Gas mileage improvements?

[Milford Cubicle II]

My dad still believes in putting in the "high test" (93 octane) gas every few fillups actually makes a difference. I tried to tell him that his car ('01 Lincoln LS) wants 89 octane and going from 87 to 93 every now and then is the worst thing he could do for mileage. Just run 89 all the time and the engine will not have to adapt back and forth!

If it's a V-8 then it definitely does need 91+

 

[srmitchell]

I just changed my 2 upstream 02 sensors in my 2001, I'll report back.
Otherwise it has-
4 hole injectors
Cold air
Cat back
4.56 gears
32's on alloy rims
And its only at 4 inches of lift.

I've been averaging 14, which SUCKS.

 

[kastein]

Jesus, I get better than that with my 96, driving it with a heavy hammer foot, stock injectors, stock intake, stock + kinked badly + caved in exhaust, 3.55s+AW4, 33s on steel rims, and 3.5/5 inches of lift. What the hell are you doing? Those O2 sensors will hopefully give you a bit of a boost.

(it's at 15-16mpg after correcting for tire diameter and my cheapass lack of a speedo gear swap)

 

[srmitchell]

I contribute it to terrible CA gas, and I believe its over geared. I rev too high on the freeway. (2450 at 65) Sh** sucks.

 

[Milford Cubicle II]

Is Cali up to 20% ETOH yet?

 

[kastein]

Wouldn't surprise me. I've been leaving mine in 3rd for the last few months though, I usually cruise at around 3000rpm.

PS, anyone who tells you the 4.0 can't handle high revs for a long period of time is smoking something. Don't be nervous at all about cruising at 3000 or 3500. I towed around 500 miles over the last weekend with the AW4 in second gear and at 4500 to 5000rpm, stock 3.55s and 33s. It's still running great, oil pressure is exactly what it was beforehand and no overheating.

 

[srmitchell]

Hmmm, whats your throttle position like to hold that mph?

Before i regeared from 3.55's, it took a lot of throttle to get it up to 65, but it seemed to have enough momentum to cruise at only about 25-30 percent throttle. Hills killed me though.

Now, It seems like I have to hold it practically at 50 percent to maintain 65. Maybe this is why my mpg's suck? It's like its reaching terminal velocity, and you have to use more gas to maintain 65.

 

[kastein]

With the trailer, probably 1/2 to 3/4 throttle to hold ~80 in 2nd gear.

Without the trailer? maybe 1/4 throttle to hold 80 in 3rd. Not really sure, but it cruises nicely. I'd have to put the scangauge back on it to find out exact numbers.

 

[Finaltheorem47]

Besides the obvious, swapping the fan out would likely be a noticeable gain. ... Except I love my mechanical fan since it sounds like an jet engine spinning up when I accelerate!

 

[WrenchedXJ]

10 Mpg in my 89 regardless of if its interstate or city. I baby the pedal, have dual e-fans, on 31s, stock 3.55 gearing, very free flowing exhaust, just changed plugs and filter. Running a full bottle of seafoam, 93, and octane booster right now to see if I can clear my injectors out. Thus far, I've been driving for 3 days, and the needle has barely moved. If this doesnt improve my mileage, I'll be swapping injectors. Just a thought...say if the previous owner decided to take a hammer drill to the inside of my cat to remove the internals(because he thought it was cool). Would that throw it off as well? I was planning already on a high-flow cat, just havent saved up the $$$ yet since trying to get everything else right....and bills.

 

[kastein]

10mpg? That is insanely low, how are you measuring it? Full tank to full tank volume difference + odometer? That's about the only reliable way there is.

Does it get up to operating temperature (200 degrees, give or take 10 depending on the season)?

I would bet on the O2 sensor or perhaps the CTS being at fault. Smell your exhaust, if it smells like gas you are running rich for some reason or another, then you just have to figure out why. Don't throw parts at it, do some measurements (I'm not really a RENIX expert, so I can't tell you what reading to expect from the CTS at operating temp) and figure out what parts to swap.

 

[yossarian19]

Correcting for my 31s, I'm averaging ~14 MPG after a full tune up, new cat & only on 2" of lift. It's all city driving though - I may "cruise" only 1 mile a day on the freeway, if that.
I'm thinking 4 hole injectors, CAI, etc when I don't have anything else to spend the money on...

 

[WrenchedXJ]

10mpg? That is insanely low, how are you measuring it? Full tank to full tank volume difference + odometer? That's about the only reliable way there is.

Does it get up to operating temperature (200 degrees, give or take 10 depending on the season)?

