fuel economy

I've search, but found precious little about improving economy (well theres that browns gas thread)

I have a 'new to me' 1995 sport. 4.0, auto and the 231 with 192,000 miles. It is lifted 3" running 31" MTs

This is my 3rd tankfull. And I've driven less than 600 miles. My last tank I got 164 miles before filling up, or 12.4 MPG even factoring in the 7% larger tires, I got 13.2mpg I've got friends with suburbans that get better gas mileage...

I drive with a gentle right foot, even if it annoys the people behind me. I rarely go over 2500 RPM

I realize I'm not driving an little import rice burner. I can accept that. but the CJ7 I sold running 32s with a 4.2 and an oversized weber carb got 16-18mpg, I dont understand...
It runs well, and has plenty of power. Idles nice, and other than filling it up, I love it. I've installed new plugs (the old ones showed it ran really rich), new plug wires, a new O2 sensor, and a new air filter. It now has a rustys catback with the flowmaster 50 on it. In less than the week since installing it, the inside of the exhaust is black. Tomorrow, I'm going to pull the two week old plugs and take a look. I also have a new fuel filter I'm going to put in tomorrow

The last tank I put a bottle of LUCAS fuel injector cleaner in it, Today, I did it again (GUMOUT) with the fill up this morning. Both of these tanks I'm running regular 87, not the 10% ethanol blend. (but thats what I ran in my CJ)

on a maybe unrelated note, I'm going to change the diff fluid this weekend, (the PO didnt have the breather tube hooked up). Something else I've noticed, (and I dont know if it matters) it wants to drive at idle. (I have to hold the brake at a stopsign) to me it seems to always be in too low of a gear, most of my driving is around town, (I spend 4 miles on the interstate at 60mph, then I'm downtown, 25-45mph) The engine gets up to temp, running 200 or so by the time I'm downtown.

Is there anything else I can do? or am I just out of luck?

You searched eh?

Scroll to the bottom of this page and look under "similar threads"...

http://www.naxja.org/forum/search.php?searchid=1978478

http://www.naxja.org/forum/showthread.php?t=64425&highlight=gas+mileage

use advanced search and search titles only for "Gas mileage" "fuel mileage" "fuel economy" the list goes on and on and on....eleventy billion threads out there.

sounds like you are idling high - might want to check/clean/replace your idle air control valve.

Check/replace Mass Air Sensor...outside shot that might help

What kind of O2 sensor did you get? Certain brands tend to be crappy - my guess thats what was making you run rich. Like you said, check the plugs again...

Switch to some new injectors. Ford Mustang ones are a popular choice - there are some recent threads on that too. Leaky injectors could be making you run rich.

Run some Seafoam thru your intake manifold - either thru the throttle body or suck it in thru one of the vac lines.

Do a good engine flush. Lots of methods - run some seafoam in the crankcase for 50-75 miles, then do an oil change. ATF works too (drain a quart or so out, replace with a quart of ATF - I think its Type F thats the really high detergent stuff), use a BG engine flush product (http://www.bgprod.com/products/engineoil.html), Auto-Rx...Any of those could help a little.

Make sure your tires are properly inflated. Sounds dumb, but that can contribute a lot. Same with an alignment.

Changing your diff fluid could help too, especially if its REALLY old and sludgy.

Any of these could only help .5 to 1 mpg each...but combined, it could get you back closer to 18-20mpg.

EDIT - one more thing - check your exhaust manifold for cracks. if you have a big enough crack, it could be throwing off your O2 sensor readings enough to cause you to run rich....I've heard of that happening too....

Before you try the items listed below, replace the Throttle Position Sensor and clean and oil the Idle Air Control Valve (IAC), also known as the Idle Air Stepper motor, both are on the throttle body. I chased a phantom mileage ghost in mine for 2 years, nearly ran out of things to replace, then I finally found a problem with my TPS, not listed in the FSM, and so I installed a new TPS and it increased my gas mileage by 50%. Granted mine is an 87 Renix, but a biased (defective ) TPS can seem to operate mostly OK (except for the high idle which you have ) , and still rob you of mileage by making the AW4 tranny shift too soon, and it underpresures the tranny fluid pump and torque converter as best as I could tell.

If those two, a new TPS and cleaning and oiling the IAC do not do the trick, then try cleaning the mass air flow sensor with an aerosol solvent which is called Mass Air Flow sensor cleaner (do not use any other solvents) to see if some dirt on it has biased the sensor reading and caused your mileage problem. IF the MAF sensor was totally bad it would barely run so don't go waisting money a new one, yet. Also need to make sure the O2 sensor wiring is good. There are some very good threads here on testing the live, working O2 sensor with a volt meter that I wrote.

Is the cap and rotor new?

I have replaced nearly everything on my Jeep, 87 4.0, and the mileage doubled on mine from 10 mpg to 20 mpg (highway) only after fixing the bad O2 wiring the bad TPS ground wiring and replacing the TPS and O2 sensor. All the other fixes I did made it run and start a lot better like new fuel injectors,...etc., but none of the other parts had much effect on the fuel economy.

And X2 on checking the manifolds for leaks!!!!!! They can cause high idles and throw off the O2 sensor and kill mileage in the process. Also try coasting in gear to make sure there is no brake drag or bad bearings killing the mileage.

JNickel101 said:

sounds like you are idling high - might want to check/clean/replace your idle air control valve.

Check/replace Mass Air Sensor...outside shot that might help

What kind of O2 sensor did you get? Certain brands tend to be crappy - my guess thats what was making you run rich. Like you said, check the plugs again...

Switch to some new injectors. Ford Mustang ones are a popular choice - there are some recent threads on that too. Leaky injectors could be making you run rich.

Run some Seafoam thru your intake manifold - either thru the throttle body or suck it in thru one of the vac lines.

Do a good engine flush. Lots of methods - run some seafoam in the crankcase for 50-75 miles, then do an oil change. ATF works too (drain a quart or so out, replace with a quart of ATF - I think its Type F thats the really high detergent stuff), use a BG engine flush product (http://www.bgprod.com/products/engineoil.html), Auto-Rx...Any of those could help a little.

Make sure your tires are properly inflated. Sounds dumb, but that can contribute a lot. Same with an alignment.

Changing your diff fluid could help too, especially if its REALLY old and sludgy.

Any of these could only help .5 to 1 mpg each...but combined, it could get you back closer to 18-20mpg.

EDIT - one more thing - check your exhaust manifold for cracks. if you have a big enough crack, it could be throwing off your O2 sensor readings enough to cause you to run rich....I've heard of that happening too....

Click to expand...