I was hoping I could figure this out without asking you guys for help...AGAIN! but, I couldn't

F-150 in signature. It starts quickly but the idle is fairly rough, not bad, but rougher than I think it should be.
I put it in gear and depress gas pedal, the engine just POPS with backfiring through the intake and doesn't go anywhere. It will die if you give it too much throttle but if you back off it will stay running.
If the engine is cold and you do this, it wont backfire and sputter and buck it actually goes with little problem but still has a rough idle. When cold it WILL backfire if you really get into the gas pedal.
It seems if it idles for a couple minutes (enough for high idle to kick down) and then try, that is when it is at its worst. It is not driveable and hate to even run it in fear of damaging pistons or vaves.
I get no codes with KOEO and the KOER it will not run through the self test and won't let me do the dynamic test....
Very strange. I am about to sell the truck, I have been stuck with this problem for months.
I have replaced fuel pump, fuel filter, MAF sensor, O2 sensor, fuel pressure regulator, distributor, cap/rotor, plugs, wires, removed injectors and cleaned them, new TPS, ICM, purge valve solenoid... I have not really thrown parts at the issue, 90% of these parts were bad and needed replacing.
Check engine light comes on but only after you give it gas under load.
I was thinking of testing the voltage on the new TPS just to make sure it is functioning? Previous owner disabled "smog" pump system so I removed crossover tube and plugged one hold with bolt but other hole is open...could that be it? Haven't checked knock sensor but, could that be it? I have checked over and over for a vacuum leak and found none and, honestly, this would have to be a massive vacuum leak to make it this severe.
Sorry for the long post. Thank you for help

 

Moisture in Distributer cap / firing order wrong / timing chain timing off plugged exhaust

 

I can't see the engine/truck info, except it is possible its a 351.

If you haven't plugged both holes, yes, both need to be plugged. It's allowing fresh air into the exhaust. That hole runs the entire length of the head. Both sides have smog bumps cast into each exhaust port that is noticeable if you remove the exhaust manifold.

Edit: Mobile FTE site doesn't show signature.

 

Yea double check your firing order and timing, correct if find it necessary.

If that checks out put a vac gauge on it check for steady needle, if needle isn't steady the vacuum reading varies widely do a compression test.

Backfiring through intake, you might have a sticky valve or related issue causing you some grief.

 

You said the MIL comes on and if that happens you have a code set in CM. That should tell the problem. Sounds like you are running KOEO but are not waiting for the CM codes after the space.

Your EGR valve may also be stuck open or low fuel pressure to cause a lean mix for the backfiring.

 

Checked dist cap and firing order and exhaust is new (including cat). How would I check for slack in the timing chain or if it is "off"?

 

Ok Ill check with the vac gauge tonight and Ill quadruple check firing order and timing..JUST to be sure.
I did replace all of the lifters and cleaned heads thoroughly when I replaced the head gaskets but I suppose that's still a possibility. How would I correct a sticky valve? Also, the backfire is so severe it sounds like it is more than one cyl if not all 8.

 

So the KOEO codes should show me what is stored in the CM correct? I got the A/C circuit code but that's because I unplugged the harness and the A/C compressor...so basically no codes relating to anything else. I thought I waited long enough but Ill check codes again and see what I get.

Thank you gents for you replies

 

Ok, so your not gonna plug that hole in the back of the head?

Just gonna leave it?

 

NO I will get it sorry I read what you put I just forgot to reply to it. I will most definitely plug it. And yes it is a 351w. Not sure if the odometer is accurate but, it reads 320,000

 

Code 512, keep alive memory failure?

 

Yea I miss read that part of your original post, you gotta plug that other hole in the back of the other head. Thought you plugged the exhaust side but left the air supply end to it open I see now that is not the case, so yea you'll wanna plug that leak.

Do that first before anything else.

 

DTC 512 is usually when battery has been disconnected.

 

That just says that either the Computer was unplugged or the battery cable was removed from the battery. It also means all the CM codes would have been cleared.

 

Ok, that code makes sense then. For a minute I thought it meant my computer was fried!
Copy that Ill plug that hole tomorrow. It turns out that the drivers side hole had clean threads but this passenger side has gunk all on the threads so I work some at getting those threads cleaned up.
Ill do that and report back.
Thank you for all your help gents and your patience