Low Gas Mileage

[tugalo]

I have an 88 Pioneer with about 5000 miles on a new engine, and my gas mileage is only around 15-16 mpg.  Altho I do mostly short trips, I think it should be higher.

 

I have checked all the sensors and their grounds, looked in vain for any vacuum leaks, and cleaned the throttle body.

 

Would a set of injectors with a lower flow rate (maybe 17 lbs.) help, or should i maybe try Cruisers trick of advancing the timing by moving the CPS?

 

Any ideas appreciated.....

[FrankTheDog]

Which drive train do you have?

[Oyaji]

The biggest thing you can do to improve economy is to tighten up your driving habits with an eye toward achieving better economy.

.

Beyond that there a number of tweaks you can do for improvements. Running tire pressure at the maximum reduces rolling resistance. Making sure of your wheel alignment not only potentially reduces drag but also extends tire life. Use of synthetic lubricants throughout your drive train reduces frictional losses by way of lower viscosity while retaining lubrication value (best check the experience of other owners on this for specifics regarding your application though).

.

Good maintenance of your vehicle counts for a lot. When I bought my 92 4x4/auto XJ, for example, the first thing I did was to go fill it up and start a logbook. My baseline highway fuel economy was only 14.5 MPG, so I went hunting for faults. First up were the plugs: not only were they terribly worn but 2 of them had broken center electrode insulators. Replacing them improved economy by 1.5 MPG. Next was removal of the catalytic converter... +2 MPG (if disallowed in your locale, replacing a plugged cat with a free-flowing metal substrate type would yield only slightly lower results). Wheel alignment gave a plus of 0.5 MPG, balancing the tires and running them at max pressure gave 1 more MPG. Tracking down a vibration that wasn't tires turned out to be a bad left front axle U-joint, that yielded another 0.5 MPG. There was something else I am forgetting, but ultimately I went from a baseline of 14.5 MPG (3 checks in a row from fill-up to fill-up, logged miles divided by gallons consumed) to a consistent 20.5 mighway miles per gallon over 115,000 miles. Dropping my cruise speed from 70 MPH to 55 bumped up economy to 23.5 MPG, too.

.

Adopting Cruiser's idea of advancing the timing may be a good one (should ask him for his results), but swapping injectors to some with a lower flow rate is not a good one. Doing so would just cause the ECU to compensate by widening the pulse width and deliver the same fuel consumption under closed-loop running (using oxygen sensor feedback), and under heavy load demand under open-loop running conditions (near wide-open throttle [WOT]) would not deliver sufficient fuel to maintain proper air/fuel mixture and might damage your engine.

.

If you are interested in pursuing fuel economy, I suggest hopping over to the ecomodder.com forum - there is a wealth of information to be had from like-minded people there, some of whom are quite expert on the subject.

.

Your fuel economy of 15-16 MPG is not really poor unless you drive like a grandpa already. I do, and I get 19.5 MPG under combined city/highway driving... but I do take a perverse pleasure in holding up traffic in the interest of maximizing fuel economy - especially  when I keep passing the guys who are stopped at the next signal but who roared past me at WOT when leaving the previous one. I find I arrive at my destination at just about the same time as the guys who "drag-race" between traffic signals, but I do it on half the fuel.    ;)

.

PS - Frank's question above about drive train details is one that needs to be answered in order to provide you with more complete advice... and you should provide tire size and type as well.

[Jeep Driver]

We've had about a dozen 4.0s in the last few years. Our last keeper got about same as yours, 15-16 and our current keeper gets around 22-23.

I drove one back from FL that was averaging 24-25.

 

I have no exlanation for the varying rates of milage, all have been good runners.

 

The only thing that I have noticed is a possible difference in auto trans, some seem to lag while others seem 'freed-up'.

[cruiser54]

I have an 88 Pioneer with about 5000 miles on a new engine, and my gas mileage is only around 15-16 mpg.  Altho I do mostly short trips, I think it should be higher.

 

I have checked all the sensors and their grounds, looked in vain for any vacuum leaks, and cleaned the throttle body.

 

Would a set of injectors with a lower flow rate (maybe 17 lbs.) help, or should i maybe try Cruisers trick of advancing the timing by moving the CPS?

 

Any ideas appreciated.....

Do you have the original Renix type injectors? 

 

Ever replaced the oxygen sensor? Does it idle good? 

[tugalo]

fuel injectors are Ford 4 holers, and the O2 sensor has been replaced with an aftermarket unit.  i have the Peugout tranny and i run 235-74-15"s.  Since I'm closing in on 72, i do drive like a grandpa....

