the k&n filter and new exhaust, you will burn more fuel.

I'm not sure if it was changed or not. The header is new. Maybe ! They reused a lot of the parts. I didn't install it. I thought that better the flow of air and exhaust was suppose to increase your mileage not inhibit it ? ? ?
Can the O2 sensor be cleaned like a spark plug ? The computer resetting isn't something I could do, Unless it is pretty simple to explain. Then I'll give it a try. I would assume the mechanic would of done it at install time though.
He is a Jeep Nut and owns numerous XJ's.

http://myweb.cableone.net/cherokeexj/gary.html

Thank for the quick response !

The freer-flowing intake and exhaust *should* give you better mileage
IF you drive conservatively much of the time. When you put your foot
in it, the additional air flow will cause it to burn MORE gas, and hence
create more power.

On average, it usually ends up being slightly better in the long run... but
it's not unusual for a person with a new performance exhaust to "like to
listen to it"... and ends up getting poor mileage... at least until they get
used to the new sound and power!

I wouldn't expect a LOT of difference either way. Getting sub-15mpg
figures around town is not at all unusual...

Den

I have a similar situation. 98 XJ with a Jasper rebuilt. New O2 sensor. About 5500 miles on new motor. Was getting about 11-12 mpg initially. It has gone up to about 15.5 mpg but now it sounds and feels like the timing is off. Reminds me of the old days when you used to be able to advance the timing by rotating the distributor. Same kind of feel. So short of taking it to a dealer, how do you reset the computer?

I'm fairly new here but on all the K5's that I had to reset the computer you just disconnect the battery for a couple of minutes.

how "new" is the engine?? i had a new engine installed and left for Moab last fall with 150 miles on it.. my mileage was average 14 mpg for the trip out and by the time i got back it was average 18 mpg.. thats with a round trip of 5300 miles though.... as pointed out the mods you made could be killing you as well but i got horrible mileage until it got about 4000 miles on it.....

mike

You didn't indicate the distances you were driving; if you are doing very short drives the engine will run rich. Similarly, if the new thermostat is too "cold" then the engine will also run rich; where's your temperature gauge pointing? Vacuum leaks can also affect mileage and may show as a rough idle. Finally, 31s are fairly hefty meats and will also contribute to a mileage hit.

There is no "reset" for the 1988 computer. The Renix system "reboots" every time you start the engine.

The O2 sensor could be the problem. 3000 miles should be enough break-in for the mileage to normalize. But 31" tires and 3.55 gears aren't helping you any. I run 31" tires in a 1988 MJ with 3.73 gears and get around 18 MPG in mixed urban/suburban driving, 19 to 20 MPG highway. The combination of 3.73 gears and 31" tires puts my overall final drive ratio almost exactly equal to stock tires with 3.55 gears. For you, running 3.55 gears with those tires, the engine is really operating at the low end of or below the most efficient RPM range.

No, the O2 sensor cannot be cleaned. It should be replaced every 75,000 to 80,000 miles anyway, so if you're not sure it was done, I would start there.

I had the dealership put a new long block into my 87 last year. I've never had the bad mileage your having. I've gotten around 20-22 since I pulled out of the parking lot that day. They also didn't replace much else. I had to install a new 02 sensor and some other things.

Where did you get your engine from? How does it sound? I started getting a knocking noise from mine in January and my mileage went down to like 17-18. It turns out that I had some bad lifters and the dealership ended up replacing all 12 (love that warrenty!). It's still making the noise, but the mileage is back up to 19-20.

It's strange for something like that to go bad in such a new engine, but you might look for something internal in yours that could be bad. I would also replace the 02 sensor - gotta start somewhere!

I think I have the answer I was looking for. My ride to work and back every day is 3 miles round trip. I do warm it up in the morning for about 5 minutes until Summer gets here. I'm probably running rich. Plus the 31s on stock gears. I think after I get around 6000 miles I'll figure it out again and do it when the weather is warmer. I should get a pretty good reading then.

Thanks Everyone !