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I have a 1987 GMC 1500 with 4.3l TBI will run at idle only with map sensor unlpugged. Here is what i have changed.
Remanned short block, replaced map sensor, distributor, wires, plugs, cap, button, fuel pump, rebuilt pressure regulator in TBI, new throttle position sensor, Idle air control valve, and Knock sensor. also when map sensor is unlpugged it runs well until you put it under a load. then it bogs down and loses all power. I am out of things to try. Please help!  

a few things...

did the ground wires on the intake and the back of the head get hooked back up properly..????   along with the battery negative to the body ground connection near the battery ????  loose or missing grounds.. it will barely run.. remember.. electrons flow from Negative to positive.. , the ECM is ONLY grounded to the engine.. not the body..    the engine is sitting on rubber motor mounts..

did you use the same timing cover and damper?? so you know the timing marks are correct.. there can be 30 degrees of difference between years... where some may have a 12 o clock TDC timing mark.. where others may have a 2 o clock TDC timing mark.. its really easy to get the parts lined up wrong when swapping motors..   you will if not sure.. pull the first spark plug and work the crank back and forth with a section of copper wire in your hand  feeling the piston movement.. or  the crank snout woodruff key lines up with the #1 crank rod journal..   45 degrees to the right of center is where tdc is..   one of the 3 crank pulley  bolts will always align with the woodruff key slot..  so if one of them is at 45 degrees to the right of center.. it will be TDC..

you can see the difference in this image..



next... you say you replaced the distributer..  if you have an original style that looks like this.. please check it for cracks as shown.



this is what the reman distributers look like..



your timing should be TDC on that motor.. once you verify you have TDC..   look at the pickup coil tips and the reluctor tips.. are they lined up perfectly????  if not.. turn the housing so they line up perfectly.. this is where the base timing should be..  this allows you to install distributers and preset the timing so you can start it without needing to turn the housing..  

next...  why am i going on with all of this..  i have worked on a LOT of these TBI.. and you have to get everything correct...

did you change the engine coolant temp sensor.?? and its pigtail connector..  at AZ stores SU102.. come with both parts in the box for under 20 bucks.. other stores offer it in a kit also.. about half the price of the individual parts.. look it up online..  most of the store clerks are amazed when they open the box.. they did not know it came that way.   the sensor can go bad.. and the connector can go bad...   unless you have a scan tool to verify sensor live data ...

you may want to check out this post i put up on another site here...

http://forums.corvettefever.com/70/8585187/c4-corvettes/which-used-scan-tools-for-82-to-94-corvettes-with/index.html

this same info covers your truck...


do you have a vacuum gauge????  since it will only idle with the map sensor unplugged...   hook a vacuum gauge to the map sensor hose..   see if you can get the engine to speed up at all..  as you increase engine speed slowly..  the vacuum should increase...  if it drops the faster you go .. you might have a clogged exhaust..

do you have a scan tool???   print the charts from the linked article..

do you have a digital volt meter????

if you have a timing light..  can you aim it at the cone spray from the injectors.. is should always be a nice pulsed cone... not off center or a trickle..    as far as i recall gm did not install test fitting in the fuel rails...   it usually requires an inline pressure tester adaptor where the fuel filter goes...    you might want to unhook the engine side of the fuel filter...  if you can get at it safely..  have somebody turn the key to the ON position while you have a drain pan  to catch the fuel pushed out. . when the key is turned on..  the pump should run for 2 seconds..    you should get significant fuel flow..

running fuel pressure is only 13 PSI.. but the pump can make 25 pounds or so..

post what you find..   these are used on Millions of gm trucks and cars..  very rarely.. the ECM will fail.. but before you replace that.. everything HAS TO BE CHECKED AND VERIFIED..  

Have replaced exhaust, cat back to tailpipe. injector pulse is good. have  not changed coolant temp sensor or pigtail. just replaced o2 sensor and modulator valve on tranny. i will check engine grounds again on monday and let you know. Fuel pressure is about 10 psi wit hengine running. No check engine light and no codes. Have old school scan tool so take that for what its worth.  

10 PSI is NOT enough fuel pressure...

13... is right on...

Fuel Pump Part Number: E3902
Maximum Free Flow Rate (gph): 35
Maximum Pressure Range (psi): 17


what brand of scan tool.... did you go to the corvette fever site and print at least one of the charts???  so you could mark down what the data stream voltages are...  

Got it figured out. The fuel pressure reg in the tho was wasted. Replaced it and runs great now. Thanks