[Oyaji]

Links to topics at the ecomodder.com fuel economy site I suggested you visit:

 

65+ Efficiency Mods - http://ecomodder.com/forum/fuel-economy-mpg-modifications.php

 

100+ Hypermiling Tips - http://ecomodder.com/forum/EM-hypermiling-driving-tips-ecodriving.php

[Oyaji]

The tires you have will hurt a little on economy, as the will nudge the engine a bit to the slow side of the "sweet spot" for highway cruising. I'd suggest 215/75R15s.

 

Also, have you "read" your spark plugs? They tell a pretty good story of the running condition of your engine. Ideally the insulators should be a light grey or tan.

 

[92tanMJ]

The tires you have will hurt a little on economy, as the will nudge the engine a bit to the slow side of the "sweet spot" for highway cruising. I'd suggest 215/75R15s.

 

That is not true. They will hurt a bit in town but not on the highway. With his tires, assuming he has 3.07 gears and doing 65, he will be right at 2395 RPM. And 2300 is about the sweet spot for I6. Now you can have tires that are too big for the gearing, like on my comanche. I was 3.07 and 31's. Doing 65 I run at about 2100, but I do 75 (speed limit is 70 in OH and KY) so it bumps me back up the 2300 RPM range and I get about 20 MPG. In my cherokee with 3.55, aw4, auto, I do about 70 and get about 22 mpg.

Clicky clicky for tire size vs gearing to give you, your rpms...http://www.offroaders.com/tech/gear-ratio-chart.htm

Clicky clicky for an indebth look at it http://www.grimmjeeper.com/gears.html

 

And as his tires are basically 1 inch larger than the ones you suggest, the amount he would save in town would not off set the cost of a new set of tires. And as being that he is 71 a taller truck is, and will be easier to get in and out off.....there is only .5 of a hight difference in the two but it still helps some. For example, how a standard toliet is 15" off the floor and a handicap toliet is 17". Those 2" make a huge difference, even though two inches is very small but I digress.

 

An other way you could get better mpg is doing the adjustable MAP sensor mod: (write ups- ordered best to eh, IMO)

http://jeep-xj.info/HowtoMAPsensor.htm

http://www.cherokeeforum.com/f2/diy-map-sensor-voltage-aduster-53381/

http://www.angelfire.com/my/fan/MAP_adjuster.html

[JeepcoMJ]

To jump immediately to the conclusion that it is driving habits, especially when this is a TECH QUESTION....really?

 

 

check the basics.    make sure timing is correct.

 

15-16 in the city is about right for a good running 4.0, but you should be able to get 19-22 easy on the highway.

 

those mustang 5.0 injectors are GOOD injectors.   They have a multi-spray pattern that atomizes fuel better.   The problem with that...you also need to increase airflow.

 

a bored throttle body would help.   Throttle body spacers DO NOT and cannot help on a MPFI engine, because the fuel is sprayed after the spacer.

 

 

The problem with the 5.0 injectors is that they have a higher pressure rating.   They are flowing more fuel than stock, and you haven't done enough to increase airflow to compensate for the higher amount of fuel...resulting in more unspent fuel.

 

 

Upgrades for airflow.

 

Well...a true cold air intake would help, but throwing on a K&N filter pack will NOT because the filters allow more particles to pass through, and are not cold air without the proper, sealed from engine bay, shielding.

 

mid 99+ intake manifold.   Great mod.   I doubt it adds 15hp/30ft/lbs torque like they say, but it does massively improve intake airflow with the curved runner design.   You would have to buy or make an adapter for the renix throttle body, which would negate any gains, as well as port match it to your head.    The best way is to use the H.O. throttle body, intake elbow, and airbox.   You can solder in the TPS wires, or buy/make a TPS adapter to bolt the renix TPS it to the H.O. throttle body.

 

Exhaust.    Not enough can be said.   Opening up the header will help...a lot.   Going much past 2.25" exhaust, though, won't.  Going too big negates back-pressure, and costs you venturi effect that helps more or less siphon exhaust through rather than the engine having to lumber and push the entire amount with no help.

 

I've ran a number of headers on mine.  APN seems to be the best.   Just moving the collectors downward with a smoother (and wide open) radius on the bends helps immensely.

 

Cams.

Not enough can be said about the proper camshaft.    Everyone forgets about it with these 4.0's.

There's a HUGE power loss in the factory cams.   They are set to have HP and TQ meet at 2200-2400RPM.    How often does the motor stay at that RPM once you attain desired speed?

 

A "torquer" series cam will drop that range.   I've always used Crane or Comp cams, torquer series, designed to drop that range down to 1700-1900RPM.   This is where I've found the needle sitting at speed.

