OK, need some SERIOUS help here please.
I'm "FCC 1st Class" Electronics Tech, but this has STUMPED me!
Went to Ford Dealer for help, but say "Factory does not allow us to work on older trucks", really? Called repair shops, but only "well, don't know, will just have to charge for time/hours, can't give estimate", so, spend more than truck is worth to fix? Naw... Let's try here.
HELP!
Problem is a real complex "String" of symptoms, but has ended up with THIS;
Symptoms are same on BOTH fuel tanks.
My 1992 Ford F350 with 460, Dually w/Auto-Trans, now is POURING gas through engine, so much so, that it will ONLY start to run smooth and with power when floored, around 3,500 rpm floored engine is pulling strong and it gets smoother the higher the RPM.
It all began with FUSE #17 blowing. Did research online, all said this same fuse for Door chimes/cabin lights also runs Tach, Gauges and Overdrive, true enough (why would Ford put Tach/Overdrive/GuagesICU on SAME fuse as door chimes/cabin lights/turn signals?)... Said that often fuse blows due to short in Turn Signal wires, horn, door switch for cabin light. Nope, all those work fine. Tore whole cabin and dash apart, no shorts. Put in heavier fuse, and it blew instantly just turning on key. I had an "Ah-Ha" thought: I tried replacing fuse when going down freeway, and Voila, engine ran perfect, gauges came on, overdrive started working again, all symptoms GONE completely... Until I came to stop light, then would ALWAYS blow again, ok only when changed fuse while running at speed. This "change fuse while at speed" worked for 1 day. NOW, #17 fuse does NOT blow and all gauges/overdrive/etc work correctly, but so much gas is going through engine that in ~ 10 seconds from start, the cylinders start to miss SUPER BAD, always the SAME cadance for the SAME cylinders (ones closest to oversupply of fuel source in rail dealer suggested). Black unburned gas pouring out tailpipe. But, when floored the truck starts building power as RPM's climb, and by ~ 3,500 rpm it runs at full power completely smooth. The Transmission seems to shift with a HARD bam into next gear, but all gears shifts work at correct rpm... Even when engine is at lower rpm's and running super rich and rough on partial cylinders.
Ok, I've read sooo much, all say "check fuel pressure regulator"; "Check Temp Sensor"; "Check EGR"; Check Fuel Return line if clogged"...
Been told maybe "Engine Computer probably bad". If so, why does it run GREAT under FULL POWER as RPM's climb? Ok, all injectors perfect, fuel pumps both perfect. Injectors getting proper "Signals", so if computer shot how can THAT be?
STUMPED.
Especially now that that FUSE stopped blowing, and all the gauges/overdrive/door chime & cabin lights are good. Did the INITIAL "FUSE" blowing issue, cause some wire or connector somewhere to burn out and now, no proper engine management controls to Injectors? But then HOW COME as RPM's raise under full throttle engine runs smoother and smoother and gets FULL POWER (this is a very strong pulling 460, someone put headers on it, and maybe other internal mods, it is way stronger than it should be if "Stock". I had a 460 in both bought new 1991 and 1995 that were allot weaker.
HELP !!!!!!!!!!!!

 

Start with pulling codes. You may know, there is a diagnostic connector close to drivers side firewall near brake booster. No check engine light doesn't mean no codes.