1998 Montero Sport LS RWD 3.0

I have been chasing this odd gas mileage problem for about a year now and I've replaced god knows all on this truck with no luck. I have replaced the catalytic converter, O/2 sensors, replaced every exhaust gasket, searched and found no evap or vac leaks, I have reverted back to the stock intake, replaced the MAF sensor, replaced the MAP sensor, purge solenoid, EGR valve is tested and works as it should, cleaned throttle body, new NGK Iridium plugs gapped to .040" (checked with feeler gauges"), installed new NGK coil packs, replaced plug wires with Bosch brand (super cheap), cleaned intake plenum and manifold of carbon buildup, installed used but in way better condition charcoal canister, replaced gas cap, replaced vacuum check valve, serviced front and rear brakes to ensure they weren't dragging, replaced fuel regulator, back flow cleaned and installed significantly lower mileage injectors, and installed a new fuel filter.

I even tried driving like a granny for awhile to see if that would work and it didn't. No matter how hard or light I drive with A/C on or not, I get the same average gas mileage. Thing is, my city gas mileage is always 13.5~14.6 and my highway mileage is all over the place. Only one code has been thrown for an O/2 sensor malfunction P0136 which would be the upstream sensor if I recall correctly, this was due to me installing an exhaust gasket incorrectly which I fixed, I then reset the computer by pulling the positive cable overnight, and gained absolutely no mpg increase, still exactly the same as before. After installing the new cat, I noticed I got 17mpg city mileage for exactly one day but after that it went back down, I got under the truck and had someone rev the engine to around 3000RPM while I checked the input/output temperature of the cat and saw a 170 degree jump at the output so I'm gonna say the cat is doing its job. I must also note that I deliver pizza with this truck and mid day I can smell what can be described as an extremely strong ethanol smell coming from the evap system where the pressure relief tube runs under the driver side firewall/wheel well. I assume this is normal as the gas tank get's pretty toasty after doing city driving for an 8 hour shift and the excess pressure needs to be vented but dang is that smell strong, my coworkers have even mentioned it.

Here's the real kicker, I've been driving back and forth to my dads house up north every two weeks lately, I take the same route and fill up at the same gas stations to ensure the same fuel is being used for testing. Here are the consistent results over the last few months:

90% mileage is highway miles varying from 60MPH to 80 MPH over the course of 237 miles. Same route there and back with absolutely no deviation.
Headed to dads:
1st fill to 2nd fill: 95miles @ 14.1~14.7mpg
2nd fill to 3rd fill: 142miles @ 16.3~17.2mpg
Leaving to head back:
3rd fill to 4th fill: 20.1~22.8mpg
4th fill to 5th fill: 16.2~17.4mpg

It is so consistent, I'm beginning to wonder if the problem is with the quality of gas I'm getting. I even thought it may be an elevation issue where I'm climbing on the way to my dads but that doesn't make sense either because the whole trip back I should be getting great MPGs. Over the course of the 237 miles to my dads I only climb 136.28 feet according to https://viewer.nationalmap.gov/theme/elevation/##bottom and it's so gradual I believe we'll find it's pretty negligible.

 

