State your engine and HIGHWAY fuel consumption!

3.0L  Less than 15mpg highway
0 (0%)
3.0L 15-18mpg highway
3 (14.3%)
3.0L More than 18mpg highway
1 (4.8%)
3.5L DOHC Less than 15mpg highway
2 (9.5%)
3.5L DOHC 15-18mpg highway
3 (14.3%)
3.5L DOHC  More than 18mpg highway
0 (0%)
3.5L SOHC Less than 15mpg highway
2 (9.5%)
3.5L SOHC 15-18mpg highway
5 (23.8%)
3.5L SOHC More than 18mpg highway
2 (9.5%)
3.8L Less than 15mpg highway
2 (9.5%)
3.8L 15-18mpg highway
1 (4.8%)
3.8L More than 18mpg highway
0 (0%)

Total Members Voted: 21

Author Topic: Let's talk about increased fuel consumption!  (Read 1740 times)


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Let's talk about increased fuel consumption!
« on: November 12, 2018, 02:54:30 PM »
Everybody's going to get a different MPG because everyone drives differently, with different loads in the vehicle and different tires at different pressures at different elevations with different weather conditions and different fuel grades and different lubricants and different traffic and countless other factors - we can generally state that if your Montero is consistently getting 10mpg it has something wrong with it and if it's getting 30mpg your pants are presently on fire. 

Let's try to help each other dial in our Montero to run as efficiently as possible.     That's going to save you money at the pump and it's going to save you repair money too because subpar combustion leads to actual damage over time. 

First thing to get out of the way is your math:   if you put different tires on your vehicle and didn't compensate for that your odometer is wrong and you're measuring wrong.   

Example:  Your Montero may have come from the factory with 265/70R15 tires which measure 29.6 inches diameter.    That means your speedometer and odometer count 682 revolutions as one mile.      But if you put 33" tires on your Montero (without changing gear ratio) and count 682 tire revolutions you've gone 1.118 miles.     
So if you fill your fuel tank, drive 200 miles according to the odometer and refill your tank to the click with 16 gallons that looks like 12.5MPG (18.8 l/100km) - but you've actually covered 223 miles of distance and you really got 14MPG (16.8 l/100km) . 

So get your math right first. 

Once you know your actual fuel consumption here's a list of factors in no particular order that can harm your fuel efficiency:

  • Your check engine light.   Is it on?  Find out what's wrong with it and fix it.   Plenty of help available, there's really no excuse.      Even if the fault you get is downstream O2 (P0138, P0139, P0140...)  the engine doesn't use the downstream O2 sensor for metering but those faults could mean your cat is plugged which definitely will affect fuel consumption.

    Fix whatever is causing your check engine light first, chances are pretty good that it's related to your fuel consumption.
  • Aerodynamic loading.    This means "Stuff that makes your truck punch a bigger hole in the air than it did when it was stock".     That can be roof rack, roof tent, big tires,  body or suspension lift, brush guard, big ass tow mirrors even.    I think it's safe to say the most common offenders here for us are going to be the big tires, body lift and the roof rack.   Brush guards don't help your aero but they alone aren't a huge factor
  • Big ass tires.  Tires are heavy and they're the worst kind of heavy because it's rotating and unsprung weight.   That means any time they have to speed up, slow down or change location relative to their vector of travel (that means go up and over a bump while you drive) they are asking for a lot of energy from your engine and that means drinking fuel.   Mud tires are even worse because they're on soft lugs and have higher rolling resistance than a smooth tire.  Tires are also almost entirely outside the slippery bodywork of the vehicle, especially if you're lifted and a tire is essentially a cube as far as aerodynamics are concerned.
  • More about tires!  Especially for the SOHC engines that don't have dual path intake runners, these engines just love to run out and their torque peak is in the mid 3's - that's where they're most efficiently turning gasoline into horsepower.   They balanced these things out pretty well with the stock tire size and stock gear ratio but when you move up to bigger tires you're taking the engine out of its sweet spot - that means sucky acceleration for you and it means worse fuel efficiency as well.   If you have stock 4.27 gears and you've increase tire size I strongly suggest considering an upgrade to 4.63 or 4.9 axle gearing.  This isn't 1980s American V8 trucks where higher (numerical) gear ratios make it drink gas - this is about putting the engine right in the middle of its sweet spot while you cruise. On Gen 2/2.5 consider 4.63 for 31-33" tires and 4.90 for 32-35" tires, and 5.29 for 35"and up.   
  • Added weight.    You can get really nerdy really fast on this.  It takes an estimated100 newtons of force applied over a distance of 160 meters to get 100lbs from 0-60mph .    With some napkin assumptions about the thermal efficiency of your Montero's engine and the amount of energy present in a gallon of gasoline that means for every 100lbs of stuff in/on your Montero it costs you 0.006 gallons of gasoline every time you accelerate from 0 to 60mph.    Doesn't sound like a big number, but remember that's applicable absolutely every time you accelerate at all - every instant that you're increasing speed.. and it doesn't even take into account the pull of gravity when you're accelerating up hill.   

