Fuel economy and odd auto tranny behavior gen 3 montero

#1
I’ve been getting terrible fuel economy in my gen 3 since I purchased it earlier this year. Averaging 13-14mpg on mostly country roads where I average 18-19mpg in my Jeep Wrangler with bigger mud terrains. I just had a big service done at the Mitsu dealer. Timing belt, water pump, front rotors and pads, tranny flush, spark plugs and wires, and valve cover gaskets. Was hoping it would be getting better mpg after. But no such luck. Got 13 mpg on the first full tank after the repair.
I’m not sure what to do next. Was thinking maybe it needs O2 sensors. Or maybe the brakes are rubbing and them replacing the rotors and pads didn’t fix that issue. The front wheels are usually quite warm even after short drives where as my Jeep wheels aren’t.
Also the transmission acts oddly. But maybe it’s completely normal. It drops revs when I take my foot off the gas. Even in fifth gear. I’ll be cruising at 2000rpm in 5th. Take my foot off the gas and revs drop immediate to 1200rpm. Is this normal?

Any other ideas of what’s killing my mileage. I already put in a new air filter. The tires are stock size and properly inflated and it has fresh synthetic oil in the engine as well.
I should also mention I drive very conservatively and usually get better mpg than what is normal for any given vehicle.

I wanna start a build on this truck but really want to get it running well first. It only has 104k on it. 2004 limited montero.

Thanks



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#2
The tranny is nornal, at least mine does it too. The torque converter is unlocking.

I hate to say it but the gas mileage is probably normal too. I see some people reporting high teens but I've never been close. 13-15 is where I'm at almost all the time. Granted I'm also in Colorado so the elevation and passes effects it more.

Maybe watch the fuel trims but unless your getting a check engine light I doubt spending money on o2 sensors will do anything for you.
 
#3
I'm not saying that the fuel economy isn't normal, but I can help you check the transmission for problems.

Move the shift lever into the manual mode and perform the same test, i.e. drop throttle and watch rpms. The 5-speed transmissions in the Gen 3 have a locking torque converter which allows for engine braking when needed and better fuel economy (no loss through the converter). In manual mode, you should be able to get the engine to hold its speed without throttle being applied.

If the transmission cannot brake or hold itself in gear, then I would start by checking the dipstick and evaluating the level/color of the transmission fluid. It should have a deep red color when new, slowly turning dark as it ages. Following that, a fluid flush/replacement is a common fix for most transmission issues on these vehicles and can be done easily in the driveway without removing the transmission. Fluid must only be the approved SP-III fluid. Any other type of Dexron/Mercon/ATF will not work in these transmissions and causes more problems.
 
#4
This is the write-up I follow for doing fluid changes. The filters can be purchased from RockAuto or other places on the internet. The Diaqueen SP-III fluid can be Mitsubishi, KIA, Hyundai, or Aisin brand. I buy in cases of 12 quarts for the full service. You will need 10 quarts to change all the fluid and about 1-2 to make the correct oil level at the dipstick.

https://www2.pajeroclub.com.au/forum/showthread.php?t=15879

https://www.rockauto.com/en/parts/aisin,ATFSP3,transmission+fluid,11387

https://www.ebay.com/itm/12-Quarts-...812199&hash=item2370314e59:g:vKMAAOSwHoFXsVTT
 
#5
I'm not saying that the fuel economy isn't normal, but I can help you check the transmission for problems.

Move the shift lever into the manual mode and perform the same test, i.e. drop throttle and watch rpms. The 5-speed transmissions in the Gen 3 have a locking torque converter which allows for engine braking when needed and better fuel economy (no loss through the converter). In manual mode, you should be able to get the engine to hold its speed without throttle being applied.

If the transmission cannot brake or hold itself in gear, then I would start by checking the dipstick and evaluating the level/color of the transmission fluid. It should have a deep red color when new, slowly turning dark as it ages. Following that, a fluid flush/replacement is a common fix for most transmission issues on these vehicles and can be done easily in the driveway without removing the transmission. Fluid must only be the approved SP-III fluid. Any other type of Dexron/Mercon/ATF will not work in these transmissions and causes more problems.
The tranny was just flushed at a Mitsubishi dealer. It does the same thing in manual mode in 5th gear. Also if I gun it in 5th gear manual mode on the highway going 65 mph or so the revs go up before the speed goes up. As if the torque converter isn’t locked. They don’t go way up. But they clearly aren’t going up proportional to the speed.

I think most people would never notice this is even happening because it still basically drives normally. But If there is an issue I like to address it before it becomes larger.

