GMC Repair: problem with 1990 gmc pick up, vacuum gauge, backpressure

Question
I have a 90 gmc k2500 4x4 pick up it has the t400 trans. The problem is when I hit the gas kind of hard from a dead stop throttle respose is nonexsistant. If I give it the gas slowly it takes off ok. It has had a full tune up a new distibutor,intake gaskets were replaced, air filter.Egr system has been gone over and all seems fine. The Exhaust seems to have a leak that I cannot find but can only hear when the truck is accelerating. At idle you cannot hear it in or out of the truck. A vacume gauge shows 22lb of vacume at idle with little or no variation, when you blip the throttle the gauge drops to zero then goes to 25lbs drops back to 17 and climbs back up to 22 where it stays. I was thinkg clogged exhaust or fuel filter or spark advanced any thoughts?

Thanks Bob

Answer
Hi,Bob

 Good diagnosis,by the symptoms you have,i would lean towards a restricted exhaust,also a plugged fuel filter will cause the same complaint,the two can cause the same symptoms and you would need to isolate which one,a fuel filter is inexpensive and if it is due for one,i would change that first and see if it made a differance,if it did not then check for excessive backpressure in the exhaust,i use an adaptor that screws into the O2 sensor and take the reading using a vacuum gauge,running the vehicle and goosing the throttle,anything over 3psi is cause for a restricted exhaust,or you can disconnect the exhaust sys from the engine manifolds and run it that way,basically an open exhaust sys and see if that made the differance,if it did i would replace the catalytic converter they are usually the ones that fail and cause the excessive backpressure,let me know how you make out,and if you need further help.Thanks

ED