1987 F-150 XLT 4x4

How does the engine run?
Does it hesitate or bog when starting off?
Any skip or miss while driving?

Does your automatic transmission have overdrive?

What size tires do you have?

What is your differential gear ratio?

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I would install new Spark Plugs as the other guys said.

Since you're driving only 2 miles each way;
your engine RPM does not get high enough to keep the spark plugs clean
and the engine may not be getting up to normal operating temperature.
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You have a heavy truck, my 87 Regular Cab LWB 4WD weighs 5,200 lbs.
Your extended cab SWB 4x4 is at least that heavy, probably heavier.
But, you should be getting 10-13 mpg average unless
you are letting the engine Idle a lot.

My 87 4.9L was getting 12.5 average with 32 x 11.50 BFG tires
and now it gets 10 mpg average with 35 x 12.50 BFG tires and
nothing is wrong with it.
My C6 automatic transmission does not have overdrive and
I ran the air conditioner when I calculated the fuel mileage.

The correct way to figure fuel mileage is to fill the gas tank to full,
up to the neck, reset the trip-meter, drive the truck approximately 150 miles
or until the gauge is down to a 1/4 tank, refill the tank to full again,
up to the neck, write down the gallons pumped and the miles driven,
reset the trip-meter again. Below is how I calculated my results:
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Corrected odometer miles traveled for accurate MPG

1987 F-150 XLT Lariat 4x4

4.9L EFI (65K miles on rebuilt and balanced engine)
C6 Automatic Transmission No Over-Drive (1.0:1 3rd and final gear ratio)
Air Conditioner (Compressor on continuously)
3.55:1 Ratio
LWB

Odometer Mileage = 132 (Front tank / City driving, no trips)

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Pi 3.1415927 X 35” Diameter tires = 110”

Pi 3.1415927 X 29” Diameter tires = 91” (Stock P235's)

Divide 110” by 91” = 1.21 (1.21 = Correction factor)

Multiply odometer reading 132 X 1.21 for 160 miles traveled.

Divide 160 actual miles by 15.9 gallons gas = 10.4 MPG.

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Therealblinky

remove the lead brick from your foot, should increase mpg instantly

FORD blue blood

Had the same symptom kept filling the rear tank (cursing at the lousy mileage) and had no idea the front tank was filling up, until I accidentally opened it at a station and it was so full it overflowed. The return from the EFI was going to the front tank. I disconnected my front tank cause it cost next to nothing to do. there is a full explanation on this site.

fordfreakout93

ajay i have the same problem like you. i have a 93 f150 and i get seven mpg. i live in minnesota. we should still be getting over 10 mpg.

drewskeez, how the heck are you getting 15 mpg. ive replaced my map sensor, the mass air flow sensor, new fuel filter , plugs wire, rotor, cap, oil change, i have proper tire pressure, and i still get 7 . i dont accelerate fast or anything.

any other suggestions?

ymeski56

Error code scans! Could be as simple as a bad ECT sensor.
wrong thermostat or worse, no thermostat.

An ignition upgrade w/ base timing bump to a error code free, reasonable cylinder compression, clean injectors, good fuel delivery, desludged, descaled, de-carboned cylinders engine, would do wonders for perfomace & MPG at the same time!
http://fullsizebronco.com/forum/show...ht=timing+bump

My 87' 302 (83K miles) get's 20 MPG highway. No problem breaking the tires loose on dry pavement from a dead stop either.

I went through a couple month period where I dropped from 20+mpg to 10-11mpg for no apparent reason. It started when I started a job I was on, & my good milage returned after finishing the job.

For those two months, I only drove from the house to the job (<1 mile), the job to the supply house (<2 miles), & home (<1 mile). The moral of the story is, that's all the MPG possible in warm up cycle, before the engine reaches normal Op. temp. (or knows it reached Normal Op temp/ bad ECT sensor).

The ECU does not "Fuel trim" until Normal Op Temp is reached & it "knows" (via ECT sensor) it has been reached. Once I started driving longer distances my good MPG returned.

ajay5605

thanks for the suggestions guys
i now have seafoam in the gas tank and in the crankcase and i put it in and ran it out of the vacuum system and gumout fuel sys cleaner in the other tank lol we will see what happens with that. and im going to be cleaning the MAF tomorrow

ill keep ya posted

1987 F-150 XLT 4x4

Just how much Sea Foam did you put in the crankcase?
Over 8 ounces?

If you did...


Anyone else do this?

fordfreakout93

How much is sea foam. Where can i get some

1987 F-150 XLT 4x4

Any Auto Parts Store or the Internet.

drewskeez

I have no clue how but that's what the math said my truck is a 2x4 so that helps and so dose the flowmaster but I drive a lot on the highway and run it at 2000 to 2500