 

Doing a lot of these mods will help.   I got 19mpg mixed driving out of my 7" lifted 87 MJ with H.O. built 4.0 that had a torquer cam, 99 intake, borla header, mustang 5.0 injectors, 62mm bored TB, adjustable map (also a good idea), aw4 auto on 33x10.50 and 35x12.50 mud terrains with MT axles and 4.10 gears.   Got 20-23mpg on highway.   Same motor put in my 99 5 speed XJ averaged me 20-25mpg depending.

 

 

 

Rather than throwing other parts at it to allow you to stick with the mustang injectors, though...put a good clean set of stock injectors back in, and see how it works. 

[Oyaji]

You guys do have a point about your experience with engine speed and economy. I guess I should point out that the factory engineers designed the entire powertrain to return best economy to beat the EPA test back when the speed limit was 55 MPH. They certainly did the best job they could and no individual can match the R&D budget of the entire corporation they had behind them, but still, as they were fond of saying (and still do in their disclaimers), "your mileage may vary"! :D  Doubtless driving a vehicle under the conditions it was designed for will yield the best results.

 

As for some of the other stuff, well... maybe more on that later.

[JeepcoMJ]

They actually design everything to pass EPA standards with best economy based off of the limitations and restrictions they were tied to.

 

 

Which is NOT to say that they were ever designed for the best fuel economy.   It actually states quite the contrary.

[92tanMJ]

To jump immediately to the conclusion that it is driving habits, especially when this is a TECH QUESTION....really?

 

 

check the basics.    make sure timing is correct.

 

15-16 in the city is about right for a good running 4.0, but you should be able to get 19-22 easy on the highway.

 

those mustang 5.0 injectors are GOOD injectors.   They have a multi-spray pattern that atomizes fuel better.   The problem with that...you also need to increase airflow.

 

a bored throttle body would help.   Throttle body spacers DO NOT and cannot help on a MPFI engine, because the fuel is sprayed after the spacer.

 

 

The problem with the 5.0 injectors is that they have a higher pressure rating.   They are flowing more fuel than stock, and you haven't done enough to increase airflow to compensate for the higher amount of fuel...resulting in more unspent fuel.

 

 

Upgrades for airflow.

 

Well...a true cold air intake would help, but throwing on a K&N filter pack will NOT because the filters allow more particles to pass through, and are not cold air without the proper, sealed from engine bay, shielding.

 

mid 99+ intake manifold.   Great mod.   I doubt it adds 15hp/30ft/lbs torque like they say, but it does massively improve intake airflow with the curved runner design.   You would have to buy or make an adapter for the renix throttle body, which would negate any gains, as well as port match it to your head.    The best way is to use the H.O. throttle body, intake elbow, and airbox.   You can solder in the TPS wires, or buy/make a TPS adapter to bolt the renix TPS it to the H.O. throttle body.

 

Exhaust.    Not enough can be said.   Opening up the header will help...a lot.   Going much past 2.25" exhaust, though, won't.  Going too big negates back-pressure, and costs you venturi effect that helps more or less siphon exhaust through rather than the engine having to lumber and push the entire amount with no help.

 

I've ran a number of headers on mine.  APN seems to be the best.   Just moving the collectors downward with a smoother (and wide open) radius on the bends helps immensely.

 

Cams.

Not enough can be said about the proper camshaft.    Everyone forgets about it with these 4.0's.

There's a HUGE power loss in the factory cams.   They are set to have HP and TQ meet at 2200-2400RPM.    How often does the motor stay at that RPM once you attain desired speed?

 

A "torquer" series cam will drop that range.   I've always used Crane or Comp cams, torquer series, designed to drop that range down to 1700-1900RPM.   This is where I've found the needle sitting at speed.

 

Doing a lot of these mods will help.   I got 19mpg mixed driving out of my 7" lifted 87 MJ with H.O. built 4.0 that had a torquer cam, 99 intake, borla header, mustang 5.0 injectors, 62mm bored TB, adjustable map (also a good idea), aw4 auto on 33x10.50 and 35x12.50 mud terrains with MT axles and 4.10 gears.   Got 20-23mpg on highway.   Same motor put in my 99 5 speed XJ averaged me 20-25mpg depending.

 

 

 

 

Rather than throwing other parts at it to allow you to stick with the mustang injectors, though...put a good clean set of stock injectors back in, and see how it works. 

 

 

This. ^ Now don't get ahead of yourself upgrading the intake and forget about the exhust 'till later'. Air can only come in as fast as its going out and vise versa

[cruiser54]

You guys do have a point about your experience with engine speed and economy. I guess I should point out that the factory engineers designed the entire powertrain to return best economy to beat the EPA test back when the speed limit was 55 MPH. They certainly did the best job they could and no individual can match the R&D budget of the entire corporation they had behind them, but still, as they were fond of saying (and still do in their disclaimers), "your mileage may vary"! :D  Doubtless driving a vehicle under the conditions it was designed for will yield the best results.

