Fooling the Computer for Better MPG

[hollyxj]

just my .02

I've wondered why the 4.0 has such crappy gas mileage, my guess is just that it is an inefficient design and prolly loses gas mileage because of that. I have a f250 v10 with an open bed and tailgate in place on average i get about 13mpg. So a vehicle that is 2x the weight of an xj, prolly the same or worse drag, and has an additional 4 cyl but only gets 3-5 less mpg doesnt really make sense unless power is lost through inefficiency..at least that is the way I see it.

Just to throw in something about air/fuel ratio, what hasn't been discussed is the atomization of the fuel itself. I dont care what ratio you have, if the fuel is not atomized completely it is not going to burn completely. So if some one was really serious about trying this project, I think it would be safe to assume that an injector swap for the 4.0 would yield better results considering the 4.0 injectors have only a single spray nozzle compared to 4 on a gm counterpart.

 

[1bolt]

First off If you are scared away because none of those fuel management systems are marketed as fuel economy devices then I suspect you shouldn't be modifying your vehicle at all, you might very well damage it.

If you want to modify your air fuel ratio in a precise and predictable way you should seek out one of the many options already out there. I mentioned only a handful of them. One of which is FREE (Mega Squirt), and the APEXI can be had for 150 bucks off ebay and controls fuel trim via TPS. Please don't be put off by the name "Mega Squirt" this is not meant to imply that it will squirt crazy amounts of gas into your engine :shiver:.

If you manage to hard wire something to your o2 circuit that leans out your mixture be prepared to pay a MORE than the cost of one of these solutions; when you have pay your mechanic to replace pistons and valves. After you go up your first long incline. (hint this is where you want to watch your EGT gauge)

I say "your mechanic" because well... Again if you don't understand how a fuel management system can be used without the words "Fuel Economy" stamped all over it....

I suspect you guys are more interested in mental "excercise" than actual experimentation. :dunno:

 

[Ecomike]

just my .02

I've wondered why the 4.0 has such crappy gas mileage, my guess is just that it is an inefficient design and prolly loses gas mileage because of that. I have a f250 v10 with an open bed and tailgate in place on average i get about 13mpg. So a vehicle that is 2x the weight of an xj, prolly the same or worse drag, and has an additional 4 cyl but only gets 3-5 less mpg doesnt really make sense unless power is lost through inefficiency..at least that is the way I see it.

Just to throw in something about air/fuel ratio, what hasn't been discussed is the atomization of the fuel itself. I dont care what ratio you have, if the fuel is not atomized completely it is not going to burn completely. So if some one was really serious about trying this project, I think it would be safe to assume that an injector swap for the 4.0 would yield better results considering the 4.0 injectors have only a single spray nozzle compared to 4 on a gm counterpart.

The 4.0, stock, has reached 25 mpg highway. I would not call that crappy mileage. I have an 89 I bought recently that was getting 24 mpg highway before the head gasket lost its integrety between cyl #4 and #5. It's in my roundtoit projects todolist currently. My 87 4x4, 4.0 is currently geting 20 mpg highway.

For typical stop and go driving the problem is the gas consumed getting back up to speed, and the gas consumed sitting in traffic, and at lights, and compromises made for power, torque, and speed, ride, reliability.... in lieu of fuel economy.

 

[hollyxj]

The 4.0, stock, has reached 25 mpg highway.

I've heard this before and based on the numerous threads of ppl complaining about mpg this is an extremely rare bird. I have my reservations as to the validity that an xj could get 25 mpg but if you say so, so be it.

in my reality numbers are around 17-18 which coincides with epa estimates (http://www.fueleconomy.gov/feg/calculatorCompareSideBySide.jsp?column=1&id=14742)

 

[BBeach]

I've heard this before and based on the numerous threads of ppl complaining about mpg this is an extremely rare bird. I have my reservations as to the validity that an xj could get 25 mpg but if you say so, so be it.

in my reality numbers are around 17-18 which coincides with epa estimates (http://www.fueleconomy.gov/feg/calculatorCompareSideBySide.jsp?column=1&id=14742)

I can get 22-23mpg with my near stock 98 auto with 3.55's and 235's. My scangauge says so at least, and when i did a fill/refill test and compared the fuel used to the fuel filled, it was within 1-2% IIRC. Im pretty sure with a standard and the right tire combo, maybe a tailwind (whatever it is from behind) and you could get 24-25 mpg.

