90 gmc k1500 5.7 tbi backfire problems

I have a 1990 gmc k1500 5.7 tbi. I had the engine rebuilt, bored .040 over
and mid range cam installed. I have replaced every item that touches the
engine already. Map, IAC, TPS, rebuilt the TBI, EGR system, Fuel tank, fuel
pump, fuel lines.
I sent this truck in once to a mechanic already to fix a lifter problem i was
having (for the life if me i could not set the lifters correct). After he returned it
back to me it drove fine. I decided to upgrade the Oil pump in the block to
ensure better oil pressure due it is going to be used alot under heavy loads.
After i swapped out the oil pump i had to reset timing on the truck and when i
set it at TDC 0* and hit the gas from a idle, it will backfire through the TBI
unit. I was first told that with the increased oil pressure that the first round of
lifter adjustments may have been to much and needed to re adjusted. After i
had then re adjusted again and checked compression on each cylinder and
all came back fine. i still cannot figure out why i have a backfire if i do a
wholeshot from stop from the TBI and if i try to drive it at all it is really
sluggish. I have tried to set it advanced between 12, 10,8,6,4,2 and back to
0* and each one is having the same problem.
I did a full tune up again with new plugs wires and cap, same problem. Dont
know if i missing something.

What cam did you put in it? Any other go fast parts? It sounds like you may be getting lean pops. What's your fuel pressure? How old is your O2 sensor? You should get it on a scanner to see what it's doing as far as timing and O2 feedback (AFR).

i dont know the exact one, but i know it was just a mid range cam. nothing fancy. the 1st o2 sensor i put in was about 1 week old, thought it was going bad so i just put a new one in about 1 day ago. the reading go from 150 range to 750-800 range .

Sorry, what I meant was while driving, at what TPS position, RPMs, MAP, SA, and the O2 feedback to see when it goes lean or rich. Depending on your cam you probably need to get the ECM tuned so your VE and SA tables can support it. Another trick is to adjust/install a higher pressure spring in the TB to get more fuel to it. Your INT and BLM can only adjust up to 12%+/- from your VE tables' cells.

i am sorry i was able to follow along until Your INT? and BLM? can only adjust up to 12%ish for you VE?

INT -= Integrator or short term fuel trims, BLM Block Learn Multiplier or long term fuel trims. So, if you're driving along at 50MPH @ 1500RPM and MAP of 45, your O2 is telling your ECM it's too lean, the VE cell for that RPM/MAP is more than 12% below what it's calling for, it can only increase fuel up to 12% more then, you'll still be running too lean.

Edit: Another good reason to know what your fuel pressure is.

Ok now i am back on track. I guess i should also explain that at this time i cant get the truck to even drive correct under load without it being sluggish. I set it at tdc and timing to zero but when i hit the gas it is so non responding its hard getting over 30 mph. i figured my problem with the sluggish started when the backfire started so i thought starting there would help narrow down the problem. I guess i should have explained the full problem from the start. Before i upgraded the oil pump it ran fine. Just the oil pressure we below where i wanted it and the idle was set very low while in gear, around 300 rpm. i increased the idle and swapped out the oil pump and that is when this started.

From what you are saying could it be that due to the increase of the idle from what it was set at, it is now over the 12% correction rate the computer can compensate for?

Darkelden said:

i wanted it and the idle was set very low while in gear, around 300 rpm. i increased the idle

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How did you do that? Did you screw in the min idle adjustment screw (T25 Torx head)? If you did, you need to calibrate the TB before the IAC tries to drive itself closed and possibly burn up its stepper motor.

Here's a .pdf on how to do it.

You need to think "what's changed" when you swapped out the oil pump and maybe go from there? Did the distributor get moved? Did you crack the cap/rotor? Stuff like that too but, calibrate your TB 1st.

i tried to do the iac idle set, i followed the instructions and set the base idle to 575 ( i couldnt find where my prom base idle was so i based it off the instructions you provided) when i plugged the scanner back in after i finished the steps it would keep it at 1000 rpm.
One weird part i did notice after i un plugged the scanner the rpms dropped from 1000 to about 600/700 and went back up when i would plug the scanner back in. i did not even get to the part to adjust my tps yet. first i was hoping you could help with the info i was able to collect

Darkelden said:

when i plugged the scanner back in after i finished the steps it would keep it at 1000 rpm.
One weird part i did notice after i un plugged the scanner the rpms dropped from 1000 to about 600/700 and went back up when i would plug the scanner back in.

Click to expand...

A lot of ALDL cables have a 10K Ohm jumper or switch from pins A to B which will raise your idle to ~1000RPM. If you take that out the ECM will go to the Desired C/L Idle Speed table.