 

As for some of the other stuff, well... maybe more on that later.

Was the speed limit 55 in 1987? It was 65 here in Arizona.

[Oyaji]

Detailed Test Information

EPA tests vehicles by running them through a series of driving routines, also called cycles or schedules, that specify vehicle speed for each point in time during the laboratory tests. For 2007 and earlier model year vehicles, only the city and highway schedules were used. Beginning with 2008 models, three additional tests will be used to adjust the city and highway estimates to account for higher speeds, air conditioning use, and colder temperatures.

Note: EPA has established testing criteria for electric vehicles and plug-in hybrids that are slightly different than those for conventional vehicles.

 

Highway: Represents a mixture of rural and Interstate highway driving with a warmed-up engine, typical of longer trips in free-flowing traffic.

 

http://www.fueleconomy.gov/feg/fe_test_schedules.shtml

[cruiser54]

That doesn't answer my question.

[Oyaji]

What it does show is the highway driving test cycle for which vehicles of that time were optimized.

 

I didn't bother to look up the 1987 Arizona speed limit because it had no bearing on the design of vehicles for the American market.

[JeepcoMJ]

Still not valuable information that answers any questions in this thread.

 

 

I will say this once.

 

Emissions testing is a load of squat. There is no benefit to economy in it. It is fictitious, made up standards, and non beneficial. Find me a vehicle that has benefited from emissions. All they do is require vehicles to pass a standard that was voted on, and often times that actually harm fuel economy.

 

 

1986 jeep MJ or xj with 2.8l. I don't think I need to say anything else on the subject.

[Oyaji]

Since the driving cycle I posted is for the EPA Fuel Economy test, it has everything to do with the topic of this thread. As can be readily seen by reading it, the maximum speed of the test is 60 MPH, and the average is 48.3.

 

That is the speed range for which vehicles for the US market were designed to deliver best economy.

[HOrnbrod]

fuel injectors are Ford 4 holers, and the O2 sensor has been replaced with an aftermarket unit.  i have the Peugout tranny and i run 235-74-15"s.  Since I'm closing in on 72, i do drive like a grandpa....

 

Getting back to the OP's questions, which Ford 4-holers injectors do you have? If you have the 24# version on a non-aspirated stock engine, they will not help your MPG, especially with the Renix computer. Also have you checked your fuel pressure at the rail with a gauge set, and what gear ration are you running?

[cruiser54]

 

fuel injectors are Ford 4 holers, and the O2 sensor has been replaced with an aftermarket unit.  i have the Peugout tranny and i run 235-74-15"s.  Since I'm closing in on 72, i do drive like a grandpa....

 

Getting back to the OP's questions, which Ford 4-holers injectors do you have? If you have the 24# version on a non-aspirated stock engine, they will not help your MPG, especially with the Renix computer. Also have you checked your fuel pressure at the rail with a gauge set, and what gear ration are you running?

Why would we want to do that? Stick to the subject at hand, that is. Isn't this forum turning from a "help the guys out" forum into an ego driven pontificating distraction instead?

[Comanchemodder]

To help compare your milage to others, check out fuelly site. This site has drivers log their economy by vehicle. Granted, ther are only 9 comanches, it would appear the average for the I6 is around 15-16 mpg.

 

 

http://www.fuelly.com/car/jeep/comanche/gas%20l6

[HOrnbrod]

 

 

fuel injectors are Ford 4 holers, and the O2 sensor has been replaced with an aftermarket unit.  i have the Peugout tranny and i run 235-74-15"s.  Since I'm closing in on 72, i do drive like a grandpa....

 

Getting back to the OP's questions, which Ford 4-holers injectors do you have? If you have the 24# version on a non-aspirated stock engine, they will not help your MPG, especially with the Renix computer. Also have you checked your fuel pressure at the rail with a gauge set, and what gear ration are you running?

Why would we want to do that? Stick to the subject at hand, that is. Isn't this forum turning from a "help the guys out" forum into an ego driven pontificating distracrion instead?

 

Yep, roger that Cruiser. Hopefully the OP, tugalo, is still on board.

[cruiser54]

Hope we haven't lost him, and I use the term "we" loosely.

[92tanMJ]

 

fuel injectors are Ford 4 holers, and the O2 sensor has been replaced with an aftermarket unit.  i have the Peugout tranny and i run 235-74-15"s.  Since I'm closing in on 72, i do drive like a grandpa....

 

Getting back to the OP's questions, which Ford 4-holers injectors do you have? If you have the 24# version on a non-aspirated stock engine, they will not help your MPG, especially with the Renix computer. Also have you checked your fuel pressure at the rail with a gauge set, and what gear ration are you running?

Not only that, but what brand O2 sensor is it?