Gas mileage going down. Weather? 02 sensor?

Does the cold weather effect mileage?

1990 Jeep comanche. 4.0 ax-15 np231 3.07gears stock tires, 2" lift

My truck got 21mpg when I first bought it, 17 or so if I beat it.

I lifted it two inches with spacers, stock tires, glasspack and a cone air filter. (I figured those mods would HELP mileage, not make it worse) now I get 17 average. And if I go wheeling or play in a field or something ive gotten down to 13mpg. I played a bit when it was stock, but it NEVER went down anywhere close to 13mpg.

No idea how old my o2 sensor is, any way I can check that? What else should I look at?

I read an engineering explanation of the intake system (Renix) in the late 80's. They said the intake for the air box, was an integral part of the emission system. The warm intake part of the system was supposed to actually be an intake air mixer, at most normal temperatures. And it's responses were programed into the computer. The Mat values, were designed to work in conjunction with the warm air mix system in the air box.
Intake air temp. MAT and engine temp. sensor have a rather large influence on the final air fuel ratio. As does the MAP and O2 sensor.
The booklet actually suggested, the warm air inlet on the air box, as a major cause of emission test failures.
I live in a cold climate and have always kept that booklet in mind, when messing with my air box. Especially in winter.
Just a theory, not a definitive answer. If you kept the pieces, you might want to try the OEM air box and make sure the warm air mix (vacuum and valve) system is working. If you go back to the original and run a couple of test tanks and it helps, post up.
I recently changed back to a set of 215 tires, while I get my rims stripped nd powder coated. I've been keeping track of the difference in mileage between the 30's I normally run and the 215's on there now (which is actually not much difference).

Not on an XJ, but might help to ease your mind...

My experience is that I lose on average, about 1 (sometimes 2) mpg in the wintertime on my 2001 4wd silverado. I have seen it do this over the last two winters. I spent a lot of time on the road with it as its my daily driver, and track my mpg just about every tankful. My fuel economy always comes back during the summer.

A couple of things probably play into it. Aside from the colder air intake temperatures under the hood and the compensation the computer has to make to maintain a smooth idle during warm up, I think it also has to do with the differences in how the fuels are blended for summertime vs wintertime. I wouldnt be suprised to see different regions fuels giving different results either. Also remember that during the winter, you'll likely have to turn on the defroster more, and at least on my truck, this also turns on the AC compressor.

I'd fully expect my better half's XJ to display similar MPG hits in the winter.

Dave

I don't think it's the 'winter fuel' I think that was put on hold this year to allow the oil companies to 'catch up' from that make believe shortage we just went thru. Depending on how you drive it might be time to run some cleaner thru the system, seafoam, Techron system cleaner, etc. If you do alot of low speed local driving you could be having a carbon build up. I drive pretty much in the upper RPM ranges for my gears locally and this last week I was commuting to a new site outside allentown which entailed a ~24mi run on the turnpike with the traffic flow at around 80mph 2x's a day. I filled up before leaving and drove down to the PX, commissary and the inlaws in NJ yesterday, 150mi round trip, then filled up again when we got back at the local $2.29 a gallon no name station, got 23mpg down and back, average speed 70mph. Prior to that I had been averaging about 18-19mpg so I think the combined hi speed runs and two cans of seafoam cleaned stuff out, idles smoother and is a bit quieter too. I did not do my annual plugs/cap/rotor/wire change this year either so there is about 30k on whats in there.

I am also thinking about going back to my original airbox for the winter time. I am changing the oil this weekend, so I figured while I was under the hood i'd take off the cone K&N. Think this will have much effect on mileage?