Originally Posted by Naeos_Valkarian

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.

Originally Posted by HunterD

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: https://youtu.be/hbsnzC3nXOU then just dive in... We all are stuck at home, so might as well educate yourself

Originally Posted by HunterD

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: https://youtu.be/hbsnzC3nXOU then just dive in... We all are stuck at home, so might as well educate yourself

Originally Posted by HunterD

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.

Originally Posted by Naeos_Valkarian

Since this thread was revived unnecessarily, I might as well chime in and update anyone who's interested in what I found. Exhaust leaks are the culprit, I have tried everything under the sun aside from welding everything together or getting an entirely new exhaust system crafted. I fix all the leaks and I get amazing MPG for about 3-4 days and then it's back to "normal". I have found that the main leak(s) are at the manifolds, no matter what I do the bolts keep backing off. Thinking about putting a tack weld on each nut to hold them in place. I have tried all manner of exhaust gaskets at each joint, copper gasket maker, and a combination(later being more effective), and no matter what, they start leaking again in short order.

I have resigned myself to a life of accepting terrible fuel economy but hey, it runs good.