Bad Gas Mileage

[Knucklehead97]

Just replaced plugs and wires with Champion Coppers and Autolite wires. I also got a cap and rotor but was scared to because I'm a little questionable on how to.. any tips? Now that the plugs and wires are changed, my idle is higher, why? Also I brought the plugs to autozone and they said they looked like my engine is running fine. I compared them to the chart and it seems so. But still I feel that I am running rich... I got a fuel injector cleaner that I'm gonna add next fill up.

[Oyaji]

I never replace cap and rotor unless I have a misfire problem that demands their replacement - I am cheap that way. :) If you are going to replace them anyway, just make sure they are properly indexed and you'll have no problem. I do this by starting with a known direction of error in orientation, then rotating them in the direction of their indexing notches so I feel them drop into place (this works for both cap and rotor).
.
Going by what the plugs tell you, your engine is not running rich. That means you need to look elsewhere for improvements. Below follow the areas for improvement I can think of just off the top of my head (there may be more I can't think of at the moment).
.
If you have a significant fuel leak, it should show up when the engine is running. With it safely parked, start the engine and trace the fuel lines fom tank to engine, looking for any drip, wet spot, and/or odor of gasoline. It should not take long - any significant fuel leak should be obvious.
.
Next is to consider the things that add to the load that the power of your engine must overcome to move the truck. You stepped up to larger tires without changing your final drive ratio. That puts your engine at slower RPM while it is still expected to provide the same power at a given speed. Practically speaking, that is not significant until you are driving at speeds over 35 (really noticeable at 55+ MPH). The reason is that aerodynamic drag is proportional to the square of the speed. Here is an example: if it takes 4 horsepower to counter aero-drag at 25 MPH, then it would take 4 * 4 = 16 horsepower at 50 MPH... and 16 * 16 = 256 horsepower to sustain 100 MPH. Your engine was designed in the era of the "double nickel" - the 55 MPH speed limit. That means the "sweet spot" for best economy in top gear will be around that speed - but because you changed your tire size, your overall gearing has changed. In order to get back into that sweet spot, you need to re-gear. That would mean stepping up to a numerically higher ratio final drive. Simple proportions will let you calculate your need: assuming stock tire diameter is 27", current tire diameter at 30", and current final drive at 3.07, let X = desired final drive ratio. Then 27 / 30 = 3.07 / X. Solving for X yields 3.41. The nearest available ratio to 3.41 is 3.54 - a little high, but much closer than 3.07, so it should net you an improvement.
.
Since aerodynamic drag is the biggest consumer of energy you can do anything about, it makes sense to start there in any attempts to improve fuel economy. There are 2 ways to to this: clean up the coefficient of drag for your vehicle, or reduce your driving speed. If fuel economy is important to you, think twice about adding things to your Jeep that increase aero-drag: lift kits, roll bar, lights on the roll bar, extended fender flares, bigger side mirrors - anything that increases the frontal area of your vehicle will add to aero-drag. If you must have these things, keeping your speed down to 50-55 MPH will help your economy a lot.
.
After aero-drag, look at frictional drag. This includes your mud grip tires that have significantly higher rolling resistance than street tread - if you can get by without them, this will save you gas. Wheel alignment can have a significant effect on frictional drag - if you haven't had it checked, maybe you ought to. Use of synthetic lubricants can  significantly reduce friction  because they are *supposed* to provide equivalent lubrication with less viscosity. (Less viscosity = less friction.)  They are available for the whole drivetrain from engine to transmission to transfer case to final drive... but better check for verification from people with practical experience with your vehicle type before using them: I have heard stories of failures blamed on the indiscriminate use of synthetics, so caveat emptor. If you live in a state that allows it, removal of the catalytic converter will net you at least 1 MPG by reducing exhaust pumping losses; otherwise, a new one and other low-restriction exhaust modifications will help to some (greater or lesser) degree. Probably the best tip in this area (and certainly the cheapest) is to inflate your tires to the maximum, and check their pressure at least weekly. It may ride rougher, but your fuel economy will improve.
.
Cheapest and best overall tip for last: if fuel economy is really important to you, then work continually on improving your driving habits for best economy. It requires attention, but that attention will not only yield better miles per gallon, it will also extend the life of your vehicle - and maybe by improving your situational awareness will make you a safer driver in the bargain.

