I have a 1998 F-150 5.4L 4x4, 3dr. cab, short bed, E40D trans, with a cap on
the bed. I bought it in 2006 with 69,000 M on it, including being driven to my
home state of Michigan, from the state of Washington.
When I got it, it was Heaven. I drove a 1987 Nissan D21 2W drive w/a 5/spd
for 19 years! It gave me 208,000 miles. Died of Cancer, salt induced!
It was getting 14mpg around town, and 17 on the hwy. At around 100,000, it
started falling off steadily. I bought some new tires, which it needed, and I replaced every fluid in it, as well as the fuel filter, air filter, and trans filter. All
the fluids I replaced were done w/synthetics. I even replaced the fluid in the
power steering pump.
When gas was $4.00/gal., I was getting 12/13 all the time, and going broke. I
have replaced the plugs twice, and the only time I have had a miss, is when I
had a heater hose leaking onto the right rear coil. I fixed that. I am currently
getting 10/11 overall. I have gotten repair manuals from the local Library, and
tested almost every bloody thing under the hood.
There are only two concrete symptoms I can currently describe that might be
of notice. Sometimes it idles slightly rough when it is warmed up, and when I start it cold, it surges forward, as though it went into gear, and came right back out. I have been reading the forum quite a bit, and I thought it could be the coils, or the O2sensors. I thought it might be the cam phasers until I saw
that it doesn't have them. Thank God.
I don't want to buy what it doesn't need. I was going to buy coils from that E-bay site, but they won't reply to my questions. I could do the O2sensors, if
that is it, but I'm stumped. I'm an old school guy who grew up changing points
and caps and rotors and plugs. I would appreciate any suggestions/wisdom.

 

You may want to try cleaning the MAF sensor located in the intake tube between the air filter and the throttle body. Use a spray made specific for that such as Gumout MAF sensor cleaner or others. Just spray it down good. easy to do and may help.

 

O2 sensors could be part of the problem. If you replace them you only need to replace the upstream ones as they're the only one that have anything to do with A/F ratio. The downstream ones just monitor that the cats are working properly.

Might also be worth pulling the TB and cleaning it. Make sure to clean out the EGR ports on the TB while you're at it.

Definitely clean the MAF like Dynotech suggested if you haven't already.

While it's unlikely at 100k your cat's might be clogged I suppose. That would certainly kill mileage. Might be worth getting a cheap vacuum gauge if you don't already have one and checking intake vacuum to rule this out.

Anyway, best of luck. I'm dealing with the same crappy mileage right now myself.

 

I notice with my truck if I get straight gas I get better gas milage then getting gas with ethonal in it.

 

Actually, I have already cleaned the MAF, although I didn't use a specialized cleaner for it. I have also replaced the PCV valve at the suggestion of one of the counter guys at a local auto parts, who owns an F-150. I also try to avoid the use of gasahol, due to it's propensity to collect water. I think I will go to the website I found, and buy the O2sensors. They are less than $30.00 each. Thanks for the help.

 

Buy ONLY Motorcraft or Bosch O2's, or the mileage may get even worse!

DG508 coils from either Global Automotive or Uneek Supply on Ebay are a safe buy. Use ONLY Motorcraft OEM plugs unless you want to spend big bucks for Denso Iridiums. The Triton doesn't like Autolite or Bosch plugs at ALL. NO antiseize, 28 ft/lb, and dielectric grease in the boots.

It wouldn't hurt to take it to a shop that has a Motorvac or BG machine and getting an intake cleaning job.

 

What did you clean your MAF with? electronics cleaner is OK, or any cleaner. But it absolutely has to leave no residue on the MAF, or you have not done any good by cleaning it. Also make sure you don't damage the wire.

 

If you are not throwing any codes, then most likely, everything is working properly. The only way to help mileage is to slow down, pump up your tires, or regear. Nothing else will help enough to see a difference.

 

I cleaned it with Tuner cleaner for TV and Stereo tuners.

 

Do you have any spark knock? Also, the PCV valve is important, it's VERY IMPORTANT that you use the correct one. I haven't found a 3rd party valve that meets Ford spec yet. So it's important that you run a Motorcraft part only.

Other valves are way to restrictive, in turn, lead to oil consumption and are hard on your engines other parts as well. Gaskets, - allot of components in general, -she can't breath!
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Yea, I don't believe in just hosing the MAF down with a cleaning agent is enough IMO. I always clean it by hand. It's not recommended, but it can be done fairly easily. Use a good electrical cleaner, -sounds like you have one. I'll use alcohol or a paint prep cleaner. I won't use nothing CRC, even MAF cleaner. MAF cleaner leaves a residue, believe it or not. That's just not right lol.

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Replace your upstream O2's, it's time. O'Riely's/Murrays carry the factory Bosch replacements.

 

The truck I just bought with the bad motor had good oil in it and still to this day doesn't burn or leak any. Two days after buying the truck it went in for an oil change and they brought it to my attention that the pcv valve looked to be original and was broken. After looking at it the pieces on the inside were actually broken off (not sure how) but they were and now I assume it had something to do with the break down of the motor at 120k. Along with poor maintenance I'm sure.

Thanks for the heads up on the crc products.. I've used them on my last 5.4L and wasn't too happy with it.

 

That may or may not be OK - a lot of those cleaners leave a residue designed to lubricate potentiometer windings. If the can specifically says non-residue it's okay.

 

I replaced the O2 sensors, no difference. Did an R&R on the EGR, cleaned the ports, valve
and Throttle Body. I wanted to do a vacuum check to see if the Cats are working, but I couldn't find a port to hook up to. There are no spare vacuum ports on the manifold. So far, nothing I do seems to help. I also contacted the guy who runs Global Automotive in Miami. What a jackass! There is NO WAY that I would buy coils from that idiot! He actually accused me of trying to interfere with his "Business", simply because I asked about his warranty procedures, and the stability of his business. What good is a lifetime warranty if the place goes under? His prices may be cheap, but I'll bet the coils are too.

 

They have been around for a while. Only other option is to go to the dealer to pick up motorcrafts for $89 each. Or autoparts stores can get you some off brands for $45 each that wont last a mile. Half the people on this site use Globals or Uneeks cop's with no issues. So far they are satisfactory, otherwise its empty your pockets.

 

Does a 98 have the COPs or the Coil Packs? My 97 has the coil packs with plug wires, and my memory seems to think the change to COPs was in 99?