    The bottom line for non-nerds is that more weight = more fuel consumption.    Less weight = good.   
  • Oil fill, condition & viscosity.    There's a number in the owners' manual for what viscosity your engine calls for,  and you deviate from this at your own peril.   Old oil is less slippery than new oil - plenty of amateur scientists on youtube doing nonetheless authentic experimentation to demonstrate the reduction in lubricity that old, diluted, contaminated, burned oil suffers.   Overfilled or oil-starved engines aren't doing anybody any favors either of course. 
  • Transmission line pressure adjustment - this is specific to Monteros which use the 4 speed Aisin transmission.   There's a second cable on your throttle body which leads to the transmission, this is called the TV cable and is sometimes (incorrectly) called the kickdown cable.  On these transmissions this cable's sole responsibility is governing line pressure inside the transmission and correct adjustment is important.    Too much slack and you get sluggish shift overlap which produces heat, wears the transmission and harms your fuel consumption.    Too little slack and you get excessive line pressure which hydrostatically loads the pump assembly and can shorten its life, produces excess heat and harms your fuel consumption.
  • Poor fuel regulation.   This can be from a plugged fuel filter, weak fuel pump,  dirty fuel injectors,  bad data from your vehicle's sensors,  electrical faults..  sometimes these won't cause a check engine light to illuminate even if they're faulty because there isn't always a sensor-to-sense-the-sensor, if that makes sense.     Your throttle position sensor could be getting noisy which will mean the vehicle has to guess about how much fuel is being demanded and when to shift the transmission - that's not going to help fuel efficiency. 
  • Inconsistent electrical/sensor readings - your vehicle uses a common ground for most of its electrical systems;  ignition events are metered by transistor short to ground, your oxygen sensors are biased against ground, your engine computer uses a ground bias... basically every reading and command is "x" volts relative to ground.   So if ground is inconsistent then sensor information is inconsistent.   That's why the manufacturer spent money on multiple powertrain grounding locations.   Over time corrosion increases the electrical resistance at every junction and sometimes repairs are done and the mechanic doesn't reconnect every ground cable - this isn't doing you any favors.  Not a bad idea to refresh and inspect your grounds above and below the engine. 
  • Tire Pressure shouldn't need to be said but.. tire pressure.  It's listed on your driver side door jamb and even if you have larger than stock tires you'll still have good results if you ensure your tires are inflated to that pressure or close to it.
  • Fuel quality is an interesting topic with these vehicles.  Gen 2/2.5 Monteros do not have knock sensors, aren't especially high compression and don't run particularly aggressive ignition timing - they're not very picky eaters.   That means you probably won't get any meaningful benefit (or any benefit at all) from running high octane fuel.  Similarly you probably won't get any benefit from whether or not your fuel contains ethanol;  pure ethanol has around 30% fewer BTU's per unit of mass so if you have E10 that means your total fuel can have at most 3% fewer BTU's..  but ethanol also improves the observed volumetric efficiency of the engine by supplying oxygen directly to the combustion chamber in the liquid fuel so your net difference in fuel consumption is vanishingly close to 0 between E10 and E0 petroleum.     
    So where does that leave us?   Interestingly enough on these vehicles if you happen to notice a big difference in performance or fuel consumption among different fuel suppliers or octane grades it might indicate more about the condition of your Montero than it does about the quality of the fuel.  Your Montero might have a fuel metering or combustion problem that's making it a picky eater and the fuel is simply bringing that to your attention - worth investigating, anyway.   
  • Wheel alignment, suspension or brake problems.  Goes without saying that if your tires are pointing in directions other than straight a good amount of your fuel energy is going into scrubbing those wonky tires along the ground and it's also going to get expensive replacing tires before their time.   A dragging bearing,  a dragging brake are also obvious causes of poor fuel consumption.    A somewhat less obvious cause can be tires out of balance or completely worn shocks.   Tires are heavy and whenever they are moved out of line from the direction of travel (bounce..) that uses energy.   All of the kinetic energy in  your vehicle comes from burning gasoline so if you have tires that are bouncing along due to poor balance or if the tire or chassis is hopping along after any disturbance because the shock absorbers are way past dead, you're using up fuel to move those along and it does add up over time.
  • Improper CAD operation - Your front axle (Gen 2, 2.5, 3, super-select 4x4 Sports before 2002) uses a pair of vacuum solenoids, a couple switches and a vacuum diaphragm to disconnect the passenger side axle shaft when you're in 2wd mode to save fuel, reduce wear on the drivetrain and reduce noise.   That's cool but the default position for the Center Axle Disconnect is connected so that means if it fails, it fails into 4wd.   If yours isn't working correctly it will leave the front axles connected together and that means you're always spinning the front driveshaft, always spinning the bottom half of your transfer case - and that always consumes more fuel.   Problems with the CAD are easy to diagnose, easy and inexpensive to fix (if caught early) but can become moderately expensive to fix if you ignore certain faults. 

That's about all I have time to type at the moment - please contribute to this thread if you have anything to contribute I'd like to have one place to point people experiencing excessive fuel consumption so they can get their truck back in tune and enjoy it for years on.   


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Re: Let's talk about increased fuel consumption!
« Reply #1 on: November 12, 2018, 07:04:59 PM »
all of these things are things I need to fix lol
think for yourself, question authority


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Re: Let's talk about increased fuel consumption!
« Reply #2 on: November 13, 2018, 05:41:15 AM »
New favorite thread.


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Re: Let's talk about increased fuel consumption!
« Reply #3 on: November 13, 2018, 02:40:44 PM »
Since I put 33's on, I've been calculating MPG based on GPS. Still not 100% accurate, but it sure made me feel better about the MPG! :-D