[HOrnbrod]

Just replaced plugs and wires with Champion Coppers and Autolite wires. I also got a cap and rotor but was scared to because I'm a little questionable on how to.. any tips? Now that the plugs and wires are changed, my idle is higher, why? Also I brought the plugs to autozone and they said they looked like my engine is running fine. I compared them to the chart and it seems so. But still I feel that I am running rich... I got a fuel injector cleaner that I'm gonna add next fill up.

 

Why do you "feel" like you are running rich? Your plugs say no. You have no clue as to what your actual MPG is, you have no baseline established because your fuel gauge is fubar, your speedo gear is not correct, and you have not checked the odo and speedometer against a calibrated known standard like GPS or even a local cop. Until you know what you have, the parts you are randomly throwing in mean nothing until you can verify an improvement based an accurate baseline.

[Knucklehead97]

It just has a smell when the trucks running. Other than burnt oil, that is. I already removed my cat convertor and I'm considering just straight piping completely. And I really didn't expect the 30" tires to change that much, but when talking about gearing it does make sense. The PO had rear tires already on it that were brand new, so I just bought to match. These trucks sure did come with small tires... even the 30" ones look to small on stock to me. And my driving habits are honestly terrible. Considering I'm new to manuals, I hsve to give it a good bit of gas starting up hills.

[Oyaji]

With regard to driving habits, you should  maybe have a read over some of the hyper-miling guides available on the Internet. The main thing to recognize and compensate for is that every time you accelerate, you burn extra gas to get your vehicle up to speed. That means that every time you slow down, you need to burn more gas  later to recover  the speed you lost to braking. Learn to read traffic and stop lights far ahead of you and slow down only when you must, and  by conserving your momentum you will save gas. Accelerating slowly instead of doing "jack rabbit" acceleration will also save fuel...

.

I learned good economy driving habits during the oil crises of the 1970s, when gas prices tripled or quadrupled  over a week or so. Two good tips I remember from those days were to "avoid the brake pedal as if it were a land mine" (exact quote!   :D ) and to drive  with shoes off for better feel  and finer control of the accelerator pedal. Even when avoiding the brakes as much as possible, changes in speed by indiscriminate use of the throttle will still cost you gas (remember - lifting off the throttle slows you down, just like using the brakes does except you  are using "engine braking" instead). Pretend you have an egg  you are trying not to break between your right foot and the accelerator pedal, and this problem largely goes away.

[Knucklehead97]

I'll practice my driving habits and everything :D and I'mrunning a sensor test sometime, checking my spark plugs again to just BE SURE, and I'm checking my fuel lines and the other vacuum lines/thevacuum bottle. My truck was in a smal frontal collision before I owned it. I think the vacuum bottle might have been cracked or something

[Knucklehead97]

Just checked my vacuum bottle. Alot of lines seem to be missing.

[Rockfrog]

there should only be one going to the vacuum bottle (black hose), then it T's off at the heater core (purple hose). Although, if you have cruise control there will be another T at the Servo close to the battery.

 

On the passenger side there will also be the EVAP lines but they are larger an unmistakeable (3/8" plastic snaking behind the motor).

 

There should only be the one 3/16" line (the black one) running behind the motor to the first vacuum tree on the manifold. The rest of the lines, With the exception of the MAP line are contained in the vacuum harnesses mentioned earlier. They pertain mainly with the EGR system (valve, transducer and solenoid) really (although the forward end of the CCV is in the two main harnesses). The MAP, FPR, and CCV orifice line are separate system lines. It's actually a fairly simple vacuum system for the time period.

 

 

I'm going off memory, I'm a loooooong way away from my MJ right now, might have missed a line.

[carnuck]

The breather in the valve cover can get clogged from blowby (previous motor) Most shops don't clean the gunk out, slap the cover back on and oil gets forced out of the new engine. People get mad and sell them because the mechanics are idiots.

