I have a '92 Custom with an in-line 6cyl 300ci engine and auto trans. I bought the truck used in '96 with 25k miles on it. It has about 65k miles on it now. In the past I used to run only 93 octane gas and I got decent milage, possibly near 20 mpg on the highway. Recently I had been talked out of (by my father-in-law no less) running high octane and switching to the factory recommended 87 octane. It should run fine, I was told, 87 octane is what the factory set it up to run on. I am currently running 13-16 mpg combined milage. Ouch. Worse yet I get pinging on hills, and I have a worsening problem of stuttering or gallopping at highway cruising speeds. My plugs, cap, and wires are less than a year old (under 10k miles), as well as fuel filter, and air filter. I used to think it was ignition, now I am being told that it may be a timing/computer related problem called "lean-burn missfire". I did get a "check engine" light a couple of weeks ago and will try to read the codes today. Any suggestions? Please help. I am on the verge of switching to a smaller truck due to milage.

 

i say go back to premium gas and see what happens. after you fill up if you can go home and take the battery cables off for 30 minutes to let the computer reset itself and see if it works.

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95 F-150 s/c XLT short 5.0 4x2 4R70W trans. 3.55
Cat-back duels with 40 series FlowMaster and echo tips
K&N FIPK, Superchip
Dunlop rover touring 255/70R15
Accel 8.8 custom wires, accel superstock coil, accel cap and rotor
Motorcraft platnium plugs
Smoke color bug deflector and vent visors
Better built tool box
Future mods:
Tint job, and a pair of painted oem style cab steps
Front and rear anti-sway bars

 

Hi Tim,

These vehicles are tuned from the factory to run specifically on 87 octane gasoline, and can exhibit some driveability & mileage symptoms when you use a different gas. Unless you use a performance chip, it's usually best to stick with regular gas for most of these vehicles, simply because that is what they were specifically tuned to use.

The first thing I would do is as was suggested, disconnect your battery, to erase the accumulated memory in the computer, and get a "fresh start". Then either go back to premium, or stay on regular. Disconnecting the battery should take care of your "pinging", unless the quality of your regular-grade gas is just too poor, then maybe use a mid-grade.

Since you've been using premium for a long time, your computer has adjusted it's fuel curves (up to the limit of its ability) a bit most likely, and it may be running a bit lean now on regular. If that is the cause of the detonation, and it can be, then doing a battery disconnect should take care of it hopefully. We suggest disconnecting the battery, and turn on your headlights for 5 minutes to drain all residual voltage from the IC's & caps. Then re-connect the battery and start driving again, and see what you get.

Those "Big 6's" can live very long lives, 250,000 miles and more with regular oil changes & maint.

Good luck!

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Mike Troyer
Performance Products, Inc.
National Distributor of Superchips
(540) 862-9515
Email: mtroyer@compuserve.com
Performance Products F150Online Superchip ordering system: F150Online Superchip Ordering System
First National F-150 Online Rally Info:https://www.f150online.com/rally2000/index.html

 

As I would agree with 95FordMan but I would suggest taking the cable off before changing back to the premium gas to see if it helps your mileage. Keep in mind that it will take a few hundred to possibly 1000 miles before the computer re-learns your style of driving to get the best possible numbers for mileage. I had an '87 with the 4-speed manual and also ran 93 octane, had to replaced the catalytic (sp-??) converters on it at about 275,000 kms (172,000 miles). People say that by using the premium gas that the excess that doesn't get burned ends up in the converters and eventually will make them unable to pass the exhaust properly and then the truck will slowly have it's upper top speed dropped because of the rising backpressure from the cats. Mine started when I wasn't able to travel faster than 110 km/hr and progressively dropped till it was so bad that I couldn't do 30 km/hr in 4th (overdrive) gear. Never was able to get the truck to run correctly above 2000 rpms either. If I would have cut the pipes before the cats, I would have had no problems driving at higher speeds until I either lost my hearing or got it fixed. Another thing to have a mechanic check is the speed density sensor, sometimes they can start reading improperly. Hope this helps and good luck.

Have Fun & Keep on Truckin'
Dean

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1994 F-150 XLT Supercab, Long Box, 302 EFI, Automatic, Royal Blue in and out, Captain Chairs. Chrome Rims 15x8's, Michelin LTX M/S 105S 235/75R15 (Summer), OEM Aluminum Rims 15x8's, Michelin LTX M/S 108S (Winter), Lightly Tinted, Soft Tonneau Cover, Ford Receiver Hitch Cover & Lock, J&J Enterprises Stainless Steel AirDam with KC driving lights, Lund Supersteps, Lighted Grab Bars, VTech Slotted Taillight Covers, Alpine CDM-7829 Head Unit, PPI 4800 Amplifier, Kicker 10" Solo Baric Sub with Custom Box and Kicker Impulse 6.5 Mid & Tweets.

 

I'm no mechanic as many of you (Mike) will attest to but.... Could it possibly be a vacuum leak? Just some food for thought! Good luck and let us know what you come up with cause many of us have the same engine!! Thanks!

Neal Coppola

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'94 F-150, 4.9 6cyl., K&N FIPK, Mangecor 8mm wires. Gibson 3" Side Swept, MSD Blaster Coil, superchip. Just stolen: Pioneer DEH-P835R Head Unit, Rockford Fosgate 200a1 pushing 2 RF 10" subs. Pirelli Scorpion A/T 255/75/15. Est. 200 hp 300ft/lb. Torque! Not bad for a straight 6!! Also driving the girlfriend's '94 Mercury Grand Marquis.