 

[hollyxj]

I can get 22-23mpg with my near stock 98 auto with 3.55's and 235's. My scangauge says so at least, and when i did a fill/refill test and compared the fuel used to the fuel filled, it was within 1-2% IIRC. Im pretty sure with a standard and the right tire combo, maybe a tailwind (whatever it is from behind) and you could get 24-25 mpg.

I think you are missing my point these are 2 vehicles I own and drive. I drive each the same way and pretty much to the same destinations and yet my gas mileage only varies by 5 mpg at best.

 

[srimes]

I think you are missing my point these are 2 vehicles I own and drive. I drive each the same way and pretty much to the same destinations and yet my gas mileage only varies by 5 mpg at best.

Look at the percentage difference, it's much bigger than "5 mpg" sounds like to many people. The releationship isn't linear. Double the weight of the xj and the mileage won't drop by half.

 

[rapom]

I have a 98 Jeep Cherokee 4.0 4x4 5speed manual with the stock 3.07 gears.
I avg 24.5mpg in the summer driving like a granny and I live in a mainly flat area. In the winter I avg about 21 mpg. I keep my tires pumped up to about 35psi. and I keep the rpm's low shifting into 5th gear at 35 mph if I'm going to be cruising at that speed. Also I try and keep my highway speeds at around 60 which gives me a nice 28 to 30 mpg. depending on the wind.
I added an overhead console which is fairly accurate and that helps out a lot in fine tuning the best mileage you get. Best mileage is when the road is flat, no headwinds, and cruising in 5th gear at about 40 where I can get about 34 to 36mpg. Mileage is absolutly terrible getting up to speed. Just wanted to let you guys know that Jeeps can get better mileage but having a 5 speed manual seems to give you more control over how much the engine revs which helps out a lot over the automatics.

 

[hollyxj]

Look at the percentage difference, it's much bigger than "5 mpg" sounds like to many people. The releationship isn't linear. Double the weight of the xj and the mileage won't drop by half.

If you look at in terms of kinetic energy

A mass of 6800(f250) to reach a speed of 55mph requires 932315 j to reach that speed
A mass of 3500(xj) to reach a speed of 55mph requires 479868 j to reach that speed

The f250 requires almost twice the amount of energy to reach a speed of 55mph. Which brings me to my original point of the straight 6's overall efficiency. I'm even going to make the intuitive leap and say this is prolly why straight 6 are not used these days as much a v6 design is.

 

[srimes]

If you look at in terms of kinetic energy

A mass of 6800(f250) to reach a speed of 55mph requires 932315 j to reach that speed
A mass of 3500(xj) to reach a speed of 55mph requires 479868 j to reach that speed

The f250 requires almost twice the amount of energy to reach a speed of 55mph.

Not on this planet.

That statement doesn't take drag into account, and wind drag is responsible for the vast majority of the energy needed for highway travel, and it plays a significant role in city driving as well.

The energy needed to maintain speed usually is more important than the energy needed to get up to speed. Of course this depends on the type of driving.

 

[hollyxj]

Not on this planet.

That statement doesn't take drag into account, and wind drag is responsible for the vast majority of the energy needed for highway travel, and it plays a significant role in city driving as well.

So you believe that an f250 without a bed cover and the tailgate in place has less drag than an xj hmmmm btw the xj runs 235 at's (9.25) compared to 33x12.5 mt's on the f250 w 20" rims and 75% of my driving is city

 

[badron]

The A/F is fine maybe at bit rich then is needed for the best MPG on a real world XJ but not bad at all. Find a way to crank the timing without blowing holes in the pistons. Don't forget to set it back before getting your tailpipe sniffed because without a EGR your NOs will skyrocket too.
Maybe a addon knock sensor that works with a addon black box and the CPS to back off the timing a bit when ping is produced.

 

[BBeach]

If you look at in terms of kinetic energy

A mass of 6800(f250) to reach a speed of 55mph requires 932315 j to reach that speed
A mass of 3500(xj) to reach a speed of 55mph requires 479868 j to reach that speed

The f250 requires almost twice the amount of energy to reach a speed of 55mph. Which brings me to my original point of the straight 6's overall efficiency. I'm even going to make the intuitive leap and say this is prolly why straight 6 are not used these days as much a v6 design is.

Inline 6's are one of the best engine design out there in terms of balance and efficiency. Think of the cars they are in, most bmw's (m3's with the s54 go to 8000rpm), toyota supras (1000+hp), nissan skyline (rb series), so on and so forth. They usually are put in cars because of the length of the engine. Most diesel's are inline 6's as well, Cummins and all.Packaging plays a large role. Something like VW's VR6 is really cool because it reduces the length yet keeps a lot of the attributes of an inline 6. Do some research on engines before you start saying inline 6's aren't used because of some inefficiency.

