my 94 f150 with a 351w is running rough. the idle jumps from about 700-1000
kinda sounds like it has a big cam. when its cold it idles alot worst. when im driving it stumbles unless im accelerating. this started happening after i installed hooker longtube headers. the headers did have a leak on the head end but im working on that now.

i have the o2 sensor installed about 6 inches behind the passenger side header, weld in a bung for the egr and the smog pump is not hooked up

my only mods are longtube headers with true duals that end at the back of the cab.

it has new
spark plugs
plug wires
distributor cap & roter

any help will be very appreciated

 

Pull codes,

getting that exhaust leak fixed should be hi priority. you also need to get your egr hooked back up. Now the o2 sensor location is important. if its to far down stream it wont get proper readings.

the jumpy idle sounds tps related though but again pull codes once the motor is warmed up.
what plugs are you running?

 

Double check the firing order. With the headers it will be easy to tell if you have a cylinder(s) not firing. When you first start it up, before the headers get too hot, wet a finger and touch the header to see if you have some not heating up as fast. If you wanta keep your fingerprints you can spray or drop water on the tubes. An ifrared temp gun would work well if you have one.

 

i got the egr hooked up.
im running autolight plugs and wires.
i took it to a muffler shop to pull codes and a bad o2 sensor code came up and a bad idle code came up. i dont remember what the code numbers were

 

im pretty sure i got the firing order rite. would it be possible that i have to get my computer tuned or programmed cuz i have so little back pressure?

 

i fixed the leaks and its still running the same. any other ideas?

 

Which are the pieces that you changed from before it ran rough to after? I'm not the techiest guy on here by a huge long shot, but I know good troubleshooting. The guys will be able to help more if you provide those details. For instance, you mention only mods being Long Tubes and True Duals, but then mention new Plugs, Wires, Cap and Rotor. Of the 6 parts, which ones changed from before and after the symptom started. Also, with the new parts, did you place the old parts back in to see if the new parts are out of box failures? I'm assuming the old parts worked well enough that the rough idle wasn't present.

It doesn't seem likely to me that the decreased backpressure is the lone cause. I know some of the guys on here recommend exhaust that is still somewhat restrictive, but I think that has to do with low end torque more than running crappy.

Sorry if I take too simplistic an approach, but when I see a progression like:
Working condition -> Changed parts 1 thru 6 -> worse condition
I don't jump to part 7 through 100000 as the cause, unless they were jostled, re-wired or otherwise affected by the process to replace 1 thru 6 as was the O2 sensor.

Since you have the single O2 sensor, I assume OBD-I. The code readers are incredibly cheap and handy. I mention that because I don't see the resolution to (or reason to ignore) your codes in the thread yet.

 

ok,
1.truck is all stock, rungs good
2.then headers and pipes go on. runs good
3.about 1 month later the idle gets rough (700-1000rpm) and i notice a very light stumble when cruising at about 38 mph
4.couple months go by runs the same
5.then its stars running worst, idle jumping all over even stalls somtimes when its cold. the stuble got worst, feels like im driving over bumps. it runs good when under a load or accelerating
6.i replaced 02 sensor, new distributor cap and rotor, and auto light plugs and wires. runs the same

i should add that i damaged the 02 sensor while putting the headers on.

and yes its obd-1

does anybody know were i can get an obd-1 code reader that is a fair price and not a p.o.s.?

 

The first part of number 3 and most of number 5 seem to be the Idle Air Bypass solenoid to me. When the throttle plate is closed, the Throttle Position Sensor tells the EEC this, the EEC then opens opens the IAB solenoid. The EEC flings it all the way open when the throttle plate goes from particularly open to completely shut as in deceleration (yank your foot off the gas).
Of course the TPS might not be sending a good signal as KemicalBurns suggested. Test the Ohm range coming off the TPS as well as the voltage to the TPS. I'm afraid I don't have enough time to look for IAB tests for you. Perhaps one of the guys will jump in.

A volt meter or test bulb can be used for the OBD-I. Even the P.O.S. testers work just fine.

Here's an orticle on getting the codes with the voltmeter or the Check Engine light flash sequence.
How-To: retrieve trouble codes- OBDI - Ford F150 Forum - Community of Ford Truck Fans

 

you may need to get the o2 sensor moved above the header collector (upstream).

Pull off the IAC and give it a good cleaning. check your timing make sure its at 10*btdc with spout connector removed. also make sure #7 & 8 plug wire are seperated. these 2 wires are prone to crossfire causing low rpm stumble.

 

ok thanks for the help i will try those. feel free to keep adding in

 

also what brand o2 sensor did you buy? its possible that you got a bad one. might also pull battery cable and wait 15mins, hook it all back up and go for a drive to get motor good and warmed up and then pulled codes again.

checking fuel pressure at the rail both key on and engine running. Has the throttlebody screw been tampered with?

 

i dont remember the 02 sensor brand. is that the screw the pushes on the throttle body? if so it has ben tamperd with after it started to run bad. how do i check the fuel pressure?

 

crap well that screw is not to be tampered with at all. this could be your biggest issue now that other things have been addressed. to readjust that properly is pretty involved, you might find it easier to swing by your pickNpull and get a Throttle body off another F150. doesnt matter if its off of a 5.0 or 5.8, they will be the same. you can if the screw has been messed with because the paint will be jacked on the screw.

you check fuel pressure with a fuel pressure gauge. there is a shrader valve on the injector rail.

 

can i masure how far the screw is screwed in on one that hasnt ben messed with, with a caliper or are they all differnt?