Poor MPG, Good O2 Data with 4406 2wd swap

[Anticitizen1]

Hello All,

I am getting terribly fuel economy in my 98 xlt v8 with the 4406 swap, about in the neighborhood of 13 mpg. My truck runs and drives well, no codes whatsoever and no skipping or misfires. Here are the specs of my rig as best as I can remember, and live data after that:

V8 GT40p with 143k miles, synthetic 5w30 oil, 4406 f150 swap. LT245 75R16 Kuhmo Road Venture AT51. K/N Air Filter (Recently Cleaned) MAF Recently Cleaned, No Roof Bars just a Lund Windjammer, Brush Guard on the front end. E87 fuel.

Now here's a couple things I already know. The Brush Guard, slightly taller tires with more aggressive tread cost me a few mpg. The aforementioned 13mpg is with slow accelerations and a light foot, still barely clinging to 275 miles per 21 gallon fuel tank and babying it the whole way. But the vehicle also got pisspoor fuel economy with the stock 235 75 r15 tires and no brush guard as well. As for tuneup and other information, the plugs and wires were changed at the dealer by previous owner in 2013, I've never touched them. I believe the original coil packs are on the truck. I cleaned both the MAF and my K/N Air Filter, no benefit was gained. I don't believe my engine has any vaccum leaks right now. I tested my o2 sensor on my MAC Tools launchpad OBD2 Scanner, and they all appear to be in good shape with good fuel trim. Here is my live data in Closed Loop mode:

Idle Speed: 699RPM-725RPM approx
B1S1: Voltage Range .105v-.825v, Short Term Fuel Trim -0.8% to +5.5%
B1S2: Voltage Range .085v-.575v, Short Term Fuel Trim 99.2%
B2S1: Voltage Range .110v-.895v, Short Term Fuel Trim -0.8% to +7.8%
B2S2: Voltage Range .090v-.390v, Short Term Fuel Trim 99.2%
Long Term Fuel Trim B1: +0.8%
Long Term Fuel Trim B2: 0.0%
MAF Air Flow Rate: 0.7 lb/min
Engine Coolant Temperature: 192 Degrees Fahrenheit (Vehicle went for 30 min ride before data testing)
Calculated Load: 34.1%@idle speed
Absolute Throttle Position:19.2%

So with this data, I believe my Upstream O2 sensors (for fuel adjustment) work properly, my Downstream O2 Sensors (for measuring CAT Performance) work properly, idle speed within reason, closed loop function and MAF appears to be getting good data and no fault codes since owning the truck. So minus the brushguard and tires, with 2WD and a light foot I would have to think AT LEAST 15-16 mpg would be reasonable to expect, but I am at best getting 13MPG. Anyone have any thoughts on this? I really appreciate all the help, and please let me know if I made any mistakes or you need something I forgot. Thank you for your time.

 

[CDW6212R]

Welcome, and I would adjust the 13mpg result by the diameter change you made. Note that about 30" tires is the most accurate speedometer size, and you went from 29" to 30.5" tires.

I've got a 98 Limited I began driving last November. Mine had 158k on it and wasn't maintained well. The plugs seem to be not worn out so I didn't change them due to lack of time. I have only changed fluids and the front O2's. The old O2 sensors were not seized in the bungs, I suspect they weren't very old, they didn't have Ford numbers on them either. I didn't see any change in mpg from any changes I've made.

I've usually gotten 16mpg or so with my first 98 AWD Mercury, it dropped to the 14mpg range with wheel/tires that weighed about 14lbs more each. I never did the O2's, and with 206k it needs some care now.

This "new" 98 Limited I am using delivering mail, it'll go 53 miles on the route and about 40 commuting, and average around 12mpg. With various mixed work driving, I range from 12-14mpg. My guess is I'm getting 9mpg on my route, and near 18mpg in normal driving. The actual figures have to be near that, or one higher and one lower, or the first lower and the 2nd higher. I doubt I'm getting 18mpg in normal driving, so my route mileage has to be at least 9mpg, which is good for an AWD 4700lbs truck delivering mail. I've gone 260+ miles on a tank with two days of route(106) miles, and put 22 gallons in once. These have 22 gallon tanks, all 4dr models.

Sorry to be long winded, I'm just laying an example to say you must have something amiss like the O2 sensors that don't test bad, or other air/fuel tune issues that are hurting mileage a lot. How much do those wheel/tires weigh? They need to be under 60lbs, about 55 was stock. My Mercury with the first 18's weighed 72lbs each, those killed performance and economy.

 

[SWIGIN]

Your math is good IF you are sucking all 21 gallons out of your tank....which you aren't. I just checked, if it takes 19 gallons to fill your tank with those miles your getting about 14.5 MPG without adding the extra miles the tires are giving you. I'd bet your closer to 15 then you think

How much does it take to fill is the number you need to divide the miles by. And don't forget your going farther then the odometer is telling you with big tires.

 

[Anticitizen1]

I knew I forgot something in my post! My estimate of 275 miles is in fact the adjusted mileage after considering tire height. I may try designing my own larger air dam for the front end, and continue looking for other troubles in the system. I just want to avoid throwing money at sensors, been there done that haha. I still have to put Bilsteins, 1" rear addco sway bar and touchscreen head unit in too. Thanks for your help guys!

 

[CDW6212R]

Keep at it, doing good maintenance stuff takes care of most problems. The O2 sensors were lower this last time from what I recall them costing 10+ years ago. I think I paid under $60 total for the front two I changed. I bought two of each and one pair advertised as OEM had the Ford emblem on them, but the # was carefully ground off and the connectors had the plastic clocking tab removed. Obviously many OEM O2 sensors may be identical but for the connector.

For the rear sway bar, try to hold off and let Addco know that you are interested in a larger one, the same as the Explorer Express bar that is discontinued. There is a new thread just recently about the EE bar, and mention that Addco might make a run sometime this Summer. A 1" bar is not big enough in the rear, it needs to be bigger.

 

[Anticitizen1]

I was looking into contacting Addco about having a custom 1 1/8" rear bar bent up, but I saw on their website they're not currently accepting custom orders. Maybe I'll just hold off and get in touch with a representative and find out what their deal is.

 

[CDW6212R]

Someone in that thread about the EE bar called Addco recently. They said they were too busy right now, but maybe in Summer they could be caught up. I don't think I need another one, but I like helping others to get the parts.