I Just Picked Up An 89 f-150 4x4 3 sp. With A 351 windsor 5.8 For 500 Bucks. Drove it in town for about a week Ran Fine ( Maybe About A Hundred Miles Or So ) .....The Only Thing I Did Was Change The oil , Filter , Air Filter , Then A Few Days After That It Would idle Fine , Then only Run Rough When Engaged In Drive with My Foot On The Brake ...Then Gradually , Poor acceleration , Really Rough Idle Now And Keeps Getting worse, Have Good Fire From All The Wires But Have Not Pulled The Plugs Yet ....Fouled Plug Maybe....Thinking Maybe Fuel Filter ...? All The Vacuum Lines seem To Be Connected And Intact ...Maybe Some Kind Of Sensor ...? Any Input Would Be Appreciated .....Tks ...!
Check the codes on the computer. Could need a full tuneup, including fuel filter. Could be a fuel pump, temp sensor, throttle position sensor, could be a any number of things.
When specifically is it running rough? Is the engine hot? Cold? Both? Is it at idle? Accelerating from a stop? Cruising on the highway? If you can pinpoint the conditions it will help with the diagnosis. Also this is a question that gets asked daily OR MORE so if you do a search you will probably find the answers you seek.
You say you get it for $500, so this truck has been sitting for awhile?
How long? If its more than a couple months the 'gas' in the tank probably looks like paint thinner and black mud. If you started running it you're stirring up a lot of gunk.
Drain the tank if possible. If not, let it idle awhile and burn off some fuel. Put in as much new gas as possible, along with some treatment. Basically it'll start and would hold idle. Good sign, idle is one of the hardest things for an injection system to manage. The more fuel you need (accelerating) the more it gets starved. Changed the fuel filter. Once that's done you should see a big improvement and can start checking little things like oxygen sensors, vacuum leaks, dizzy, timing, etc etc.
Don't be too upset about it. At $500, and the price of 351 parts, you'll make a couple simple fixes and end up with an even better deal.
Thanks For The Input Guys ..! I'll Change The Fuel Filter And Check The Fuel Pump & Pressure & Take It From There....It Had Been Sitting For Just A Bit ( According To The Previous Owner ) But All Ran Fine , As I Said , For About , I'm Guessing For A Hundred Miles Or So . I Did Run Both Tanks Back And Fourth With Fresh Gas ( Premium ).....But Maybe That Was Just Enough To Stir Up Some Crap That May Have Been Hibernating At The Bottom Of One Of The Tanks As It Did Come On Gradually And Eventually Got Worse ...Again , Tks. For The Heads Up ...!
You need to check fuel pessure, the system should build and hold about 40psi with vacuum removed from the regulator. If it doesn't get that high you could have a clogged filter, bad switching valve, or weak pumps.. though if it does the same on both tanks it's probably not the pumps.
Think of it like a river bottom OP. Can be clear as day, but once you toss something in or step on the bottom the water is filled with dirt. Would cause the gradual decrease in performance you talk about. Like clogging jets in a carb.