You've certainly done your due diligence in keeping track of the variables in your case. I have to say - don't ever get a Prius or some other hybrid, because you'll be obsessing about your mileage even more
The mileage you are getting is within what is expected for a Montero. For example. Last weekend I drove 330 miles on 20 gal of fuel (premium from a some Mobil station in the middle of nowhere). When reading your post, when I came to P0136, it was like - there it is, there's your culprit... But you fixed it and no, it's not it. The one thing that comes to mind is possible variation in ethanol content in the fuel. The stickers on the pumps say "May contain up to 10% ethanol". Notice the "MAY" part. Which means that it can be 10%, 7%, 5% 2.5% or 0%... That all depends on what the refinery is mixing on a specific fuel run. I'm not working with fuels, so I do not know how often they may adjust those ethanol percentages, but when it says "May contain..." To my ear it means - content could reasonably vary. On another forum that I frequent, there are literally word battles over the inferiority of ethanol mixed fuels. Those arguments are supported by ables, Mega Juels of energy, Kcal calculations, etc. and nearly the same level of OCD (please don't take it the wrong way, I have that too) of trying to keep up with fuel records and mileage tracking. Anyhow, all this makes me wonder if the mileage differences you are observing are related to fuel alcohol percentage. The only way to find that out, is by - 1. Finding a gas station that sells Ethanol Free gas and driving on that for awhile, and/or 2. Taking a fuel sample every time you fill up and testing it for ethanol content (water column method). Then compare the mileage vs ethanol contend results.
One othe quick thing - is there any difference in prevailing winds on your route? Also, don't drive 85 MPH if you are trying to get better mileage. I'm sure you're well aware that 55-60-65 is optimal speed for that. Going over that speed increases wind resistance significantly and with that fuel consumption. Additionally, if you're traveling faster than that (say 65/70), then the smallest variation in speed (even 1 mph) will change fuel consumption. I don't have numbers to back it up, but it's physics and there is some rule for fluid dynamics that relates to that.
Anyway, that is some interesting puzzle you got there
I hope my rant makes sense and is at least somewhat helpful.

 

I never considered wind resistance, this is beginning to make a bit of sense then. on my trip up to my dads, not only am I doing 80mph during that 95 mile stretch and I'm getting that wind resistance but I'm also driving uphill albeit gradually, so i'm basically seeing a couple parasitic factors here. After that 95 mile stretch the speed limit goes down to 70mph and then again to 60mph for awhile which I suppose is why I start seeing the increased mpg on my way up there and I see it reverse on my way back. This may explain the varied highway mileage but not my 13ish city, that's an entirely different set of circumstances.

I would have to say at this point I'm obsessing over the mpg because I don't want to say I wasted money on parts I never really needed. The ol monty is running better than ever so maybe I should just count my blessings.

 

Ok so this morning I started it up and BAM! That pesky P0136 code came back on, I think I found my culprit. Not all Denso O2 sensors are created equal apparently and it's time to order a new one...again.

 

Ok, so I finally had some time to look further into this before I go about replacing the sensor. I decided to watch a few videos on O2 sensors and tested them and they are reporting accurately (good thing I didn't just replace it). While out delivering pizza today I got a wild hair to start a data log in Torque Pro for my Fuel Trims. If anyone here can make heads or tails of this, I'd be eternally grateful. If not, well, I need to attend a few youtube classes on Oxygen sensor diag (looking at you ScannerDanner and South Main Auto Repair).

Here is is the Excel log file:
https://drive.google.com/file/d/1moU...ew?usp=sharing

 

Nuevos, I looked at the data you provided. It is quite interesting. What I see is that at idle and when you just start accelerating, combined fuel trims shoot up pretty high. When engine is running at higher RPM (above 2000) almost always fuel trim is going down closer to 0 value. To me this is an indicator that you have a vacuum leak. Possibly a cracked intake air hose. When engine revs up, it shifts a little possibly letting the crack open/close slightly and thus affecting air supply and fuel trims. Occasionally I see a pretty high bump in fuel trim when engine is running at higher RPM, but that is more random. It can be due to same engine shifting when hitting a bump in the road or making a turn (?). I'll try to attach the file with your data "tweaked" with conditional formatting. You can see the patterns in RPM and Combined fuel trim nearly always going in opposite directions. I hope it works for everyone.

I do have a question related to your data stream: I see Fuel trims (Long and Short term) for Bank 1, but nothing for Bank 2. Do you have an option to collect Bank 1 and Bank 2 Short and Long term fuel trims or is this a limitation of your software. Don't worry about O2 voltage data. The sampling frequency is too low to make much sense of data in this case. The O2 sensors are working, so no worries there.