I would bet on the O2 sensor or perhaps the CTS being at fault. Smell your exhaust, if it smells like gas you are running rich for some reason or another, then you just have to figure out why. Don't throw parts at it, do some measurements (I'm not really a RENIX expert, so I can't tell you what reading to expect from the CTS at operating temp) and figure out what parts to swap.

Measuring at fill up. Fill it full, run it out, divide miles by gallons put in at next fill up. It runs 210 year round. exhaust is indeed rich. I believe I will start by following EPA rules, and putting a new cat on it. From there work to the O2 sensors. The injectors i'll probably just take out and clean, 232k original miles.

 

[kastein]

I would actually not put a catalytic on it until you whip the running rich issue. Call me a lawbreaker, but what running that rich will do to a cat will make it worthless, so it is better to put the cat on after fixing the other problems so you get more life out of it. It'll produce WAY too much heat and can actually melt the cat core and other things. I do run cats on all my rigs, but they don't run that rich.

You only have one O2 sensor on a RENIX rig, so at least you have that going for you. Buy yourself a cheapish multimeter and check out Ecomike's RENIX thread, I forget the title of it but he knows his stuff about fixing those beasts. You should be able to figure out what sensor(s) are bad and replace only those instead of replacing things till it works, possibly wasting money, though at 232k you are probably due for an O2 sensor anyways.

 

[DaYeep]

(it's at 15-16mpg after correcting for tire diameter and my cheapass lack of a speedo gear swap)

Did the speedo gear swap help you any as far as mileage? Have yet to do this swap...wondering if it would help?

I have changed plugs, wires, cap, camshaft sensor, crankshaft sensor, TBI sensor, synthetic fluids, cold air intake, O2 sensors, good injectors, name it...except the mech. fan and the cat back (cat not clogged). I have a 6.5" lift, 33" with 3.55 gears. I use additives regularly. I take it easy on the skinny pedal, and consistently get 10.5mpg (and yes, I know how to calculate it). For those of you who swear by a stock Jeep, good for you...I rock crawl several times a month and DD the rest of the time (my commute is 2 miles each way). If this is the best I can do, I accept it...LOVE my XJ...but dream of going over 170 miles on a tank! Don't intend to regear, as I plan on doing a full axle swap and don't want to waste my money on D30 & D35 upgrades. Ideas? Thanks!

 

[kastein]

Did the speedo gear swap help you any as far as mileage? Have yet to do this swap...wondering if it would help?

nope... the speedo gear just tells the speedo/odo what speed you are going, nothing more. What I meant by that is that I calculate my mileage based on what the odo says and how much I put in the tank to fill it back up, then divide by stock tire diameter (28) and multiply by my tire diameter (33) to get actual miles traveled. Haven't verified the distance provided by this method against GPS because quite frankly I don't care that much, it's not my DD (anymore) so I don't really care as much how much gas it uses.

 

[DaYeep]

nope... the speedo gear just tells the speedo/odo what speed you are going, nothing more. What I meant by that is that I calculate my mileage based on what the odo says and how much I put in the tank to fill it back up, then divide by stock tire diameter (28) and multiply by my tire diameter (33) to get actual miles traveled. Haven't verified the distance provided by this method against GPS because quite frankly I don't care that much, it's not my DD (anymore) so I don't really care as much how much gas it uses.

Yeah, well I'm there with you on not really caring....it's my daily driver and I would enjoy figuring out a way to get better mileage, but is not that big of a deal to me either.

 

[CharlesS]

The factory original fuel injectors are showing signs of leak by and possible heat soak once the engine is shut off.
Decided it is time to replace the fuel injectors.
There is presently a sale on most of the Precision fuel injectors so I purchased a set of the 4 hole fuel injectors for my '98. The sale price is $119 which knocks $70 off their normal price.
Once I install them I will post the improvements.

 

[redrider2911]

'90, 4.0, AW4, NP231
7" lift w/ 35s
Stock 3.55 gears (i know, i know. Im waiting for the money to build the DD)

I had been getting 7.8 MPG driving around town since I bought the jeep and threw it together. It had been sitting for 8 years. Last week I fixed the exhaust manifold leaf, replaced cap, rotor, and plugs, then found some orange ford injectors out of a t bird with a 4.6. 4 Hole injectors. Now Im averaging about 13 MPG. Not a bad improvement but I still need to check that brake caliper.

 

[qbert2500]

Take out the spare tire and jack (have AAA road service or someone to bring you these when needed). Fill gas up half tank at a time.