Low Gas Mileage

[Randy in Maine]

My Comanchee ('90 4.0L with 5 speed and 3.08s) was not really driven for a few years before I got it.   The things I have done to improve my fuel economy over the past 12 years include: 

 

Went to stock sized tires (215/75/15) properly inflated and correctly aligned.

Sent my injectors off to have them cleaned and calibrated.  I use www.cruzinperformance.com but there are others.

Went to NGK triple ground plugs.

Fixed all of the vacuum leaks I had.

Changed out the O2 sensor.

 

I get about 21-22 mpg in over all driving, but I just go 65 mph (speed limit here).

[Oyaji]

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

 

Yes. Assuming an engine running on all cylinders and in a reasonable state of tune, fine-tuning driving habits unquestionably offers the biggest opportunity for fuel economy gains. You don't need to take my word for it - go Google it for yourself and read any site geared toward improving fuel economy.

 

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. 

 

None of this has any effect except at wide-open throttle (WOT). Tugalo is seeking fuel economy gains, not maximum power with the throttle mashed to the floor.

 

Some of what you mentioned is erroneous and misleading. Benefit from 4 hole injectors would be predominantly from low RPM running to help beat the idling exhaust emissions test. (Air-shrouded injectors offer the same benefit, but despite seeming promising turned out to offer no benefits elsewhere and thus were dropped for their additional cost and complexity.) However, it is true that reducing emissions under part-throttle running can offer a fuel economy benefit.

 

The main thing to keep in mind about bigger throttle bodies,cold air intakes, and other "snake oil" "upgrades" is that any benefit they would offer is only available at WOT; many times such parts offer no benefit whatsoever as proven by dynamometer testing (some products even return worse test results than stock). Any other time, the limiting factor for air flow is the throttle plate - because it is mostly closed for most engine running conditions. Further, the amount of fuel being injected is not dependent on the injector, but rather the control unit (which for part-throttle operation will compensate for sets of different flow rate injectors by simply taking feedback from the oxygen sensor into account and computing the proper duration injector pulse width). Too little or too much and the engine won't run properly. The engine needs to produce only the power required to equal the load, so increasing injector and air flow rates are not at all the issue here.

 

I'm sure you know that, but I'd guess that like many people you got sidetracked by the allure of parts that seem to offer a logical benefit, and you lost sight of the issue at hand: economy.

 

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. 

 

For someone who willing to modify his vehicle from stock, changing camshaft profile indeed offers potential for economy gains. To reap such benefit, changes are required elsewhere in the powertrain too, to change overall gearing. Getting such dialed in at the factory requires a large budget and time, but a motivated and meticulous individual can accomplish the same thing with patience, knowledge, a willingness to get his hands dirty, and deep pockets. I won't presume to know whether Tugalo is interested in going to those lengths, but considering his age I don't think it is likely.

 

I may be interested in pursuing such changes though, and would be very interested in hearing any details about cam choices, tested and proven degrees of advance on stock and aftermarket camshafts to increase low-RPM torque, part-and full-throttle torque and power curves under steady state dyno loads, and the changes to overall gearing required to take full advantage of the modifications.

 

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.

 

 

The upper fuel economy results you mention are phenomenal, particularly in view of common experience. Would you share your method of determining those figures?

 

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. 

 

I think this is the best advice you have given, and I completely agree.

[HOrnbrod]

None of this has any effect except at wide-open throttle (WOT). Tugalo is seeking fuel economy gains, not maximum power with the throttle mashed to the floor.

 

I have to agree with that statement. I've done all the modifications Jeepco detailed above, and none of them benefit fuel mileage. But at 80% throttle and up, it brings up a big smile to my face and mileage be damned.

[Oyaji]

My Comanchee ('90 4.0L with 5 speed and 3.08s) was not really driven for a few years before I got it.   The things I have done to improve my fuel economy over the past 12 years include: 

 

Went to stock sized tires (215/75/15) properly inflated and correctly aligned.

Sent my injectors off to have them cleaned and calibrated.  I use www.cruzinperformance.com but there are others.

Went to NGK triple ground plugs.

Fixed all of the vacuum leaks I had.

Changed out the O2 sensor.

 

I get about 21-22 mpg in over all driving, but I just go 65 mph (speed limit here).

 

Rock-solid advice - and very nice economy figures, too!

 

I am curious - did they flow-match your injectors when you sent them off? How much did they charge you for the service, and what fuel economy gain did the matched injectors return for you?

[Randy in Maine]

I did most of those changes at the same time back in 2002 and it went from about 16mpg or so to "touching 22 mpg" stil today.  More importantly it really did not idle well and was sluggish getting up to speed when I got it.  That was the most immediate change.

 

Here is link top the cruzin site....back when I did it I think it was about $15 x 6 injectors.  Might be a few bucks more these days.

