yeahford

my 98 f150 reg cab short box 4.6l 4x4 is having some issues. truck has 185k miles. the truck doesn't have much power. it is getting terrible mpgs 5-7 city driving. the only this has done to it is that it has dual pipes with separate mufflers. the only code i am getting is for lean fuel mixture. it is progessivly getting worse. i changed the fuel filter once and and the truck ran great. next day it ran the way it was before the filter change. i proceeded to check the dpfe, vacuum lines, and spark plugs; all were good. i then replaced the fuel tank, fuel pump, and filter(again). no improvement showed.

i don't know where to go from here, i am more than capable of fixing the problem myself but cannot identify it.

note: the egr tube was busted at the manifold and was welded back on, may leak slightly, don't know if this contributes to the issue.

also truck was like this when i bought it at 176k, but ran decent( not quite as much power as it should) and got decent mpg(12 city). has been terrible the last 4-5k miles.

please help, i don't want to get rid of it due to this issue.

yeahford

Bring this old thread to the top. The truck was parked and hasn't gone far since April. Now that we have a bunch of snow in Northern Michigan, I am beginning to miss her very much.

I really need some thoughtful help here guys. The old ford is just sitting there rusting.

I can answer any questions to best of my knowledge to help figure this out.

Please help me out. I couldn't even imagine how much a mechanic would charge to track this down and certainly wouldn't be able to afford it.

TomB275

Some ideas:
New motorcraft spark plugs, make sure air filter is clean or new, replace all O2 sensors (made big diff for my 4.2...no codes), make sure no vacuum leaks, if EGR leaks with valve always open this will cause poor mpg and power, make sure brakes are not dragging...feel wheels after driving and/or look for evidence of heat.
I would suspect the EGR is stuck open.
Others probably have more ideas.

yeahford

Thanks for the reply Tom. Do you have any idea about the change in performance due to the filter change?

Frank_Ford

I second the advice of Tom to look at the EGR valve. Change that tube (cost about $50) and make sure it operates freely. You can take the EGR valve off and clean it but don't get solvent on the diaphragm because the solvent will damage it. Make sure it is operating freely when you put it back together.

yeahford

Alright guys. It sounds like the egr could be "contributing" to the issue. Something to do with fuel is the main issue though. Changing the filter honestly made this truck run like it was brand new. I was excited when i parked it for the night because this had been an ongoing issue since i bought the truck over a year prior. Hop in the truck the next day to head to work and it was right back to its old games. Terrible power loss from the night before.

TomB275

If the filter change was so helpful, perhaps particles that got past and are plugging the injection system.

plumberboy

I also agree with the EGR not working. Vacuum leaks are tough to find. I would closely inspect the tube on the passenger side that goes to the back of the engine and the PCV valve.
You can use brake cleaner to locate vacuum leaks. My 97 was giving me issues last year. I found a tube that looked ok. By using brake cleaner I found it was leaking through the side wall of the tube.
You can buy a cheap vacuum gauge and T it in a line to check how much vacuum there is.
If you take your EGR off, do it carefully. The studs break very easily. EGR passages get clogged and cause bad milage too.
YouTube and this site has really good instructions on how to clear them out and testing / locating vacuum leaks

yeahford

Would you say that the particles that got past would cause the exact same running condition? Also, i changed the fuel tank then the filter (again)

TomB275

My son's VW had a rusting tank and fine particles got past the filter to the injectors. Rust particles can be very small.
When you replaced the tank was it rusted?
You seem to be doing the right things so multiple issues are possible.