Current MPG: 12.7 City & 16.3 Highway

For starters, I will state everything I have(n't) done to solve this issue.

#1: New NGK Iridium Plugs gapped to .041 and rechecked with a set of feeler gauges.
#2: New NGK Coil Packs.
#3: New Bosch Wires
#4: New Denso O2 Sensors. Upstream: 234-4645 / Downstream: 234-4633
#5: New Walker brand catalytic converter (Part#: 16556)
#6: New exhaust manifold gaskets and 4x downpipe gaskets/cat gaskets
#7: Thoroughly checked for vacuum leaks and solved a couple I wasn't aware of prior
#8: Cleaned my functioning spare EGR valve by soaking it in simple green for two weeks (The diaphragm was not submerged)
#9: Cleaned the MAF sensor and performed the unplug test to see if engine state would change ( Engine Ran rough once unplugged)
#10: Cleaned the throttle body and idle air control valve body
#11: Replaced the IAC with a Standard Intermotor brand from Oreilly Auto Parts (works perfectly fine, no dying at redlights anymore)
#12: Tried replacing the purge valve solenoid and EGR solenoids with used units, no change after swap.
#13: Cleaned MAP sensor (testing with voltmeter not performed just yet)
#14: New air filter installed

Interestingly enough, for two days after I replaced the catalytic converter my MPGs went up to 17 in the city and then plummeted back down to 13. If anyone has any ideas please chime in, I need to fix this before I burn out my new cat. At this point I'm pretty sure the purge canister isn't clogged/full, according to the reference thread, I should be seeing other symptoms if this were the case, and drive ability is not affected in anyway, drives great.

Also, what the heck is this line used for. I took it off and the plastic piece on the line is completely hollow/empty.

 

Sounds like you know what you're doing and have decent tools. Are you able to pull live data from your Montero while it's running? Your concern about burning up new cat is only valid if the engine is running extremely rich. Can you share fuel trim values (both short term and lomg term) while st idle and with load applied (driving). Any CEL error codes since you've done all this work?
About plastoc tube - where did it come from? It might be useful to post a photo of it in its "original" habitat.

 

No codes are present, LTFT was around 4.7%~5.5% last couple times I checked, STFT is around 1.6%~2.1%, seems to be thinking I'm running too lean while driving. I do have a short ram with a oilable cone filter, I wonder if this is introducing too much air which is throwing off the computer? I will have to try installing the stock filter to see what happens. I will have to find a solution for this if it winds up being the issue. The throttle response is so much better with the short ram installed, and I love how this engine sounds with it installed.

As far as the plastic tube is concerned, here is a picture of where it resides:
Weird vacuum line

 

First is the easier part - mystery rubber ring. I believe that vacuum hose is for feeding vacuum to purge solenoid. The rubber ring is (was) used to keep two additional vacuum lines (EGR valve feed and some other one nearby)together and to prevent them from rubbing on the intake.

The fuel trims you have are completely normal. You will NOT burn catalytic converter with that. To melt/burn the cat, you need UNBURNT fuel getting out past cylinders and into the cat. You don't have that situation.

Now onto my pet peeve... Sorry to tell you, by the hot air intake you installed does not do what you intended. I think you wanted a COLD air intake, but you got yourself a HOT air intake. The way your cone is mounted, it is sucking HOT air from the engine compartment. Hot air has lower density, thus lower amount of oxygen in it, thus lower "usefulness" for burning fuel. Even if air is flowing easier through that filter, that benefit is completely outweighed and canceled by the air having significantly lower density. You are not getting more air to your engine, but you are getting hotter air fed to your cylinders. If you do mount the cone outside of the engine compartment, or at least build an air dam that will shield the filter from the engine, the computer can adjust required fuel amount depending on how much air engine is ingesting. Remember, the MAS (Mass Airflow Sensor) has one job - to figure out how much air (by weight) is entering into the intake manifold. Air mass depends on both air by me and air temperature. The best person who can explain this to you is Gale Banks. Check out his videos on air density, he does not do anything without checking and running some numbers first. You can start here: then just dive in... We all are stuck at home, so might as well educate yourself

 

I understand you trying to educate me on the finer points of intake design and functionality, and perhaps someone else who reads this thread can learn from it but I'm already fully aware this is not the proper setup. The only reason I setup the intake this way is because it was easy and yielded better throttle response. I'm not trying to get more power from using it, just a better driving experience. Not trying to be rude, but I've already had a lot of people scold me for it not being setup "properly".

On another note, interesting to know that plastic piece is just used to keep them rubbing together. Kinda odd way of doing it I think. I guess I'll have to keep looking for other reasons my MPGs have tanked so hard.

 

Do you think it's possible that MPG calculation is based on slightly wrong amount of fuel recorded? For example, if you top off one time and didn't top off next time, your estimates will vary greatly. Drive around more (more fill-ups) and see if it gets more consistent.

 

I deliver food as my day job so I've been getting some good mileage. As far as fill ups are concerned, I fill it up till it stops pumping then give it a few more blips to ensure it's full. Good idea though. I also reset my computer by disconnecting the battery over night, we'll see what happens.

Thanks for the ideas.

 

Somewhere I've seen following method to try to get consistent fill levels - you pump fuel till the first click off, then wait 30 sec (to let system burp), and pump again till another click off. That seems to produce more consistent fill up level. I have to say ive never used this myself, since my OCD can't handle weird dollar amountd

 

I always stop on at least an increment of .25! I know the feeling. I will post an update here if anything changes, perhaps it will help others with the same issue later down the line.

 

Alright, I replaced the charcoal canister and MAF sensor with a junkyard unit. My avg MPGs went up from 13.2 to a whopping 14.6 in the city. Another thing I found today is that there seems to be a length of hose missing from the vent system. On the side of charcoal canister there is a purge solenoid and a one way valve much like a PCV valve. A rubber hose extends from the one way valve down the side of the wheel well and along the firewall where it just terminates right there. Is this line supposed to go anywhere?

Edit: Let me clarify that I actually got under there and looked really hard for some kind of connecting point but there is nothing there, no hardline or softline to connect to.