I have a 1986 F150 2 wheel drive with 5.0L/302 V8 EFI with 4 speed OD transmission. About 4,000 miles ago I replaced the plugs and wires, cap and rotor, and air filter. It ran fine after this. After it started running rough I replaced the fuel filter and ran a can of "Seafoam" cleaner through it (in the gas tank). There was no change with the new fuel filter, but maybe a slight improvement with the Seafoam (it could have been wishful thinking). Most of the time it runs OK, but sometimes when I accellerate it runs rough. I don't think it is not firing, but a fuel issue. It seems to do it worst between 25-35 mph in third gear and 55 mph in overdrive. If I let off the gas pedal it smoothes out. About anytime I accellerate in OD it does it. Sometimes it idles rough, but most of the time it idles OK. I checked the codes on it and it flashed "11" twice, which I understand means no faults detected. I want to check the throttle position sensor (TPS) but am confused by my Haynes book about how to hook up my multimeter. Does this sound like a TPS problem or something else? Where do you put the multimeter probes to test the TPS?

Rick

 

From those conditions, lower engine RPM with higher load, I think of ignition misfire. What spark plugs did you put in 4,000 miles ago? The exact Motorcraft plugs spec'd by Ford for your truck, or some whiz-bang get more power and better mileage wonder plugs, or ?

 

My truck did the samething until I changed the ignition module.So dont count out something in the spark.

 

I've had bad luck with cheap wires, usually a missfire under load is a plug wire. Make sure none of them are close to the exhaust manifold and burnt.

 

The plugs are Motorcraft--the same number and gapped according to the sticker under the hood. I checked the plug wires. One on each side was close to the exhaust manifold, but there didn't appear to be any damage.
I tried to move them a little farther away without changing the routing. I checked all the plug connections again, and couldn't find any loose ones. The plug wires are USA made but I don't remember the brand. They aren't Motorcraft, but they weren't the very cheapest at the auto parts store either. If I remember right, they are copper core. The cap and rotor are Chinese cheapies; it's hard to buy all-American anymore. Someone suggested the ignition module--is there an easy test for this?

Rick

 

Most auto parts stores can test them but I'm not sure if they will pick up an intermittent missfire. I've never heard of a module causing a miss under load though.

 

Did you ever get anything figured out? I have a pretty similar problem. Anytime I have the gas pedal held around 15-20 hundred rpm the truck rounds real rough. I can smooth it out if I stop giving the truck gas or if I floor it to more than 2500 rpm. But even in both of those scenarios you can still feel the "misfire". I've replaced the plugs (Motorcraft platinums same I've ever used everytime before), Wires (O'Reillys omnispark tri core or something like that), distributor, cap , rotor button, Ignition Coil, intank fuel pumps, inline fuel pumps, canister fuel filter, inline fuel filter, and the FPR.
my next thought was to replace the injectors or have someone more experienced tear into the motor for an internal misfire cause. But before I did that I figured I'll look around here for answers and see what someone a lil older knew that I probably didnt.

 

If you have EFI you need to pull the codes. The older trucks didn't have check engine lights, so don't go by that and say you do not have any codes. You may indeed not have any error codes, but on a system that complicated you need to check.

 

I don't remember the number is exactly but I have an ego code for running lean and EGR code for a open circuit and a knock sensor code because there was no response during self test

 

The scanner at the shop says that my codes while running are 25 31 and 41

 

You most likely got the code 25 because the goose test wasn't performed when it called for it.

Code 31 is a EGR code. You could try disconnecting the vacuum line to the EGR and plug it, and then see if the problem goes away. If it's still there with the EGR disconnected, I think we can rule out the EGR causing the problem.

Code 41 is the oxygen sensor saying the engine is lean. This could simply be a vacuum hose off or cracked somewhere sucking air into the engine. A lean mis-fire could be the cause.