 

[BBeach]

If you look at in terms of kinetic energy

A mass of 6800(f250) to reach a speed of 55mph requires 932315 j to reach that speed
A mass of 3500(xj) to reach a speed of 55mph requires 479868 j to reach that speed

The f250 requires almost twice the amount of energy to reach a speed of 55mph. Which brings me to my original point of the straight 6's overall efficiency. I'm even going to make the intuitive leap and say this is prolly why straight 6 are not used these days as much a v6 design is.

I didnt go through the numbers because its not worth my time, but like someone else said, you're neglecting a lot of other features. Bottom of the line, how fast do you drive in your jeep, what conditions, etc. I know my scangauge tells me 18-19 mpg when im doing 80mph. Aero has a large part to play in this. Look up drag in wikipedia, the power required to keep a vehicle in steady state is related to the cube of velocity. The force of drag is proportional to the square of velocity. Maybe you're just going too fast and getting lousy mileage. You gotta balance load when talking about mpg.

 

[BBeach]

And on top of all of this, how are you guys getting your mpg numbers? If it's the fill then refill method, make sure you're going to the same pump, same temps, same station, etc. (not positive on all of that but so ive heard plus from personal experiences i remember different stations stopping earlier than other when filling up). And if its overall mpg, my total isnt that great. Its lousy in the city so it ruins my average.

 

[Ecomike]

First off If you are scared away because none of those fuel management systems are marketed as fuel economy devices then I suspect you shouldn't be modifying your vehicle at all, you might very well damage it.

If you want to modify your air fuel ratio in a precise and predictable way you should seek out one of the many options already out there. I mentioned only a handful of them. One of which is FREE (Mega Squirt), and the APEXI can be had for 150 bucks off ebay and controls fuel trim via TPS. Please don't be put off by the name "Mega Squirt" this is not meant to imply that it will squirt crazy amounts of gas into your engine :shiver:.

If you manage to hard wire something to your o2 circuit that leans out your mixture be prepared to pay a MORE than the cost of one of these solutions; when you have pay your mechanic to replace pistons and valves. After you go up your first long incline. (hint this is where you want to watch your EGT gauge)

I say "your mechanic" because well... Again if you don't understand how a fuel management system can be used without the words "Fuel Economy" stamped all over it....

I suspect you guys are more interested in mental "excercise" than actual experimentation. :dunno:

Mega Squirt is not free. How did you get the idea it was free? I see some of the programing stuff is free, but not the hardware that it requires. Cheapest do it yourself unassembled ECU kit is $140 that I can see for the out of date kit. It is also a MAJOR ECU /PCM replacement that is not EPA or emissions compliant or legal for DD use over the US highways, and it is not just a sensor interface for "fooling the existing computer" to improve gas mileage so it is not really what the thread title asked for discussion about, but it is intersting stuff for any one who wants to start over from scratch and scrap the entire ECU/PCM system :shiver:. Frankly I use and need my DD reliabilty too much to jump off the start over from scratch experimental stage with a do it yourself ECU kit, especially since the Renix ECU tables and algorithms are a big unpublished mystery to everyone.

I found some APEX stuff, but not anything like you mentioned???? so where is this APEX sensor interfaxce that involves the TPS signal. This here horse is getting thirsty!!!!:dunno:

 

[Ecomike]

I've heard this before and based on the numerous threads of ppl complaining about mpg this is an extremely rare bird. I have my reservations as to the validity that an xj could get 25 mpg but if you say so, so be it.

in my reality numbers are around 17-18 which coincides with epa estimates (http://www.fueleconomy.gov/feg/calculatorCompareSideBySide.jsp?column=1&id=14742)

I see several XJ years in your link above where the EPA listed 21 mpg highway under the old method which was not the absolute max highway MPG, but a typical average highway mileage based on average highway conditions. It says higher MPGs are possible depending on drivers habits, speeds, acceleration and road conditions, etc. so 25 mpg is not a stretch. I started another thread here about 6 to 8 months ago as I prepared to buy a reported 24 mpg, AW4, 4.0 89 Renix XJ jeep, 2WD as I was skeptical too. But far too many members here posted to that thread admiting to have achieved 20 to 25 mpg highway mileage with a 4.0 L jeep. Some were manual trannies, some were one time long flat road runs out of town, tires varied some, etc, but the fact remained that far too many had achieved regular 20 mpg highway runs, and some had reached 24 to 25 mpg including the guy that started this thread. His mileage dropped from 24 to 20 mpg when he replaced the old O2 sensor with a new one!!!!!! Thus the reason fot this thread asking about tweaking and verify O2 sensor data. I suported his idea, theory, based on the small variations that can and do exist between identical parts, like O2 sensors that can cause small variations in performance.

