5 and 6 are running side by side if I remember right. I will check that when I get home from work.

 

This may sound like sillyness but I have seen several guys fix a miss under load problem like this by simply re-routing the plug wires. A good way to check for it is to find a place that is pitch black at night, turn all the lights off on the truck and have a look under the hood while sombody loads the engine down with the brakes and gas at the same time. If there is any arcing between wires or between wires and the engine you will have a lightshow and the fix will be obvious.

 

I have not done the pitch black check yet, but my ALT is bad. This maybe causing this issue also, correct? Battery is still 12 volts when the truck is off, but with the truck on and at idle it drops to 11.12 volts and my battery light came on in the instrument cluster.

 

I replaced the ALT with the one out of my 94 F150 and the voltage output did not change much. Engine off 12.20 volts engine running 11.75. I guess the ALT out of the 94 F150 is bad also. I am taking the original ALT to Autozone to test it.

 

I took the original ALT to Autozone for testing. It passed. I had them run it twice. It passed both times. My battery was so low on voltage/amperage that the truck would not start today. I put the charger on it and it is currently at 25%.

Autozone said it maybe the belt also. The belt tensioner pulley was moving back and forth a huge amount. So I pulled the tensioner off of the 94 F150. I can't get it to fit on the F350 correctly, but the belt is tight. Same problem. Less voltage on the battery when it is running then when the engine is off.

I am going to check the ground wire at the block next.

 

ALT and Dash Cluster swap have been resolved, but the mis-fire is still happening.
In park eazing into the gas it is fine up to 1500 rpms.

At 1800 rpms there is a small bounce back to 1700rpms
At 2000 rpms it is bouncing back to 1500 rpms
At 2500 rpms it is bouncing back to 2000 rpms
It continues to bounce about 500 rpms with steady gas.

At WOT the mis is not noticable.

Pretty bad mis-fire.

 

5 and 6 were not running side by side at all. I ran my hands along each wire and did not feel any arching. I have now plugged the smog inlet into the head to take the smog air out of the problem. It is still missing, but it seems like it is decelerating when I hold the throttle at a steady position. It is like it is bouncing back and forth when I hold it anywhere over 2000 rpms.

 

While driving it when I floor it.........it hauls butt! Slow to mid range mis fire and the noticable bouncing tach when in park and slowly pressing the gas. it still idles fine right about 900rpms.

 

Just thought I would add my input into this since I am having the same exact problem on my 1993 F150 351W. I also had a bad exhaust gasket so I replaced it and didnt bother to properly reconnect my smog equipment, the truck ran fine for months after that then all the sudden it was really bad. I managed to get it to run a little better by cutting off a cat but it sounds like you dont have any.

For me it seems like if I run below half of my front tank it starts running worse. I make a short trek to the gas station put 10 gallons in and its great to 40-50% throttle.

I have gotten down to the following things from reading online and peoples input. Fuel pressure regulator, fuel pump (too high or too low will cause these issues), EGR related issue, vacuum leak (just found this diaphram thing in the pass. side firewall has a broken line), air injection is pumping fresh air to my O2 sensor.

Hopefully this gives you some ideas. Good luck.

 

Now it is idling poorly even at idle in park. Hopping around between 500 and 1000 rpms. This was even when the engine was cold in the closed loop. Problems are changing. Before the idle was ok when the engine was cold then when it warmed up (out of closed loop) it would idle poorly.

Earlier today it idled great (closed loop or not), now it idles very very badly (almost died 5 times during a 2 minute idle). Same problem with the bouncing tach with gas pressed slowly. I held it at 3500 rpms and it was sparaticly to 3000rpms. It was not a constant drop to 3000. I mean if it is a mis it is not every revolution, not even close to every revolution. If I keep it at 3500 rpms it drops to 3000 rpms about every 20 to 30 seconds.

I do have cats they have about 15,000 miles on them. They were installed in 2008. My exhaust system is true dual with longtubes, but I do have the smog, egr, 02, and cats. In my county they put the sniffers in the exhaust to check emissions, so I have everything that is required. I have either replaced or swapped used the following parts:

TFI module, coil, AIC, fuel regulator, rear fuel pump, battery, alternator, grounding strap, TPS, vacuum lines, and the fuel filter.

I pulled the engine in 2008 and went thru it and modified it. It ran great until about 6 months ago. I am probably going to pull all the plugs again and replace the wires even though they very new.

 

I checked the fuel pressure today at the rail. With the truck running it is 35psi. Should it be higher?

 

Fuel Pressure stayed at 10 psi 3 hours after I turned the truck off. I had started hearing a small ticking noise recently from the right side exhaust. This side has a bad habit blowing header gaskets, so I checked the header bolts. The one in the very rear (hardest to get a stubby wrench on) was a little loose . I was able to tighten it a 1/4 turn. The rest of the bolts were tight.

This header is in a tight situation, so I went and inspected it from underneath. I found a black spot, that inidicated that there is an exhaust leak. One of the pipes has a hair line crack in it. It is bad enough have black on the pipe next to it. This is the problem. One thing that my EEC4 pcm does not like is an exaust leak on the header. I had some heat wrap in my toolbox so I wrapped the cracked area with it. I know this not a permanent fix, but it made almost all of the problems disappear. It still does not idle well in gear, but the hesitation thru the rpm ranges is gone!

I will pull the header and weld the crack up. Thanks for all of the help!

 

I too would suspect fuel pump issue, the next time it's acting up check the fuel pressure.

 

The truck has dual tanks. The rear fuel pump is 1 year old. When the hesitations occur switching from between the 2 tanks does not help. That was leading me away from the fuel pumps.

 

I pulled my gooseneck today with a horse in it and the hesitation is still present. Before I hooked up to the trailer I drove to get some fuel. The truck did not hesitate the whole way to the gas station (6miles). On the back from the station it (right at start up leaving the station) the hesitation was back. I had to pull the trailer a total of about 20 miles. Half thru the trip I pulled the timing spout to see if the problem some timing adjustment made by the computer due to the cracked header. It ran better but the hesitation was present, just not near as bad......... actually the last 3 miles it really did not hesitate at all..... any input?