I’m wondering if the flush even caused the problem because I didn’t notice this before the flush. But I only owned the car 1 month before the flush and I’m used to driving manual cars. The fluid looked dirty to me which is why I had them flush it but they told me it really wasn’t that bad but they still suggested I flush it for preventative maintenance.


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haolepinoy

Incomplete Idiot
#6
To get a good bead on your mpg woes you'll need to check your fuel trims. You may be running rich, in which case you can then start chasing gremlins in the MFI system. Healthy fuel trim numbers should give you 16-19 mpgs.
 
#7
I live in Denver as well (altitude sucks for mileage) but what I've found with mine is the length of drive makes a huge difference in terms of mileage (on the 3.5 not the 3.8). For example, when it was my daily driver, I'd regularly be 13-14 MPG, and on occasion as low as 12. But, daily for me means heading to Home Depot, or a quick jaunt to the store, all the drives were <10 minutes. When it switched over to my wife's daily drive (>20 minutes) we saw it pop up to 15-17. This is consistent with long trip MPG.

I know the other variable is driver, but generally I get better MPG than my wife when we're on long trips so I don't think that's it. I'm curious if the warm-up process is particularly fuel rich on these trucks, because the exhaust definitely smells rich on mine when I take short drives (<5 min). This may be normal, or it may be something else like O2 sensors. I get an occasional P0125 and depending on the source I've heard rumors that can be caused by a dying O2 sensor. I've watched the fuel trims and O2 voltages while driving - but - I don't really know what I'm looking for so I'm not sure if it's good or bad there.

These trucks are getting old, so it may be time to figure out how to do a fuel/air system refresh, or at least a proper debug list.
 

jeep-N-montero

Expedition Leader
#8
The tranny was just flushed at a Mitsubishi dealer. It does the same thing in manual mode in 5th gear. Also if I gun it in 5th gear manual mode on the highway going 65 mph or so the revs go up before the speed goes up. As if the torque converter isn’t locked. They don’t go way up. But they clearly aren’t going up proportional to the speed.

I think most people would never notice this is even happening because it still basically drives normally. But If there is an issue I like to address it before it becomes larger.

I’m wondering if the flush even caused the problem because I didn’t notice this before the flush. But I only owned the car 1 month before the flush and I’m used to driving manual cars. The fluid looked dirty to me which is why I had them flush it but they told me it really wasn’t that bad but they still suggested I flush it for preventative maintenance.


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Unless a vehicle has had flushes at regular intervals since new it can actually do more harm than good, this could very well be your problem, a quick google search will explain why.
 
#9
The tranny was just flushed at a Mitsubishi dealer. It does the same thing in manual mode in 5th gear. Also if I gun it in 5th gear manual mode on the highway going 65 mph or so the revs go up before the speed goes up. As if the torque converter isn’t locked. They don’t go way up. But they clearly aren’t going up proportional to the speed.

I think most people would never notice this is even happening because it still basically drives normally. But If there is an issue I like to address it before it becomes larger.

I’m wondering if the flush even caused the problem because I didn’t notice this before the flush. But I only owned the car 1 month before the flush and I’m used to driving manual cars. The fluid looked dirty to me which is why I had them flush it but they told me it really wasn’t that bad but they still suggested I flush it for preventative maintenance.


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Unless a vehicle has had flushes at regular intervals since new it can actually do more harm than good, this could very well be your problem, a quick google search will explain why.
I had heard of risks of flushing and asked the service techs at Mitsubishi if they would recommend a drain and refill instead of flush because I heard a flush can cause damage. They said that they don’t actually put pressure on the system to flush. They let the transmission pump, pump it out and refill so that’s what they recommended. Hopefully they were right.....


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#10
I live in Denver as well (altitude sucks for mileage) but what I've found with mine is the length of drive makes a huge difference in terms of mileage (on the 3.5 not the 3.8). For example, when it was my daily driver, I'd regularly be 13-14 MPG, and on occasion as low as 12. But, daily for me means heading to Home Depot, or a quick jaunt to the store, all the drives were 20 minutes) we saw it pop up to 15-17. This is consistent with long trip MPG.

I know the other variable is driver, but generally I get better MPG than my wife when we're on long trips so I don't think that's it. I'm curious if the warm-up process is particularly fuel rich on these trucks, because the exhaust definitely smells rich on mine when I take short drives (
These trucks are getting old, so it may be time to figure out how to do a fuel/air system refresh, or at least a proper debug list.
My commute to work is about 7 miles. 12 minutes. So maybe that’s part of it. I’ll have to try to figure out what the fuel trims mean. It’s a bit over my head. I watch the voltage on my scanner sometimes and it just seems super random. I feel like it would need to be tracked on a graph to really see if it’s working correctly.


