GMC Repair: low power at highway speed, vacuum gauge, muffler shop

Question
Hi, I have a 93' GMC one ton Dullie, I pull a large utility trailer all the time with it. the truck has enough power to get up to speed, but once at 55 mpm thats it, no matter how much gas you give her she is slow to resond. I found out that the engine was way out of tune and had a good tune up done, plus fuel filter and had a new catalytic converter put on, but that was months before the tune up. The guy at the muffler shop told me if the engine was out of tune the new one will go bad quick, how can I check if it the converter is clogged?
Or could I have a fuel pump that is weak.

Answer
Hi,Mike

You may have a restricted exhaust,but dont rule out fuel pressure,test the fuel pressure i believe your truck is throttle body injected,and needs a min of 10psi fuel pressure.
 In order to test exhaust sys backpressure i use a vacuum gauge with an O2 sensor adaptor,removing the O2 sensor and screwing in the adaptor,attaching the vacuum gauge and running the truck,goosing it to wide open throttle and reading the gauge a min of 3psi,anything over that and the exhaust is restricted,ive seen an old catalytic converter come apart and the material get logged in the muffler causing the same restriction.
 Another way to test backpressure is to disconnected the exhaust from the manifolds,or disconnect the converter,and basicaly run the truck with open exhaust,if performance is now ok,then you have a plugged exhaust sys,either the cat or the muffler.if you need more help feel free to contact me.Thanks

ED