89 GMC 5.7 TBI hesitation isssues - i have tried almost everything

I need some help if anyone may know whats going on.
I recently purchased an 89 GMC Van G2500 / vandura with the 5.7 l engine with TBI in it.
The owner stated her mechanic said it had collapsed lifters , which from my experience with Chevy / gm is very uncommon. I did notice some rust markings however like the motor had overheated pretty bad , so i tore into it.
Here is all i have done: I went and replaced the lifters , i replaced the timing chain , and i have replaced alot of the sensors. i replaced the Map sensor , the idle  air control sensor , the throttle position sensor , temp sensor , and on the tbi the fuel regulator with gaskets and spring.
I also replaced the plastic vaccume lines with silicone lines , and the line under the frame that is metal and has a small portion being fuel line going to the emissions canister ( also replaced approx 1 ft by the canister as well before it goes  to metal line ) I have played around with the timing on distributer and it seems to get a bit beter , but the vehicle stilll runs really sluggish.
i disconected the vacumme line from the Map sensor as well as the one going from the vacume sensor to the egr valve , and the Van seems to run and drive now - but still sluggish.
Prior to this it would start and idle , but would barely move when put in Drive and acted as if the TBI was flooding the engine with fuel before it could dump in the engine.
It also appears that there may be too much vaccume coming from the TBI ,and  or manifold

any help on this would be greatly appreciated - Im to the point where all i can think of is that the ECU isnt responding properly , or perhaps the Injectors were replaced with higher fuel output vs the stock injectors..

I really dont want to throw more $ into this Van , and with all my GM experience have diagnosed as best as i can - Even shops cant figure this one out  

did u disconnect the timing bypass wire when u set or play the timing? if u didn't then whenever u move the timing the computer is trying to compensate. and it can only compensate to a point  

Where is the timing bypass wire located ? The only wires im aware of are the 2 going to the distributer the plug in  

is the exhaust restricted... clogged cats???

have you got a scan tool that you can look at the live data with????

any chance that you have air injection ... can you test the airflow out of the diverter valve into the exhaust manifolds..

the air into the tubes that go into the exhaust manifolds can dilute the exhaust stream past the oxygen sensors.. making it hard for the ECM to control the fuel mixture...     the upstream air should shut off in 30 seconds...   you actually have to test it... with your hand.. unhook the hoses to the check valves.. and plug them for this test..

it is also possible that when the engine overheated.. that parts of the engine / fuel system wiring harness were damaged...

you need to be able to see the sensor voltages...

i would also. say to pull the engine cover.. the distributer cap.. and the rotor... take a bright flashlight.. look straight down the distributer shaft..  if you see a riveted on plate with fingers that are the reluctor.. look closely.. at the magnet that is visible in the middle...   if you see cracks in the magnet.. replace the distributer..   reman units have a different solid reluctor.. not the riveted together piece that fails when the magnet cracks...

i would also inspect the ignition coil.. look for circular dust patterns..   where the high voltage is leaking out..   or do a coil out put test.. it needs to be able to throw a bright blue spark almost an inch..   orange.. and its bad...  price them around.. prices vary a lot..


the vacuum line to the map sensor need to be hooked up... and before you do.. verify that it is free flowing..   and that it is attached to manifold vacuum...

you also have to jump the A and B terminal on the ALDL connector under the dash to set the timing..

or line up the timing marks..  on the balancer..   15 before as i recall.. then turn the distributer housing so the reluctor teeth line up exactly with the pick up coil teeth.. then lock it there..  you should be able to get the timing really close.. that way...


one more think...     did you replace the fuel line O rings.. where they connect to the back of the TBI unit...   they are available...  there is a large Oring and a small O ring.. failure to replace these  may result in a fuel leak..

please don't use regular black plumbing rubber orings..   reddish color or tan is what they should be..