I just purchased a 2003 F150 4x4 with a little less then 104,000 miles.
I noticed that it tended to have a stumble when accelerating so we changed all plugs. Everything else looked fine but we started getting a misfire. Changed #5 coil and fixed the misfire but I noticed that it still will stumble and run rough when you turn a corner and accelerate at lower rpms 1500-2000 or so but run good at higher rpms. Changed fuel filter. Scanned it today and not showing any codes. Used Autolite Platinum plugs because the parts house I got plugs did not have Motorcraft. Was that a big no, no? What else could you tell me? I ran an older 4.6 175,000 miles with little trouble. Thanks so much for your trouble! Also can't seem to find any vacuum leaks.

 

I don't say this often, but search. I tried it, it works... Under 2 seconds, -

https://www.f150online.com/forums/19...me-advise.html

 

^^This
lol

 

Well surprisingly enough I have looked through posts on here regarding similar issues but never found one that was acting just like my 4.6 does. Instead of taking the time to make a wisecrack you could've actually been helpful!

 

Clean the MAF

 

Alright, - under normal acceleration (in a straight line lol) and at about 45mph right after the shift into OD, what's it running like ? - Is that real bad spot ? No problems there , what ? Check it if you can't recall.

 

Visteon or Motorcraft cops and Motorcraft plugs. Period. Order them online.

 

This pickup has a 5 spd. trans. and after I turn a corner I downshift to 3rd gear which is prob. about 20-25 mph and that is where it starts to stumble. I don't have a tachometer, so I don't know what rpm. After I get the rpm up, it runs like a champ. Thanks for replies! Sorry jbrew, I shouldn't have snapped at you. I know you run into people frequently that do not take the time to do basic research and I understand how you would feel about that. Looks like I may have to reluctantly change to Motorcraft but the truck had motorcraft plugs in it and it was running almost exactly as it is now with tune up so I suspect the plugs are not the culprit, but I don't know that for sure.

 

Well, it could be tired coils or dirty injectors. Visteon 60-1000 coils from Ebay, Techron in the gas. Could possibly be a sticky EGR valve.

Autolite plugs have heat range issues.

 

Thanks I believe I'll try the Techron and check the EGR and the MAF. At least I can do that with little expense.

 

If it only happens after turning corners, I have no idea. Never heard of that. It doesn't happen if you maintain a straight line for a period of time ? Yea what does all that mean lol.

 

jbrew: It has this mis-fire or stumble anytime the engine gets lugged down. That normally doesn't happen in straight line driving but it certainly shows up when you accelerate out of a corner and seems to do it worse after the engine is completely warmed up.

 

Huh, yea, I don't know. Let us know when you figure it out.

 

Just had a weird thought. Check the intake downstream of the maf. If it leaks air at one of the connections, it upsets the air/fuel ratio.

I had that problem on my sons 89 Escort GT back a while. tightening the clamps on the air tubes fixed the problem.

 

That reminds me of one other thing for some reason lol.

Look toward top of your AC accumulator (pass side firewall, big aluminum thing). A dip in the ignition harness is right above. Over time that harness can dip even lower or if for some other reason the harness is contacting the top of the accumulator here and there. What happens when it does, it can bear the foil and eventually, a few wires. Without the foil or the insulation in tact, this can cause a magnetic interference which creates havoc upon the ignition system.

It's happened before with past members trucks. You post that it happens after corning most often. That may be shifting the harness to a position close enough,- causing this disturbance.