OK I posted about this a few years ago but still haven't found the problem, My 88 F250 7.5 liter runs awesome 99% of the time, The problem I am having is when towing heavy or just working it hard it does great until I let off the throttle, then it backfires and looses power, If I push the clutch in and rev it to 4K and let of it will backfire again, after a couple of times doing this it clears up and runs fine again.??
It has done this for the 4 years I have owned the truck, I have installed a cat back exhaust, hollowed out the cat, new map sensor, coolant sensor, K&N filter, tps, and plugs,wires, cap, and rotor. I changed most of these because they were 20 plus years old not because they were bad.
The only code I get is 33 for egr not opening

 

could be an internal problem how many miles on the motor maybe floating some valves at that rpm

 

Given the age of the truck I'd suspect a PCM problem, the electrolytic caps in it only last so long and they are known for breaking down and causing problem with these units. Pull it out(after disconnecting the battery) and open it up to see of there is anything obvious, bulging caps can be replaced but if they have exploded they can damage the circuit board and other components and then it needs to be replaced.

 

Truck has 158,000 miles on it, I never run it over 3500 rpm and very rarely above 3 grand, I just rev it to 4k for a split second to clear it out. I've had it since it had 100k on it
If I get a few minutes this weekend I will pull the PCM and look it over.

 

Thought I would post an update, The problem kept getting worse to the point it was happening daily. I accidentally broke the connection on the valve above the passenger valve cover going to the secondary air injection line in the left exhaust manifold So I unhooked and plugged the lines to both manifolds. It hasn't had a problem since and runs great. Downside is I have to have the system hooked up for emissions so I still need to fix it.

 

Do they conduct strict visual inspections in Utah?

 

Yea they are pretty strict in the county I live in.

 

Did you eliminate the tubes going to the manifolds? You can sandwich the tubes going to the manifolds with quarters, between the tube itself and the manifold (Put a quarter on the manifold, then torque a quarter down inside the nut, between tube and manifold), and no one will know you did so. If, by blocking the air injection, you solved your misfire, then there was obviously air being introduced through the manifolds which leaned out the o2 sensor readings. You could have a bad solenoid, sticky air valve, or a leak that simply fouled the o2 reading. Either find and fix the problem, or do as I said above to pass the visual inspection.

 

Everything is still in place, I just plugged the rubber hoses on the valves above the passenger valve cover, I was thinking of fixing the plastic nipple I broke and hooking the lines back up but just getting a shorter belt and bypass the air pump. Then when it is time for emissions hooking up the pump.