Abysmal MPG

I bought a '99 4runner, Limited, 4x4, supercharged (1st gen supercharger) with 205K on it a few months ago and I've been working on it to get it in running shape (Previous owner parked it because the water pump was leaking).

So far I've done the timing belt/water pump.
Replaced Radiator(manual trans model)/condenser and installed an external trans cooler
Replaced plugs/wires (Original plugs were still in it)
Oil Change

I'm getting a P0325 and P0330 code which would be knock sensors so I'm planning on doing those pretty soon but I just drove from Central Va to the coast of Ms and for the first leg of the trip I averaged ~17 MPG and then mileage started dropping and my last tank I averaged 8.2, no loss of power, I was continuing to cruise at 80-85, occasional shuddering from ~2.5-3.5K RPM.

Everything looks good on the scanguage except LOD, cruising it's showing ~50-60% load and with any hill it automatically pegs out to 99% load.
How does the scanguage pull the data to determine Load?

Also I understand there's a TSB for the fuel sender, when I first started out I would drive until the low fuel light came on and then put ~14.5 gallons in it but after the 3rd fill up I noticed that the fuel gauge was acting funny and it was starving for fuel with the needle indicating over 1/4 of the tank was left, I would assume a new sender would fix this issue?

 

Based on the research I've done regarding fuel sending units and the TSB for 1999 and 2000 runners, I don't believe it is worth the hassle to replace the necessary components to fix the fuel gauge. My gauge acts funny most of the time and I just keep track of how many miles I've driven. The fuel gauge issue should not affect MPG. I'm seeing normal MPG's with fuel gauge issues. As far as the sudden drop in MPG, it might be possible that it is knock sensor related but I am not certain. Maybe someone else can chime in and confirm

 

I'll have to check out the TSB and all related parts, I just did a cursory google the other day because something didn't seem right.

Where the line from the tank goes into the filter was damp when I pulled into the shop today so I'm thinking that maybe it was leaking there, I tried to replace the fuel filter but the front line is rusted to the filter so I've got to order a replacement line as well as the harness for the knock sensors tomorrow gonna try replacing the harness before the sensors.

 

Knock sensors are the reason for the doo doo MPG, ECU goes to fail safe which is basically give it all the fuel it can give, and keep timing at set value in order to combat detonation. Replace your knock sensors and sub harness and all will be right with the world again.

 

Timing fluctuates according to the scan gauge, do you think replacing the knock sensors is necessary? I got a sub harness, going to install it later this week but because of both codes being thrown I think that the sensors might still be good and the harness is the issue.

 

yes absolutely, it's not worth having to go though all that twice. Even though the sensors are pricey, your talking about almost 20 year old electronics that have signaled an almost an infinite amount of detonation to the ECU, time for fresh parts.

 

I have the type 4 (2nd gen) supercharger on my rig. Find out if you have the larger capacity injectors. As far as I know you can't put a URD 7th injector on the type 1 (1st gen) s/c, so the larger injectors is the only solution to the fuel problem(s). Also don't forget to ALWAYS use Premium fuel. 93+ octane at sea level, 91+ octane above 4000+ft. I would agree that while pricey anti-knock sensors would be advisable, especially if you aren't always using Premium.

Another possibility for the crap MPG is going 80-85mph. Any time I go faster than 65mph I can watch my fuel needle drop, especially over 70mph. I've just gotten used to the saying out west here of 5VZ-FE-MPG-LOL. It seems counter intuitive to me but I get some of the best mileage while driving in the mountains 7000ft-12800ft, 25mpg is fairly common. Down here on the immediate east side of the Rockies at ~5000ft, I get about 13 city/16 hwy.

As for the sending unit's fuel level gauge problems, it may also be more common with s/c installs. I didn't have any problems at all with it until after my s/c install. My main problem these days is that every once in a while it'll read E on startup & slowly climb up to what it should read in about 10-15 mins. I kinda just wrote it off as possibly being related to having to switch to a high pressure (90psi) fuel pump for the s/c. I'll have to check the TSBs, every time I check using my VIN it always comes up blank.

 

it will definitely use more gas with the supercharger you have to pay for the extra 70 horse power. it's all good don't sweat the small stuff lol larry b

 

also when you put in the new timing belt you turn the engine in the reverse direction 360 turn to seat the belt to line up correctly right because if you didn't that would explain the timing codes.

 

I went ahead and ordered both knock sensors, came out to $105 per sensor but it will give some peace of mind.

How do i check the injectors? I'm assuming I will see a difference in color (I.E. not brown?) on the injectors when I have it torn down to replace the knock sensors?
From the reading I have done on URD's website it appears as though I can run the 7th injector since it's a non-EGR motor, I'm pursuing running water/meth though instead of the 7th injector.

I've always run premium through it in the 8-9 tanks I've run through it, it did have one tank or regular out through it because my dad borrowed it and didn't listen when i told him it requires premium.

My average speed on my road trip stayed pretty close to the same between tanks and it dropped from 17 at the beginning of the trip to about 8, the only difference was altitude, I went from ~700ft to sea level, after talking with Gadget at URD I've replaced the fuel filter (OEM, previous owner apparently didn't know/care about maintenance) got a walbro 190 from URD (To be installed sometime in the next week when i have time, currently not driving the 4runner) wiring harness for knock sensors, knock sensors, and am going to do an oil change on the SC when I pull it (gonna try to rebuild it in the next 5K miles, no clue on what maintenance was done on the SC), we believe it's a fuel issue as the plugs/wires are new as is the air filter.
My N/A tacoma (2003 4 door 4x4) gets 13.5-14 on average because I drive like an A-hole and have load range E tires on it, I would expect similar from a stock SC'd 4runner, more if I baby it like I have several times and seen very little difference in economy (recently) when I first got it I could baby it and get ~19 according to the scan gauge.

Maybe in the future I'll get another sending unit, i'd rather spend the money I have on the more serious mechanical stuff as much as it bothers me that the fuel gauge isn't reliable.

It shouldn't be getting single digits, especially when 2 months ago I was getting 15-17 pretty regularly.

Rotated it a complete rotation clockwise, check alignment, and then did another complete rotation, checked alignment again, and then buttoned it up, did the same procedure on my tacoma and it never had any codes for timing.

 

Pulled the S/C today, hopefully I'll get it put back together tomorrow evening.