My reality, currently is 14.5 mpg average (lots of stop and go traffic), and 20 mpg on a single sustained 60-65 mph highway trip of 70 miles, but that is nearly double what I was getting when I bought this jeep, and it is still stock! Multiple reports of stock Jeeps getting 24 to 25 mpg tells me I still need to look for undetected mpg robbing gremlins in my 87. Back in 98 I got to drive a 96 XJ jeep for about 2 years and it seemed to average 20 mpgs.

 

[Ecomike]

I have a 98 Jeep Cherokee 4.0 4x4 5speed manual with the stock 3.07 gears.
I avg 24.5mpg in the summer driving like a granny and I live in a mainly flat area. In the winter I avg about 21 mpg. I keep my tires pumped up to about 35psi. and I keep the rpm's low shifting into 5th gear at 35 mph if I'm going to be cruising at that speed. Also I try and keep my highway speeds at around 60 which gives me a nice 28 to 30 mpg. depending on the wind.
I added an overhead console which is fairly accurate and that helps out a lot in fine tuning the best mileage you get. Best mileage is when the road is flat, no headwinds, and cruising in 5th gear at about 40 where I can get about 34 to 36mpg. Mileage is absolutly terrible getting up to speed. Just wanted to let you guys know that Jeeps can get better mileage but having a 5 speed manual seems to give you more control over how much the engine revs which helps out a lot over the automatics.

Great info, thanks! I am a little surprised at the peak mileage being at about 40 mph, especially with 3.07 gears. You must be nearly idling (maybe 1400 rpm?) in 5th at 40 mph? I would have thought your peak mileage would be closer to 60 mph! What tires are you running?

 

[1bolt]

system :shiver:. Frankly I use and need my DD reliabilty too much to jump off the start over from scratch experimental stage with a do it yourself ECU kit, especially since the Renix ECU tables and algorithms are a big unpublished mystery to everyone.

I found some APEX stuff, but not anything like you mentioned???? so where is this APEX sensor interfaxce that involves the TPS signal. This here horse is getting thirsty!!!!:dunno:

Seriously are you purposfully being obtuse because you don't want to recognize that this goofy discussion was a waste of time? Because it was attempting to "invent" something that already exists? I know I know they are for "performance" and that scares you because it doesn't say "for fuel economy"... :doh:

(its Apexi with an I)


I hate to repeat myself but if you need a fuel management system that has big bright colorful "improves fuel economy" stickers on it, you're best off not playing with things that can blow your "must be reliable" daily driver up. hasta

Do you really not get that any one of these can lean the air/fuel mixture? That's what you wanted right? Or no?

Now you say you want a 50 state legal CARB certified device all of a sudden? Where the heck did you pull that out off? You guys are talking about soldering f***king resistors into the o2 circuit a few pages back!? Is that going to be Carb certified????

It's like talking to a wall, fuel management can lean your mixture... period end of story arty:

Oh yeah and before I lose it, Mike the EPA did not invent stoichiometry
stoichiometric combustion is *the theoretical ideal air fuel ratio at which the most complete combustion is made* (this is not necessarilly the most miles per gallon) that's what stoichiometric means; it has absolutely dick all to do with the EPA. Now please stop reading whatever wacko web site you got that EPA lunacy from.

 

[kennzz05]

I have a 98 Jeep Cherokee 4.0 4x4 5speed manual with the stock 3.07 gears.
I avg 24.5mpg in the summer driving like a granny and I live in a mainly flat area. In the winter I avg about 21 mpg. I keep my tires pumped up to about 35psi. and I keep the rpm's low shifting into 5th gear at 35 mph if I'm going to be cruising at that speed. Also I try and keep my highway speeds at around 60 which gives me a nice 28 to 30 mpg. depending on the wind.
I added an overhead console which is fairly accurate and that helps out a lot in fine tuning the best mileage you get. Best mileage is when the road is flat, no headwinds, and cruising in 5th gear at about 40 where I can get about 34 to 36mpg. Mileage is absolutly terrible getting up to speed. Just wanted to let you guys know that Jeeps can get better mileage but having a 5 speed manual seems to give you more control over how much the engine revs which helps out a lot over the automatics.

THE SEMI I DRIVE GETS 50+ MPG ON A SMALL DOWNGRADE AT 60 MPH WITH 80,000 LBS SO YOUR POINT IS...



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