I have a 1990 F-150 XLT Lariat. I have had the truck for around 1 and a half years. I have a 16.5 gallon tank and could usually get around 270 miles on a tank. But lately over the past few months my mileage has progressively gotten less and less. I ran out of gas the other night with only 215 miles on the trip counter. I use this method of gauging because my fuel gauge tends to stick and not be very accurate

I change my oil every 3000 miles, the last change I also replaced my air filter & crankcase air filter. The latter had all but disintegrated. The engine had been running a bit rough and the new filters seemed to help smooth it out some but the gas mileage is still bad. My next move is to change my plugs and inspect my distributor,& check my 02 sensor. Does anyone recommend a certain type of plug over another, or does anyone have any suggestions of other things I could possibly check. Also my fuel tank will only take gas if it is pumped in very,very,very slow. This has been a constant problem since I bought the truck.

 

Ive always used Motorcraft Plugs ( BSF 44C ) gapped at .046 . Ive never been too impressed with all these fancy plugs that claim signifigant gains in mileage . The Motorcraft plugs seem to last about 15 to 18K miles before they need replacing . How many miles on your truck ? I have the same model & engine truck as you & I thought the tank was closer to an 18 gallon tank vs a 16 ?

 

I agree with using Motorcraft, or Autolites. Sounds like you take good care of your truck. Don't know what your odometer reads, but I would do this: have your fuel pressure regulator checked, when they die mileage goes way down; replace your dist. cap and spark plug wires if they're original; check your timing...should be at 10 degrees BTDC; go to (http://www.dalidesign.com/hbook/eectest.html) and learn how to run the engine self-test to get error codes through your dash check engine light...that will tell you if any sensors, O2 etc., need replacing. The self test is the best diagnosis I've found for my 90 F150 XLT and its not hard to do. Once you're assured all systems are GO, consider side-gapping a set of Motorcraft or Autolite plugs. Put a lot of pep back into my truck, and boosted mpg up to 17 in town and 19-21 down the road (depending on whether the tailgate is up or down). I've run a set of sidegapped plugs for about 5K miles now and no problems. It reduces plug life, but plugs are cheap compared to gas these days. You can search this site from the main page to get more info on sidegapping plugs. There's a website that shows good photos of it, but can't remember the address. Let us know how it comes out.

 

My truck just rolled over 106,000 miles a couple of days ago. Forgot to mention it was a 4 wheel drive. How much does tire pressure and running with the tailgate down make? I recently put new tires on my truck. Went from the stock size to a 31X10.50 BF Goodrich All-Terrain. I thought my mileage might drop a bit, but this is ridiculous! You are right about the tank size. It is 18.2, I must have read the wrong line in the owners manual. According to the tank size and the mileage I was getting before I was getting around 14.8 mpg on the average. I would be very happy to get back to those numbers.

My mpg started getting worse before I had the new tires put on. It had been hanging pretty steady around 13.7mpg till about 2 weeks ago when it plummeted to 11.8mpg. I drive 204 miles a week back and forth to work plus errands around town, so I have really started to feel the crunch on the wallet. I don't know a whole lot about mechanics and I really can't afford to take it to the doctor, so all first aid will have to be performed by me unless it turns out to be critically wounded. I don't mind messing around with it though. It really is gratifying to fix you're own sled.

 

To Joe M,

Have you adjusted your odometer to take into account the increased tire size. If you haven't, your mileage caculations will be off. Your odometer and trip meter, if left unadjusted, will read less kilometers than those actually travelled. divide your new tire size by your old tire size and then multiply that value with your odometer reading to get a more accurate figure for km travelled. Your speedometer reading is also affected in a similar manner.

WHaynes,

93 F150 4.9L 4x4
4" ProComp StII Lift
BFG 33in 4.10's
Detroit TrueTrac's

 

tm,

How is the best way to check a fuel pressure regulator yourself. I don't trust most mechanic shops anymore. There is only one locally I will trust to do work on my truck, but if I can check the stupid thing myself it would save me some dough.

Plus, I have begun to smell gasoline every now & then. It doesn't happen very often, but when it does it is noticeable.



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You can buy the gauge at parts stores, but it must be one that reads higher pressures. If you've got a 300 I6, I believe the correct fuel pressure is in the neighborhood of 40 to 45 psi...but doublecheck this in a Chilton's or Haynes manual, I may be off a little. The gauge screws onto the Schrader valve on your fuel rail. Again, if you have a 300 I6 the Schrader valve is on the back of your fuel rail...back up by the firewall. Don't know the exact procedure, as I haven't done it myself...had mine tested at a shop I trust for $15. A Chiltons or Haynes manual should list how to do it, or maybe someone else will post the procedure. When your fuel pressure regulator is bad, your truck will drink gas and run real rich...mine didn't run bad when this happened, but I could literally watch the fuel gauge drop while driving...plus the tailpipe mouth was sooty black. Regulators don't cost much, about $35 to $50 I think, and it didn't appear real hard to change it...I had the shop do it cause I didn't have time myself, the truck was already there and their price was real reasonable.

 

The fuel pressure for the 300 should be 40 to 60 Psi . If the regulator is the problem most of the time you will see fuel leaking when you remove the vacuum line from the regulator .

Joe M , I have 31X10.5 on my truck & I run 30 psi in the front , 25 back , they wear evenly . What I did to correct the speedometer was replace the speedo gear . Ford makes several color coded , different number of teeth gears that you can use to get your speedometer pretty accurate . I happened along a radar cop one day & told him I wanted him to check my speedometer after I had changed the speedo gear . I made 3 passes at different speeds & wasnt off any more than 2mph . Each tooth of the speedo gear will add 2.5 mph . I cant recall what gear I have in my truck ? If you count the number of teeth on your gear that is in place now , Ill have a look at mine in the next few days & see what mine has & post it . Do you have 3.08 gears in the rear end ?