GM-GMC: 1988 1-Ton 4x4 Pickup loses power, intake manifold gasket, new distributor cap

Question
Our 1988 1-Ton 350 Pickup seems to lose power when you start going around 40 miles an hour. The hotter the motor is, the more power we lose. When you stomp on it or try to get to a higher speed, the exhaust will backfire-crackle-pop, etc. We have put on a new cadillac converter exhaust, we've tried a new distributor, cap, roter, spark plugs, wires. We've had this in the shop for 3 weeks and nobody can seem to figure it out. Do you have any suggestions or can you lead us in the right direction for someone who may know the answer? Any help is much appreciated. My husband is getting really frustrated.

Answer
Angela,

Make sure that the air and fuel filters have been changed or checked for restriction.  Verify that the oxygen sensor is working correctly.

The fuel pressure should be checked to ensure that the fuel pump is working properly, along with the fuel pressure regulation valve.

The intake manifold gasket may have a leak and that will upset the fuel air ratio and affect the power of the engine.  Try spraying some carb cleaner along the area where the manifold attaches to the cylinder head.  If the engine changes speed, a leak is likely present.

If the engine has a lot of miles on it, the timing chain may have jumped and caused the valve timing to have become retarded.

As you can see, there are a lot of potential areas to check.  Keep checking the different systems on the vehicle and the problem should be revealed.