Thanks all, Grouse, I am very familiar with that motor and here is how I would approach it if it were in my shop today.
First things first, compression test, spark quality test, timing belt alignment, and, a fuel pressure test, and this includes a leak down test that verifies that the injectors or fuel pressure regulator are holding.
Once that was checked out and verified I would hook my MEDS unit to it and look for history codes and make sure the following sensors were reading correctly, throttle position sensor, temp sensor, manifold pressure sensor, crank sensor, oil pressure sensor, shift position sensor, idle speed control, intake temp sensor, static and running outputs would be checked.
There are two sensors that will keep it in limp mode, oil pressure, and, shift position sensor, they will not let it reach full output.
IF it were reading hot on the temp sensor, an alarm will be blowing full blast.
IF it were reading cold, it would only run richer than normal and soot the plugs to heck wouldnt have it.
If all things are reading correctly and it still doesnt go to full throttle then a rigging check is made to make sure the throttle is opening completely.
ALL of this is ASSUMING it is propped correctly and I sure would hope someone didnt nilly willy put a prop on it because it fit at the moment before being sold.
I saved the injector test for last for a good reason, number one, for whatever reason on that series of motor regardless of which badge it wears they are notorious for getting dirty, and I mean dirty, especially the bottom two, poop flows downhill and when it settles at the lowest point it will be in those two injectors, certainly the bottom one or number 4 and they are a sumbitch to clean, why????
Instead of having two orifices to spray out of, they have four small orifices to administer fuel through and being small they plug very easily.
Yammie injectors take two to three times the effort to clean versus injectors that have two ports.
Funny thing is, those injectors are used in many many applications including way back to the 1998 Chevy Tracker !!!!!
We have been finding the last four years regardless of whos motor it is and what injector is used, if that motor has two hundred hours on it its time to pull them and test them, yes, our fuel today is that crappy!!!
Okay, time for worse case scenarios, slipped timing belt, dowel in camshaft has worn its slot out and cam timing is all over the place.
Failed ECU, it is usually caused by a gummed up idle speed control, it is the only thing driven by the ECU that can eat up amps, and when it does, it overheats the ECU taking out the logic module but it will run good until you get into the upper RPMS and it will lay down when it sets a bunch of codes that can only be cleared when you shut the motor off for at least two minutes and the ECU resets itself and it will run fine again until you hit the magic RPM.
This is a test I learned the hard way, you will know when a idle speed control is not working and about to take the ECU out, when you switch the key on, listen for it to make three clicks, it will do the same thing when you shut the key off AFTER it has been started.
If you cant hear the idle speed control clicking you are on borrowed time before the ECU goes south and that mother hummer is north of 1200.00 bucks!!!!
That friggin idle speed control is 433.00 plus freight!!
SO, I have to admit my MEDS unit is not perfect as it will not read engine hours on the older Yammie motors, SOOOOO, today, I spent another 500.00 on a yammie specific tool just to read engine hours. 
IFN any ya all win the lottery and need to invest in something, please remember this old tuckered out poor farm boy lol.
