Ford EFI conversion problems/backfire,miss

Has anyone run into this problem?:HMMM: I just did a stock EFI conversion on my 5.0 ford I used all stock ford parts to do a conversion and have this problem with the motor backfires through the throttle body and misses and seems to flatten out at around 2800 rpm and generally seems to fall on its face above this rpm. I have replaced all sensors : CTS,ATS,O2 sensors, Throttle Positions sensor, and the distributor also the cap rotor , plug wires,plugs and even a new computer A9L also i replaced the fuel pressure regulator with a new one and checked the pressure it checked out at 32psi running 39psi key on, I took it to a mechanic and he kept it for 2 weeks and replaced all these parts and it still wont run right!! I used a wiring harness from an 89 Mustang donor car and went to Fordfuelinjection.com and followed all his instructions on the swap which was very helpful with getting the harness stripped down to the bare minimum of wires that would be used, yes I have a good ground to the computer and the O2 sensors ,the motor runs great below this rpm and if it were being driven by my mother you probably wouldn't even know there was a problem sense the motor idles fine . Oh did I mention the motor has a B303 fordmotorsports cam in it. other than that the motor is stock crate motor 1993 model the timing is set a 10' btdc with the spout connector out with the spout connecter in and revving the car up the timing seems to go up to around 28 btdc then it comes back to around 0 btdc when you rev the motor up close to where it starts to backfire at around 2800 to 3000 rpm . What the hell is going on !!!! and how do I fix it.?:mad: :mad:

Do you have the right firing order for the cam? Its different from the older 302's.

DennyW What do you mean an open? sorry never heard that term. will check vaccume today. what info would you like I will get anything you need I am getting worried due to the fact that I and the car are already registered in the Knoxville NSRA meet in 8 days and I dont want to go empty handed!!!

R.POPE The firing order is for the H.O. motor and my mechanic says my motor is a H.O. because of the roller cam in it which I comfirmed when I had the intake off. What else could it be?

Backfire is either lean fuel or erratic spark. The Ford injection is sequential, and timed to each cylinder. Could that be messed up? Crossed injection, so to speak? Might have the same result as crossed plug wires.

Well I checked the vacuum it is at 10in Hg when the miss starts at around 3000 rpm.Shouldn't that be higher? I checked the wiring harness on the engine that goes to the injectors against the schematic and the color of the wires to each injector match up. I also tried unhooking the fuel pressure regulator vacuum and plugged it off to give the injectors full pressure(39psi) at high rpm, but the miss is still there, note: the backfire only seems to happen when I'm accelerating under load( low vacuum?)

What does it do if you take out the timing connector locking timing at 10*?
If it is better then advance the distributor to give it around 34 * and see if thats better---if it is replace distributor

I had a customers Cobra with stroker SBF with ford electronic dist and it did the timing retard like your talking about--had a msd6al and a dist change cured it, but don't just throw parts at it try the manual timing like I described

jerry, I took the spout connector out and set timing way up around 28 deg. the engine started to die so i knew i couldnt go any further . When i rev it up it started to miss and sputter like it always has. I put the timing light back on and checked the timing at part throttle and the timing is still moving back twards 0'tdc during the time that it seems to start to miss. the distributor is a new reman. i put in last night and it acts the same with the new one.

Yes I have the plug to read codes
coolant temp 195
Tps sweep on old tps replaced with new.
Don't have MAP I have a BAP the harness is off a 89/90 mustang.
Checked voltage on sensors they vary with temp and positions. But I can say they are getting voltage from computer.
the TPS voltage was a tad high 1.2 to 1.25 but it isn't adjustable like some of the older ones were.
The wiring diagram is on veryuseful.com for 89/90 mustang
I have air temp sensor on intake runner#5 I think the newer models 94 and up had a air charge sensor mounted in the air box is this what you are talking about if so no my wiring harness doesn't have a plug for that.
Intake manifold ,injectors (19lb.),harness, distributor, BAP,fuel rail all from donor 89 mustang. fuel pump Summit Racing, mass air from 90 crown vic.
computer reman. from local auto parts chain. I hope this helps. If I missed something please let me know. p.s. I did read the codes twice but most of them were disconnected emissions and others were very vague in the code book for OBD1. probably just me, I need a complete explanation I'm not a professional!

Well I / we found the problem I took it to 3 mechanics before the last one took it on as a personal attack to his craft and would not give up!! and found that one hot lead from the computer to the start/run circuit was connected at the starter solenoid and the Lokar neutral safety switch was also connected to the same lug and was back feeding 12 volts directly to the + coil wire so when the distributor got above the suspect RPM all that voltage was apparently causing spark scatter or maybe the coils field was breaking down prematurely but when we removed this wire from the start/run circuit the miss/backfire went away!!!
Thanks everyone for your imput without it I would not have eliminated those things and the mechanic would have spent at least another 10 hr. starting from the begining to eliminate all the other things first.