New member here & happy to be a part of the F250 forum here! I have a 1990 F250 Lariat XLT dual tank 7.5L 460 EFI w/57K miles on it, and I love this rig but recently it started misfiring (idling/running rough) and I need some advice. I originally suspected that it was clogged fuel injectors but I had those professionally cleaned and after reinstalling them there was no change. I've also thrown a bunch of new parts at this problem (see list below), so I'm running thin on troubleshooting ideas. Note: the truck sat for a while (8-9 months) with gas in the tanks, but I added fuel stabilizer to both tanks during that time & replaced the fuel filter prior to running it again. Also, it doesn't seem like dirty fuel because the engine misfire behavior is consistent (not random or sporadic). I replaced all of the following as part of my initial troubleshooting process (nothing helped so far)...

• Fuel filter (below driver’s side door)
• O2 sensor between the exhaust manifold & muffler
• MAP sensor
• Distributor cap & rotor
• Fuel injection pressure regulator
• Cleaned/reconditioned fuel injectors
• New plugs
• New air filter

Thanks for any advice on this!

 

Spark plug wires often get overlooked when they could be part of the problem. If and when you decide to replace them , be careful how you run the wires. When trying to make the job look neat , that creates mutual induction misfiring problems.

The firing order for your 460 engine is' 1-5-4-2-6-3-7-8.
That means plug wires for cyls 7&8 should not be placed close to one another.

 

Thanks - I'll check that out! I'm also going to replace the Idle Air Control Valve and check the timing & compression as well.

 

hey man did you get this figured out? i have 2 if these trucks both mine are 91 my 2wd one gave me a hell of a run finding out some issues with it but its a monster now

 

Did you notice the condition of the plugs removed at the time you replaced them?
See if you can isolate any and all vacuum lines from the engine. Pinch them off one at a time as close as possible to the source of vacuum supply. Sometimes the vacuum reservoir can rusts and can develop a vacuum leak.