99 Montero Gen 2.5 Fuel Efficiency Issue

jms_brns

Observer
I've recently noticed that my fuel efficiency has drastically dropped in my 99 Montero 3.5l. I've noticed the difference for a couple weeks now, but it may have been going on longer than that, as I don't really keep a very close eye on my fuel economy.
Work recently done by shop:

  • water pump
  • timing belt
  • thermostat
  • camshaft seal
  • crank gear
  • harmonic balancer
  • new 32" BFG A/T's

I tend to be a pretty aggressive driver so I don't expect to get Prius numbers, but on my last tank I calculated getting 7.5mpg. :yikes: This is while driving solely to and from work about 4.5 miles each way. Last time I checked I wasn't towing any 40' yachts behind me... I've also noticed when I fill the tank from almost empty it usually only takes about 18.5 gal which is nowhere near the 24.3 capacity. Does anyone have any ideas what could be causing the horrendous mileage issue and tank capacity discrepancy???

Thanks!
 

Buddha.

Finally in expo white.
You mention new taller than stock tires. Are you taking into account that you are actually driving more miles than your odometer is calculating?

also,

Check engine light on?
4.5 miles of city or highway driving to work?
Engine temp coming up into normal range?
 
Last edited:

jms_brns

Observer
No, this calculation is without accounting for the taller tires, I figured that would have some effect but not nearly enough to make the 7.5mpg acceptable. My check engine light is not on and my commute to/from work is on city streets. But while the commute is on the streets there is almost no traffic and I'm usually going around 45-50 mph. The temp gauge usually hits running temp around 1-1.5 miles into the commute.
 

Buddha.

Finally in expo white.
Your right I did the math, taking into account the taller tire doesn't add much. I'd check the tire pressures. I'd go around the truck after your commute and feel all four hubs and see if one is super hot to see if you have seized bearings or dragging brakes. Parking on a slight incline and putting it in neutral to see if it rolls should be a way to check for dragging brakes as well.

Does it seem to run fine? Accelerate fine? Make odd grinding or squealing sounds?

Don't mean to question your intelligence but are you sure your calculating mileage correctly? How many miles do you get out of a tank?
 

jeep-N-montero

Expedition Leader
Are you sure your timing belt isn't off timing somewhere and that the new thermostat is the correct OEM temp? A lower temp thermostat will lower mpg but add a few hp. Even with the 33x9.50 tires on my 98 I still get around 16-17 mpg not towing.
 

jms_brns

Observer
Your right I did the math, taking into account the taller tire doesn't add much. I'd check the tire pressures. I'd go around the truck after your commute and feel all four hubs and see if one is super hot to see if you have seized bearings or dragging brakes. Parking on a slight incline and putting it in neutral to see if it rolls should be a way to check for dragging brakes as well.

Does it seem to run fine? Accelerate fine? Make odd grinding or squealing sounds?

Don't mean to question your intelligence but are you sure your calculating mileage correctly? How many miles do you get out of a tank?

I'll check for drag issues later today, but there are no irregular noises or anything else. It seems a bit less responsive on acceleration but not much. I'll get another calculation next time I fill up but on my last tank I went over the math about five times hoping I had done something wrong.

Jeep-n - What would be other symptoms of the timing being off or thermostat issue? The truck idles smooth around 500-700 rpm and the temp gauge stays the same as it did before the belt/pump/thermostat job.
 

jeep-N-montero

Expedition Leader
I'll check for drag issues later today, but there are no irregular noises or anything else. It seems a bit less responsive on acceleration but not much. I'll get another calculation next time I fill up but on my last tank I went over the math about five times hoping I had done something wrong.

Jeep-n - What would be other symptoms of the timing being off or thermostat issue? The truck idles smooth around 500-700 rpm and the temp gauge stays the same as it did before the belt/pump/thermostat job.

Loss of power/acceleration is one symptom of timing being off. As for the thermostat, if a lower temp model was used then it will open sooner and not allow the engine to get up to optimal running temp and decrease gas mileage.
 

mapper

Explorer
I've also noticed when I fill the tank from almost empty it usually only takes about 18.5 gal which is nowhere near the 24.3 capacity. Does anyone have any ideas what could be causing the horrendous mileage issue and tank capacity discrepancy???

Thanks!

How can you be sure you calculations are accurate if you think the tank isn't being completely filled consistently? Maybe try a different gas station?
 

raysobi

Adventurer
I felt like I am having the same issue, but I will judge again after I finish my timing-belt replacement etc..

I am not the dreamer and tried to compare Montero to my 4 banger 2WD Scion on the gas mileage, but my Montero should do a lot better than 9-10 MPG, and the Monty is totally stock with no upgrades.
 

mapper

Explorer
I'd say 10 MPG is indicative of a problem.

The only time I've had a tank below 15-16 is when I've been cruising in the 80mph range with a loaded truck, A/C on, some head wind, and a few mt passes. I'll get 19 plus at more relaxing highway speeds. 17-19 is my normal range. I've got 31" tires and 182k on the truck and I've been running regular gas (85 octane). Just switched to premium fuel and I'll see what that does.
 

twocommas

Observer
I started to manually track my mpg and realized it's pretty low as well.
The last fill-up had an average of 13.7mpg on 87 octane, 50% slow city/50% highway miles, with no load in the car, 31"tires.
A recent 100% highway mpg was only marginally better.

That seems low doesn't it?
 

TheMole

Adventurer
Just curious if you guys are driving in 4wd or 2wd mode. I've heard that the mpg between the two are minimal, but noticed that the RPMs are slightly higher in 4wd at the same speed which could affect mpg.
 

jeep-N-montero

Expedition Leader
I started to manually track my mpg and realized it's pretty low as well.
The last fill-up had an average of 13.7mpg on 87 octane, 50% slow city/50% highway miles, with no load in the car, 31"tires.
A recent 100% highway mpg was only marginally better.

That seems low doesn't it?

That right there is your problem, your ignition timing is being retarded by the knock sensor due to the low octane fuel you are running. These trucks were designed to run best on premium fuel, if your fuel is not allowing the ignition curve to run at its best then your power/mpg will suffer a bit.
 

twocommas

Observer
jeepN, I will give it a try at the next fill up. But it looks like mapper, a few comments back, has a stock Montero/31" tires/similar engine mileage and he is getting a lot higher mpg running on 85 octane.
 

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