My 93 f150 5.0 4x4 automatic misfires through the air box and stumbles when accelerating or under a load. even if i am cruising at 40 mph and back off the gas for just a minute as soon as I get back on the gas it will stumble and then clear out. In the morning when cold it will start and then stall immediately, it starts right back up and stumbles idling funky for about a minute or two before leveling out to a somewhat decent idle. As soon as I hit the gas to pull out of my driveway it will stumble, pop through the air box and take off. Sometimes It will run as if a cylinder is dead and then clear out. I put a new fuel pump in the mid ship tank that went bad but made no difference. I also put in a new map sensor that went, ignition coil, plugs, wires, cap and rotor within the past month. I have a manual and checked the EGR valve, the new map, iac and tps all seem to be working.I keep checking for codes and keep coming up with nothing. The truck it self has over 200000 miles on it and supposedly has a rebuilt motor in it. It does not smoke, burn or lose any oil between changes, and when I changed the plugs they were all in great shape and looked as if they were burning identically. Any help would be great. I really like the truck and would hate to get rid of it.

 

Sticky intake valve or weak valve spring? Just a thought!
I think you can check that with a vacuum gauge.

 

Did you route the plug wires exactly as per procedure to avoid inductive crossfire? They have to crisscross each other in a very specific way.

 

I will read my manual and see if I can check the valves without disassembling the heads. I did read last night in some older posts about the wire issue which I will check out today also. Thanks for the input guys.

 

A compression test may be able to give you a clue to whether the valves are okay.

 

Do a dry, followed by a wet compression test. What octane do you use? Check your manual, the proper wire routing should be illustrated.

 

I am running regular, and I will do a compression test as soon as I can. I never thought to do one. Since the truck doesn't smoke or burn any oil and the plugs are clean with no carbon or oil build up and burning the same every time I change them I just assumed that the compression was fine. Does that make sense? But I am definitely going to do one, It only makes sense the more I think about it. I also rerouted the wires properly today and unfortunately it made no difference.

 

I really appreciate all the input you guys are giving me. I welcome any and all advice or suggestions. Thanks again everyone.

 

Hook a vacuum gage like I said to manifold vac source, if it fluctuates wildly it may be in the valves. The dry then wet compression test is also a great idea.
Hope it's something simple!

 

My first thought would be a hanging or poorly atomizing injector(s). A look at the plugs may or may not identify the offending cylinder depending on the level of intermittence. If you do identify which one, I've had bad plug wires out of the box. Then there's the possibility of cracking the insulator on a spark plug during installation.

 

jgger I am going to check the manifold vacuum and the compression Saturday, I will post my findings. I am also going to check out the plugs like ymeski mentioned and the resistance of the wires. Hopefully I come up with something.

 

I would try adding a container of fuel injector cleaner to a full or nearly full tank of gas. Fuel injector cleaner has solved some really weird problems for me.

 

Definitely! 2 "Hail Mary's" & a bottle of "Lucas" Injection cleaner wouldn't hurt if it is injector related. There's an injector cleaner that screws to the fuel rail where you would normally check fuel pressure that work very well, but I have yet to find the stuff. Also the reason i asked about octane is because a "stock" engine w/stock base timing will become poisoned w/ long term use of higher octanes as they are only engineered for 87 octane being a low compression rate engine. Constant incomplete burned fuel residual, Carbons up the combustion chamber surfaces. Then the build up begins to wick fuel introduced into the cylinder and uncontrolled detonations occur. The same thing occurs when a plug isn't firing on a consistent basis, but only to that cylinder. Another possibility is accidentally altering a plugs gap or contamination w/surface muck, during installation. When doing the wet compression test on below normal cylinders, if the compression bleeds down slowly implies rings. Quickly implies a valve problem. A can of "Seafoam" introduced through the air intake via the brake booster Vac hose at idle & norm. operating temp. will take care of combustion chamber build up. After introducing the can's contents, shut the engine off for 15 minutes, then restart and rev the engine till the truck reappears from the smoke cloud, then take it for an aggressive drive.

 

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