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#11
Short commutes in our '05 are similar to the MPG you are reporting. Not great, and in the winter its worse. Full tank highway driving long distance we do see 17 mpg to 19 mpg consistently.
 
#13
Have you run and injector/fuel system cleaner? A can of BG 44k did wonders for pepping up my 01 after purchase. I still sent my injectors off for cleaning/spray pattern/volumetric testing, however, it proved unnecessary according to the results. He sends pics of the spray patterns, and I had scribed cylinder # as well, so I am certain that they went back into the same postion. The injector guy said that he likes the BG 44k fwiw http://www.fuelinjectorspecialists.com/

Buildup on intake valves can dramatically affect fuel economy, just ask anyone with a direct injected motor. I would run a cleaner prior to doing O2 sensors, but a set of Denso plug n play versions from Rock Auto are around $150 for all four. It is a relatively simple but knuckle busting contortionist sort of task. I have not done mine, but did pull them and carefully anti-seize rhe threads to aid the task in the future. I simply didn’t have the additional funds and only had 138k. My 02 sensors are getting a bit tired at 178k according to voltages(see below)


I get low 12’s to 15 on strictly around town Moab tanks. Monty likes a good warmup, and stop/go driving, AC cranked and short hops (3-8 miles) contribute to crappy mileage. That said, I regularly get into the 20.6-21.8 range with speeds in the 63-71 range. I am at 4200 msl, but I think our altitude density helps with the fuel economy (Less 02/less fuel). I fill at the same stations/pumps, so I think the trip leg mileage is pretty accurate

We have an 80 mph speed limit, I drive 78-83, and get high 16-low 17’s on a Moab-Grand Junction run. I can see 18-low 19’s in 75 mph states. I had a heck of a time getting more than 18 while visiting your neck of the woods on a recent PA trip, despite the low speed limits.

Torque Pro has a data-logging and graphing feature for 02 sensors. Your upstream sensor will fluctuate to keep the catalyst at operating temp through the use of transient lean conditions, the downstream sensor should be pretty steady within a small range. I fiund this article to be a good overview. http://www.autotap.com/techlibrary/understanding_oxygen_sensors.asp

Have you checked your trans fluid, in N while idling? I did the DIY flush and added until it was correct, three days later it felt “weird”. Check the fluid and it was nearly a qt low. No leaks btw. It has been stellar since(34k)
 
#14
The flush is different than changing the fluid. Flush will always put the same worn fluid back into the transmission. Most shops are against replacing fluid because of the cleaning detergents in some fluids causing more damage. However, changing to new fluid and filter with the correct SP-III fluid should not cause any problems for this transmission. I changed it at 170K on my '01, 110k on my '05 and I will do it again on my '05 when I get a chance.

As the fluid gets old, it loses its friction properties which causes slippage in the clutches and odd torque converter behavior. My 01 had multiple flushes done for what felt like a rumble strip/gravel road as I accelerated under light load up hills and around 40mph once it got to 150k miles. The flush would help for a few months before the symptoms returned, only worse each time. At 170k miles I found someone else describing the same problem and the fluid change procedure. I didn't change the filter that time, but I cycled the engine to pump the fluid through the cooler hose 2qts at a time, shutting the engine off when flow stopped to add more new fluid until it was all the same deep red color. Afterwards, the truck drove like new. No more shaking, rumbling or odd transmission behavior.

I drove my 05 in manual mode in 5th to confirm what you are seeing and they seem to unlock the converter when decelerating in the calibration. When I shift to 4th manually at the same speed, it locks and holds the engine speed until I shift back to 5th. To tell if the converter is locking in 5th, you will see the rpms rise then drop slightly as the converter clutch grabs. You may also hear a change in noise as the engine loads up with this.

@MoabRefugee , I would say the qt low is caused by the fluid entering all the passages in the transmission while warming up. The Torque converters also take some oil as they fill. But as you saw, low fluid/old fluid/too much fluid/wrong fluid can all make this transmission act abnormally. This is especially true for the torque converter.
 
#15
I stand by my assertion that a proper fluid change with the proper fluid is the absolute best performance upgrade you can give these trucks. Much like Michael Brown, my truck went from pretty crummy, to pretty awesome, with one change. I'm actually planning on doing it again (a year later) on the basis that my truck ran something else for some unknown period before I purchased it and I figure the 7k miles with fresh fluid picked up the remaining junk so another change should put me back to factory fresh.
 
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