 

http://www.cruzinperformance.com/fuelinj.html

[Virginia]

I've had a sudden drop in my mpg's.  Started during the winter.  Went from 14.9mpg to 12mpg.  I do have a hold in my exhaust pipe before the muffler but don't know if that would cause it.  I also know that after I had my water pump replaced, my temp guage doesn't get much above about 180 which seems cold to me since it used to run about 210.  I've put new tires on and went with a slightly narrower tire since i'm not really doing any offroading.  Haven't switched gas.  I need to look at my air filter but even when I was having awful blowby, my mpg's never dropped this much.  

 

I'll run some injector cleaner but seems to me that clogged injectors would have cause a gradual drop, not this off the cliff drop.  TIres are stock and properly inflated if not slightly over.  I'll check the plugs and change the O2 sensor.  Any other ideas?

[cruiser54]

Get a factory stat 52028186.

 

Keep in mind that we now have "winter blend" fuels and that can affect MPGs.

[benjy_26]

Injector cleaners don't really do much on engines with modern, high pressure injectors. On a Renix TBI engine, perhaps, but with the 4.0, the spray alone will keep your injectors humming along. Just remember to use quality fuel from a busy station. As far as mods for economy, I may have fallen bass-akwards into it, but I get 23-24mpg on the highway with my 4.7l stroker in an auto equipped 2wd zj running 32" KM2's and a locked 8.8 rear end. I have a cone filter (more for convenience than anything), Neon injectors, a large bore throttle body, a 99 WJ intake manifold, a Banks header, and a magnaflow muffler, all stitched together with 2.5" mandrel bent tube. I did dump my exhaust before the axle (why carry extra weight and have two more bends adding restriction?), and I use nothing but Mobil 1 synthetic fluids in my vehicle. I also have a MSD 6AL box along with a Blaster SS coil, their 8.5mm wires, and their hi-po dizzy. I'll dig up my cam profile if you guys want, but I know I'm right at the limit when it comes to lift while using stock style springs.

 

 

I know all of these mods make for a huge wallop above 4500rpm, but interestingly enough, cruising on thr highway at 55mph, I have a pretty flat power band, I hardly ever feel the tranny downshift unless I mash thr pedal, and the engine is pushing me along while spinning right below 1500rpm. I do get pretty terriblr milage in stop and go driving, but the highway numbers keep my combined average at around 16mpg.

Injector cleaners don't really do much on engines with modern, high pressure injectors. On a Renix TBI engine, perhaps, but with the 4.0, the spray alone will keep your injectors humming along. Just remember to use quality fuel from a busy station. As far as mods for economy, I may have fallen bass-akwards into it, but I get 23-24mpg on the highway with my 4.7l stroker in an auto equipped 2wd zj running 32" KM2's and a locked 8.8 rear end. I have a cone filter (more for convenience than anything), Neon injectors, a large bore throttle body, a 99 WJ intake manifold, a Banks header, and a magnaflow muffler, all stitched together with 2.5" mandrel bent tube. I did dump my exhaust before the axle (why carry extra weight and have two more bends adding restriction?), and I use nothing but Mobil 1 synthetic fluids in my vehicle. I also have a MSD 6AL box along with a Blaster SS coil, their 8.5mm wires, and their hi-po dizzy. I'll dig up my cam profile if you guys want, but I know I'm right at the limit when it comes to lift while using stock style springs.

 

** Forgot to mention, I did get my intake manifold and exhaust manifold port matched to the head.

 

 

I know all of these mods make for a huge wallop above 4500rpm, but interestingly enough, cruising on thr highway at 55mph, I have a pretty flat power band, I hardly ever feel the tranny downshift unless I mash thr pedal, and the engine is pushing me along while spinning right below 1500rpm. I do get pretty terriblr milage in stop and go driving, but the highway numbers keep my combined average at around 16mpg.

[cruiser54]

BG44K in a full tank of gas. Run til 1/2. Fill again. Run til 1/4 tank. Injectors will be clean. 

[benjy_26]

Oh, and as far as the OP's engine, he did mention that it's a fresh build with 5000 miles. I don't know what y'akl's experience has been, but I've noticed that (to a point) an engine does get more powerful and more efficient once its racked up some miles. I wonder what the OP's break-in procedure and maintenence schedule is like.

[cruiser54]

Possibly needs to be broken in more...

[benjy_26]

Possibly needs to be broken in more...

My thoughts exactly... and maybe with a different type of oil than he's been using.

[metrictonner]

88 4.0 15-16 city? Sounds normal to me.  The computer contols this based on sensor readings. Screwing around with things doesn't do a thing.  A consistently cold water temp signal will cause a rich mixture.  Check out your block water temperature sensor.

[gogmorgo]

Do you let your truck idle to warm up? Getting zero mpg for any length of time can hurt the numbers quite a bit.

 

My mpg takes a hit in winter in part because of that, in part because of the cold (leading to richness), and in part because the lower traction leads to a bit of... we'll call it friskiness. 

[Virginia]

Get a factory stat 52028186.

 

Keep in mind that we now have "winter blend" fuels and that can affect MPGs.

OK