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98 f150 4.6 8.2 mpg!?

4K views 21 replies 8 participants last post by  AutoGear 
#1 ·
I've had this 98 F150 4.6 for a year now but didn't drive it much. Now that I am driving it I find it gets around 8.2 MPG, mostly city driving but it gets about the same when towing my 3200 lb race car with a u haul car trailer at 70-75 MPH. It runs good under either circumstances but the mileage sucks! It's all stock and no codes have been set. It has 247K miles on it.
Any ideas on what to start checking on this thing to improve mileage? Vacuum at idle seems to be good and throttle response is what I'd expect so I don't think the cats are stopped up. The f150 forums have been no help. Starting to wonder about those guys! LOL A lot of post on there about losing 1-3 MPG after an oil change. HUH?
Seeing a Chevy or Dodge truck with a real V8 in my furture. lol
 
#2 ·
The modular motors simply weren't designed for trucks. Most of the early 2v's make all their power between 3-4k rpm. Ford sticking with them so long says quite a few things. It's almost like if Chevy stopped making the sbc and just tried to make the best all around motor they could out of the 4.9L Caddy.

With that said, start with a basic tune-up (plugs, wires, filters, etc) and be sure to clean your IAC valve. What mileage are you trying to achieve?

Some other things to note is that the 4.6L is a bit of a mess of vacuum lines as far as modern vehicles go. Even though you said you have seemingly good vacuum, double check all the lines for cracks or breaks. They're made of a very brittle, hard plastic. On my mod motor, I cut off all the line ends with about 2" of the old line left over and used the original ends to reroute it all with standard rubber vacuum lines from the parts store.

If you still are finding yourself having issues, these motors are notorious for carbon build up. Remove the intake plenum and check the plenum, throttle body and intake for thick deposits. I would suggest cleaning them all afterwards and running a can of some of the seafoam or some kind of other alternative through the engine per the product instructions.
 
#5 ·
A 4.6 pick up, from personal experience, and in good running/tune condition should be good for about 12-13 around town and as high as 17 open road, hauling no load to speak of, 9-10 or so with a 7000# trailer behind it. A 5.4 pickup (again, personal experience) is only about a mpg less. Both of my F150's, a '97(4x2) and an '01(4x4), were great running trucks. Even the current 6.8 F350 (4x4) runs about 10mpg city driving, 13+ open road @2000+/- rpm with cruddy 10% ethanol gas. As far as a modular engine not being a good pick up truck engine is pure unadulterated bull pucky!!!! There are just too many millions running with modular engines with many thousands of miles on them.
 
#6 ·
I never said they weren't reliable. Just not a great option for doing any kind of heavy duty work. The v-10 isn't bad at all, and the 3v are okay motors but the 2v 4.6L and 5.4L (the vast majority used in light duty pickups) are absolutely anemic. The 300 I6 and windsor V8s were much better truck engines.
 
#8 ·
Check the MAF and clean it CAREFULLY.
Also how about some platinum plugs?
Even more so....02 sensors. They can be sluggish to switch and give real poor mileage. They are a maintenance item...just like a spark plug. Change the upstreams...you may be suprised.
My Nascar F 150 here in the picture gets around 17.5 on the interstate at 75 MPH with my tools in the back.
It also pulled my poncho on the trailer for several hundred miles at my last move and I thought it did pretty well for 281 cubic inches.
It is now aproaching the 200 K mileage mark.
Motorcraft parts and Mobil 1 baby
 

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#10 ·
Check the MAF and clean it CAREFULLY.
Also how about some platinum plugs?
Even more so....02 sensors. They can be sluggish to switch and give real poor mileage. They are a maintenance item...just like a spark plug. Change the upstreams...you may be suprised.
Thanks Tech, those are all good suggestions that I missed. Might I add that the MAF cleaning is very important if you're using (or have used) one of the K&N-style oiled reusable filters.
 
#11 ·
Plugs, filters, clean intake with Seafoam. Check your throttle body too, and clean the back of the butterfly with a little carb clean. I have full size 2000 with a 5.4 and although it seems weak pulling a trailer it gets 13 mpg all over, every day. K&N air filter, Mobil1 and Platinum plugs. I started some research for a power chip, but haven't decided which one to get, but that's next.
 
#18 ·
#19 · (Edited)
Yes I meant the upstream oxygen sensors. The control the fuel trim which in turn is a fine tune to a achieve a stoichiometrically accurate mixture. 14.7 to one is ideal. A lazy 02 can stay low , reading lean, causing the mixture to go rich, wasting fuel.Plugs that are tired can cause a poor burn leaving unspent oxygen causing a mixtureto swing rich also.
Electronic Engine performance basics are the same across all makes and models. Specifics and designs vary, like fuel pressure , injector design etc.
I did see some chevy specific references, which happens here a lot, but all in all if you have any experience in electronic fuel injection, principles are relayive and one can fix anything as long as you can ID the components and you have PIDs to use for testing.
Basics like healthy spark, good fuel pressure and injector pulse, are all very basic forms of strategy that every EFI vehicle has.Also tune up parts play a very important basic role. Start with the basics.
 
#22 ·
Dumb question; are you sure your odometer is reading properly? If you change the height of your tire or regear the axle and dont compensate, it will be off. This would throw off your mileage calculations to some degree. Maybe you're fuel economy isn't as bad as you think it is if this was the case. I doubt this would be the sole cause of the issue however.
 
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