[Knucklehead97]

there should only be one going to the vacuum bottle (black hose), then it T's off at the heater core (purple hose). Although, if you have cruise control there will be another T at the Servo close to the battery. On the passenger side there will also be the EVAP lines but they are larger an unmistakeable (3/8" plastic snaking behind the motor). There should only be the one 3/16" line (the black one) running behind the motor to the first vacuum tree on the manifold. The rest of the lines, With the exception of the MAP line are contained in the vacuum harnesses mentioned earlier. They pertain mainly with the EGR system (valve, transducer and solenoid) really (although the forward end of the CCV is in the two main harnesses). The MAP, FPR, and CCV orifice line are separate system lines. It's actually a fairly simple vacuum system for the time period.  I'm going off memory, I'm a loooooong way away from my MJ right now, might have missed a line.

coming from thevacuum bottle in the passenger side of the front bumper; I have it seems 4 or 5 lines. Three of which are connected. One smalll one that I have NO idea where it goes. And theres a decent sized hole I'm the canister that looks like theres supposed to be a line going to it. Either way, somethings missing

[Knucklehead97]

Since I couldn't get a GPS app on my phome and don't know anyone with a gps. I set a trail from my house to my sisters on mapquest and it said it's 6.11 miles from my house to hers. In my truck it said almost exactly 6. So at most I'm off by .2 miles. I'm working on figuring out this vacuum bottle. And testing all the sensors. I'm going to order a throttle body gasket and take it off and clean it when I can. If anyone had a vacuum bottle diagram to help with where the extra hoses go, that'd be great.

 

Thanks

[Rockfrog]

Does the vacuum harness for the front diff run to the bottle? that would explain the extra lines ... I have a 2wd so I wouldn't know on that one.

My vacuum system is down to the just "needs to be there" now (so CCV, MAP and FPR). Last two trips were 26.4 mpg (493km using 52.6L) and 25.4 mpg (471km using 52.1L) I ran it like a raped ape a little for the last one. With 4wd you likely won't get that high but it should be in the 24mpg range on the high side I would imagine.
Still have the stock IAC, MAP, CTS and TPS but the O2 sensor was replaced 2 months ago. Cleaned the snot out of the IAC and throttle body a month ago, and ran a bottle of Seafoam through one of the EGR hoses (half by vacuum pull and the other half in the tank), used to have a sticky IAC but with regular driving that issue is slowly going away.

This is what you should see to the vacuum bottle ... with a second main line coming from the heater core to the heater control valve in the heater lines (that's the purple one I mentioned)


But looking at this diagram ... it looks like, as with all things Jeep ... there were a few options. although looking at them, they are each just more complicated versions of the simple ball style system (I'm guessing you may have the one pictured far right ... which looks to feed into just two lines outgoing really).
 

I moved my reservoir to the corner between 3 and 5 and deleted everything past it. I'll go snap a picture of it now with it cleaned up better.


the third bottom option looks like the greyed out harness is the vacuum shift motor harness for the front axle CAD.

[Oyaji]

Since I couldn't get a GPS app on my phome and don't know anyone with a gps. I set a trail from my house to my sisters on mapquest and it said it's 6.11 miles from my house to hers. In my truck it said almost exactly 6. So at most I'm off by .2 miles. I'm working on figuring out this vacuum bottle. And testing all the sensors. I'm going to order a throttle body gasket and take it off and clean it when I can. If anyone had a vacuum bottle diagram to help with where the extra hoses go, that'd be great.

 

Thanks

I don't think I would  place complete faith in Mapquest (or any other map, whether on the Internet or otherwise).

.

Just sayin'...

[Knucklehead97]

The second option looks closest to mine. I checked more into the big hole with no line going to it, there was a connectector for the small line that wasn't going anywhere. But the end was broken off in the small line. The small lines seem to run to places where one turns into a blue line and connects with some very colorful red, yellow, and green lines andthen runs down to the front diff CAD it seems. The other line runs up and goes into the firewal it seems with a white line. And the white line goes to a vacuum motor thing (i'm guessing?) Which is hanging on one of those massive vacuum lines going towards the front of the engine. That second line, i have no idea what its running to which is why I said vacuum motor looking thingy.

[Knucklehead97]

I'm going to double check the miles whe I can find a gps or something

[Knucklehead97]

Tested almost all the sensors today, finally. Except for the CTS and EGR valve. Everything passed good except for the o2 sensor. It failed drastically. Didn't increse or decrease from 4.814 almost ever. It maybe moved a click every 3 minutes. So I picked one up and am installimg tomorrow/seeing what my gas milweage is. Hopefully it'll be decently better. Thanks guys.