6 MPG Gain in my Cherokee 4.0!

[Miller88]

Gas mileage on my Cherokee has been pretty poor. I have been able to get 14 MPG on the way to my parents and back (140 mile round trip) but that's about it lately.

Luckily haven't been driving it much as it has had some issues. Got the issues fixed so I can drive it and then decided to do the following:

- Aligned it true straight again. It wasn't bad this time, but it was about 1/8'' toed OUT. I replaced the front springs and lowered it about 1/2'' in the front (it goes off road, keeping it in alignment is a pain)
- Switched from 5w-40 synthetic to 5w-30 conventional
the big one ...
- Replaced fan clutch!

The fan clutch, for the past 3 years, has been roaring. I used one from a grand cherokee with a full size fan to provide a bit better cooling. Never realized just *how* much gas this was taking.

I made these changes about a month ago. Have ONLY been driving it to my parents and back. Last three tanks were
- 19.5
- 18 (stopped and played around a bit off road on the way there - still EXCELLENT for doing off roading)
- 21 (this one is surprising as I went from load range D tires on alloy wheels to Load range E mud tires on steel wheels - about 10 pounds difference per wheel AND I didn't use overdrive unless I was doing on flat ground. As soon as I came to the smallest kill, I'd put in in D)

Didn't think a fan clutch would do that much!

Now that I feel I've made up for the miles put on my car by the Jeep being out of commission, I'll go back to driving the car every day.


I have been doing research on the 4.0. Apparently it's happier when running at 2000-2300 as far as fuel economy is concerned. It can lug around all day on the rocks and not complain. I would have to agree - the best mileage I ever pulled out of a 4.0 was in a TJ Rubicon model (4.10 gears, 31'' tires) - 23MPG.

20MPG for something that is lifted, has heavy wheels with load range E mud tires and a heavy bumper is pretty good!

[Daox]

Congrats on the better mileage. Do you think temperature had something to do with it too? It does seem like a huge gain for what was done.

[Miller88]

Originally Posted by Daox

Congrats on the better mileage. Do you think temperature had something to do with it too? It does seem like a huge gain for what was done.

The fan clutch was responsible for 3-4 of it. It's pretty routine to take a 3 or 4 MPG hit when switching to the grand cherokee ZJ fan clutch.

I think it was a compounded issue. The fan clutch moved such a massive amount of air that the coolant never came all the way up to temperature (causing it to go open loop quite a bit).

The temperatures had a bit to do with it. I Think running a 5w-40 oil didn't help. At MOST temperatures, it's significantly thicker than a conventional 10w-30.

[redneck]

It sounds like your clutch fan seized and was stuck in the engaged position.

Replace the clutch fan with a electric fan.

>

[Miller88]

Originally Posted by redneck

It sounds like your clutch fan seized and was stuck in the engaged position.

Replace the clutch fan with a electric fan.

>

That was one purpose. The ZJ Grand Cherokee has a full sized fan and the Cherokee fan wasn't big enough to ever cause it to disengage.

I have an electric fan next to the clutch fan.

Complete electric fan conversion kits run for $200 or so. I could piece something together from a junkyard, but I'm still on the original 14 year old alternator and wiring.

[Oceanic815]

That's a good gain for an XJ; let see if you can keep it up. Are your mud tires oversized to help with the mileage gain?

[Sonny Cash]

thank god you figured out the fan clutch was sucking your mpg. i wish the earlier jeeps where more fuel efficient soley because i would love a black wrangler from any era my uncle took me to rauche creek he has a 14' lifted with 34's and the trails there where so fun. there was a stock wrangler clearing everything we did and he only got stuck 1 time. interestingly so we chipped his jeep for the ride down to PA (all hwy) for economy and got 15 mpg. so on the way back we where curious and chipped it to performance. got 20mpg

[Miller88]

Originally Posted by Oceanic815

That's a good gain for an XJ; let see if you can keep it up. Are your mud tires oversized to help with the mileage gain?

The mud tires are a total of 4'' taller than stock, but a bit narrower. 225/75-15 was original size, I now have 215/85-16 on it.

Originally Posted by Sonny Cash

thank god you figured out the fan clutch was sucking your mpg. i wish the earlier jeeps where more fuel efficient soley because i would love a black wrangler from any era my uncle took me to rauche creek he has a 14' lifted with 34's and the trails there where so fun. there was a stock wrangler clearing everything we did and he only got stuck 1 time. interestingly so we chipped his jeep for the ride down to PA (all hwy) for economy and got 15 mpg. so on the way back we where curious and chipped it to performance. got 20mpg

I want to go down there, but I wouldn't drive my Jeep down. Would be afraid of breaking something and not being able to drive it back. If I can secure a truck and a trailer I would!

[baldlobo]

the fan clutch is a fuel sucker; driving in winter with a full grill block you'll notice it draining the tank faster. My tj goes from 350km summer to 200-250km ish in winter; but that's because of the colder air, the 5-7 stiff fluids(8 if you count the fluid clutch for the fan).

[oldtamiyaphile]

Originally Posted by Sonny Cash

i wish the earlier jeeps where more fuel efficient soley because i would love a black wrangler from any era my uncle took me to rauche creek he has a 14' lifted with 34's and the trails there where so fun.

My black TJ is on 31's and used solely for off roading, still returns 20MPG.