 

Thank you so much! I have tested the intake hose a multitude of times with both ether and brake clean and haven't had any results thus far but as you said it may flex a certain way. As far as the data stream is concerned, I selected Bank 2 ST and LT but for some reason it didn't log, I probably hit the wrong button as this was a spur of the moment choice and I was in a hurry to make my next delivery before the boss got mad lol. All these numbers are confusing but I'm willing to learn, this information will help greatly!. I am using Torque Pro with a bluetooth scanner.
You'd be surprised what features they've added to that app over the last few years.

On another note related to a vac leak, I have been hearing what sounds like a vac leak when I apply a little throttle but only from the front of the engine bay which I find odd. I've sprayed brake clean and ether just about everywhere I can think of to try and find the leak but nothing so far. I'm beginning to wonder if I have a leak in the intake manifold or plenum somewhere where I can't get enough ether/brake clean in order to test it. I was just about convinced it was a pulley somehow hitting the right frequency to make a similar noise but since you mentioned the possibility of a vac leak I'm going back down the rabbit hole.

EDIT: Ok so I just went outside, started the engine, and proceeded to soak that intake hose in ether while I flexed the intake hose in all sorts of ways while keeping an eye on my scan tools RPM readout and spraying more as it dried out, even switched to brake clean and did the same thing. No change in idle rpm beyond what's normal. I'm feelin a little light headed from the ether atm lol.

EDIT 2: Wondering if it could still be an exhaust leak somewhere, I have replaced every gasket but that doesn't mean there isn't a crack somewhere before the sensors.

 

It sounds like the Torque Pro software you are using is a really good stuff. Do you have an option to GRAPH fuel trims in real time with it? If so, I'd suggest you do the following:

1. Get a small propane torch (the kind used for sweating cooper pipes - https://images.app.goo.gl/DL97oF6zgmMgLbKu5 ) Any kind of similar torch will do.
2. Have a fire extinguisher near by (you will not need it, but you should have one when working with fuels...).
3. Start engine and let it idle.
4. Turn on live data for Short Term Fuel Trims on both Bank 1 and Bank 2 on your Torque Pro software. If you can make it show as a graph, it will be easier. If it only can show numbers, it will work too, just might be a little harder.
5. With engine idling, note the fuel trim numbers. Then open the propane **** on the torch (NO FLAME!!!!, just fuel) and stick the end of nozzle near the manifold. You need to have propane flowing good for this - wide open, not just partial.
6. Keep it in one spot for several seconds and watch your fuel trims. If numbers stay steady, move to a different spot. Work your way all around intake manifold, throttle body, any vacuum lines, intake air hose, etc. Wiggle hoses and air intake while watching fuel trims.
7. If you manage to get close to the spot where the vacuum leak is happening, the fuel trims will go down while you are feeding propane to that spot. As soon as you remove propane, fuel trims will go back up.

This might sound on the crazy method, but it is actually a very safe way to test for vacuum leaks. The propane delivered in this way is very safe, since it will mix with air and gets diluted past flammable point outside the engine. One thing you got to pay attention is belts and spinning fan. Got to be very careful with those. I've done it on my Montero and I found the cracked air hose which I could not diagnose with any other attempts.

 

You are just awesome man, I will go pick one up tomorrow and give this a try!

 

Ok so I got caught by a member of the apartment committee and they apparently were none too happy that I had a potentially explosive container on the property. I had to turn in the propane torch to avoid a hefty fine or getting evicted.
(It's my fault I didn't check the rules first or ask before I did this)

As a result I got creative and did this:


I put sandwich bags over the ends of the intake hose, used the existing clamps to seal it off as best I could, then I attached a hose to the crankcase breather port on the intake hose, I proceeded to pressurize the intake hose and sprayed it down with soapy water much like you would a tire. I saw no leak coming from anywhere after extensive testing. When you pressurize the hose it actually expands so it presents an opportunity for a crack to open up quite easily. A visual pre inspection under a 1200Lumen lamp showed no internal cracks or damage either.