Ok so I'm beginning to suspect the issues I'm seeing are with the fuel I've been getting. I just took a 500 mile round trip and the first pit stop (92 miles) saw me sitting at 16.8 MPG highway, I filled up at Walmart and took off. Next pit stop was at 142 miles and I saw I was getting 20.9 MPG highway! I filled up on my trip back at another walmart(143 miles) and again got 20.8MPG. Then lastly, I got back and filled up at walmart to discover yet again, I was getting 20.5 MPG Highway. Then today, on a mixed tank of different sources of walmart fuel, I actually managed to achieve 15.7 MPG city (89.2 miles) during my delivery shift, which is a couple mpgs better than I was getting before my trip. I will report back tomorrow after my shift with my mileage info, however if I'm right, I expect to see somewhere around the 13ish mark again.

Edit:
I must note that I've been getting the same grade of gas each time. 87(up to 10% ethanol)

 

Did you ever figure out what that hose is supposef to be for? I dont have that on my full size Montero, so I can't tell you what should connect thete.

 

No I'm afraid not. I got back under there and look some more and honestly it looks like it just terminates in that area. There is actually a small O shaped plastic mount that the hose fits into along the inside of the fender well.

 

Alright so update time. I waited two days to gather more data. What I've discovered is that I'm still lost, I filled up yesterday after my 114 mile shift and got what I expected which was 13.5 MPG, I filled up today after my 98 mile shift and got 16.7 MPG. My last three shifts have been with regular old 87 grade Walmart gas from the same store. I checked for vacuum leaks today after work and exhausted half a can of brake clean and didn't notice any change in idle rpms using a bluetooth scan tool and my ears, I saturated the intake hose, my new catch can and all the vac lines I could find. I'm left wondering if it's the MAP sensor or a faulty O2 sensor(s).

Edit:
So after writing this I decided to get out of my chair and go test the MAP sensor. I watched a youtube video by "Ratchets and Wrenches" on how to test this sensor. When idling, the sensor voltage sat around 1.7v to 1.8v. When I slowly gave it some throttle the voltage never even budged, so revved it suddenly and the voltage spiked to 4.3v and then settled around 2.03v. Upon dropping the throttle back to idle the voltage did not decrease, in fact it would not drop below 2v again. Sometimes when I would suddenly rev, the voltage wouldn't jump at all. My conclusion is that the MAP sensor is faulty. Going to buy a $200 NTK replacement. There goes a portion of my paycheck.

 

Nice detective work. Hopefully you finally found the culprit responsible for the variable fuel consumption rates.

 

So I decided to try using the Standard Intermotor brand from Oreilly autoparts just because it's more available, comes with a lifetime warranty, and was slightly cheaper(Can also get my money back a whole lot easier too). I installed it after work and tested it the same way I did before and saw the same behavior with the new one with the only difference being that the new one wasn't as sluggish as the old one and always responded to throttle blipping and WOT conditions. When I first tested the sensor while it was idling, I saw it sitting around 1.4v~1.5v, slowly increasing the throttle did not change the reading at all. Blipping the throttle would see it raise to 3.7v to 4.5v then settle around 2v ~ 2.2v and it would stay there at idle after that.

I'll keep an eye on my gas mileage and see if this new part changed anything and I'll post the results after sufficient data has been generated.

 

What year is your Montero? Die2s it by any chance have a drive by wire throttle control? If it does, perhaps that is the sensir loop that is malfunctioning?

 

It's a 98 and it does in fact have drive by wire throttle control. There is one from the gas pedal then there is another cable leading to to the transmission, I don't know much about the transmissions electrical components but I believe this cable helps time the shifting. Also what is the sensor loop that you mentioned?

 

Nope, that's not it. I'm referring to "drive by wire" as in - there is NO trottle control cable. Instead you have a accelerator position sensor connected to PCM (via electrical wire) and an electric motor that opens and closes throttle plate based on PCM output (again - wire connector only). Nearly all newer cars these days have that. I hate thit setup - you can't work on them, or at least it's harder to do that. You got regular good old school throttle control. I was hoping that if you have that DBW system, perhaps ether accelerator sensor or control motor is bad, since MAF signal doesn't respind to slow acceleration, but will when you blip the throttle.

 

Interesting, I always thought that "drive by wire" was literal. Heh, shows what I know. Figured I'd mention there was no change in gas mileage after my shift today, 14.6 MPG was my average. I beginning to suspect there may be an issue with the new O2 sensors I installed, I was thinking about it at work today and how strange it was that I got 17 city for a day or so then it went away. My engine is burning oil from the valve stem seals, I'm wondering if this borked the new sensors. I changed them years ago but I didn't realize they had gone out again until a few days after I changed the sensors.