College kid needs MPG help...please

Hey guys...

..I only work on Bowties and Isuzus so bear with me on Jeep stuff. I've had my 94 XJ 4.0 aw4 for 8 months now and it seems as if the mileage gets worse everyday. If I have to spend all my money on gas, I won't have any cash to lift it and have fun.

The motor runs absolutely fine at all rpms and nothing seems to drag (brakes). I have 119,000 k on it 10k are mine. stock gears 3.55's, 235/75 wrangler radials.

The only thing I can think of is the new 90w gear oil in both diffs and it runs pretty cold 180* when cruising.

ANY and I mean ANY HELP WOULD BE GREATLY APPRECIATED.

So what kind of numbers do you have? It's hard to tell if it's bad without that. Do you drive around town with the a/c on? What gear do you use around town? Since it's an AW4 get it out of (D) and into 3rd.

Make sure your tires are inflated; lower pressure = more drag on the pavement

Give it a thorough tuneup - new plugs and wires, cap and rotor, oil, drain and refill transmission, and t-case since you said diffs are done already.

Do you have a lot of 'stuff' on the Jeep? Roofrack, lights, etc? Those will drag as well. Jackrabbit starts (heavy right foot) will eat gas as well.

Yeah, before we can comment we need to know where we're starting. When explaining your current mileage, be sure to say what type of driving, not just how many MPG you get.

XJs are, after all, 4WD trucks, not econo-boxes. You're not going to get 50 MPG. However, you should be getting upper teens in mixed urban/suburban driving, and maybe 19 to 21+ highway if you don't have a really heavy right foot. Don't forget that your tires are larger than stock, which means you drive farther than your odometer reads. The difference isn't huge, but it's there. You have a '94? Let's assume the stock tire size back then was 215/75R15 (and that's a pure guess ... which would have been safe for earlier years but by '94 they might have already made 225 the standard size). Using the tire revolutions-per-mile method, it looks to me life about a 4.5% difference. That's close to one MPG right there.

hell! my 99 has had the same tank of gas for 2 weeks now!! excellent milage!!!! oh yeah.... it's disassembled in the dealers shop..... i knew something was fishy there....
but anyways.. the "heavy right foot" make more difference in the XJ than any other vehicle i have ever owned.. i drive hard.. but my 87 trail rig gets about 3 MPG better than the 99 just because i dont drive it as fast or as hard.. tire inflation plays a big part in it as well....

mike

At 119,000mi I'd add O2 sensors [my 98 has two], good top engine cleaning, FI cleaning, throttle body cleaning then I would check my milage. The O2 sensors can make a big difference. 180F, wonder if it is staying in cold loop mode [rich].

Im with RichP. 180 is a little cool for an XJ. I have a 90 w/211,500 (and counting) and it doesnt idle smoothly until it gets to at least 200. I would think about putting a higher temp tstat, maybe a 195. Good Luck.

90red

Thanks Guys. I am currently using a GDI 3-core rad with the Jet 180 stat. It leaks a bit when it it sits overnight from what looks like the t-stat housing. I hope its not the waterpump. I like how it stays cool but if it starts to get hot, it gets to 220 damn quick. That scares me.

Could just be a warper thermo housing.. .mine was.. n i had to get a new one...

I agree with what has been said here, especially about heavy right feet and tire pressure, but I have to disagree with the recommendation to get the AW4 out of D and into 3 around town. Keep it in D, which allows the Jeep to go into OD. Engines get better milage if they are lugging just a little. Not letting the tranny shift into OD is going to keep your revs up, and that is going to use more gas.

Tim C.
90 XJ
01 XJ

you guys should be happy... i got 11mpg on my 400 mile interstate trip the other day.....

If it's seeping around the tstat housing I would pull it, coat the mating surface with indian head, stick the gasket on, coat the other mating surface with indian head and reassemble. Also coat the end of the threads with anti-seize then reassemble. Nothing gets by that indian head once it cures. I have been using that stuff for years on any cooling sytem seals including the ends of the hoses, I coat the first 1-2" of the hoses before reassembling. The ONE $#*$% I didn't it leaked after replacing a radiator and of course it had to be the biggest PIA to get to, the lower hose going into the water pump.

First off you have the wrong thermostat in your engine. The engine is designed to use a 195F thermostat. The engine computer does not go into 'closed loop" operation until the coolant temp is 180F. You could have the engine runing in open look which means it will be running rich.

The advice on here is to buy your thermostat at the dealer, while you are there pick up a few extra gaskets.

You may want to replace the fan clutch for the mechanical fan, they only last 5 years and there is no good way to test them.

It sounds like you may have a thermostat that has failed and is opening early or late. I'd replace that and the fan clutch at the same time.

If it continues to run hot you could have a lower radiator hose with a bad or missing internal spring. The hose can collapse and prevent the flow of coolant.

start with the basic things.

Othere things to help your mileage is a tune up, new air filter, etc.

I replaced my dead coil and at the same time replaced the plugs which were WAAAAAYYYY overdue for a change. There were carbon deposits that almost filled the plug gap. I overgapped a set of new Bosch platinums to .o45 and my mileage shot up 3mpg instantly. Like everyone said, start with a basic tuneup (plugs, wires, distributor cap) and clean out your TB really well and start from there.

tacottle said:

I agree with what has been said here, especially about heavy right feet and tire pressure, but I have to disagree with the recommendation to get the AW4 out of D and into 3 around town. Keep it in D, which allows the Jeep to go into OD. Engines get better milage if they are lugging just a little. Not letting the tranny shift into OD is going to keep your revs up, and that is going to use more gas.

Tim C.
90 XJ
01 XJ

Click to expand...

Sorry, but I disagree with your disagreement. Engines do not get the best economy when lugging. They get the best economy -- all things being equal, which they never are -- when operated at the RPM where the engine developes maximum torque.

XJs in general don't have enough gear to operate efficiently at low speed. I've made test runs going one way in 5th gear (manual tranny) and the other way in 4th -- gas mileage in 4th was the same or better. Other people have duplicated this with automatics.

Very definitely, the XJ will deliver better economy around town of you lock out overdrive and run in 3rd gear.

It's not going to have nearly as many heating issues when run at 2000rpm either...as long as your fan clutch is working, that is. Lugging in too high a gear will do that - tested and proven numerous times in and around Phoenix in the summertime.