Hello all! I need some help and/or advice regarding my truck because its driving me mad! I currently have a 1993 Chevy Suburban with about 161000 miles on its original 454 engine. It has always run smooth and strong until the past few months. In December of last year, the fuel pump went out and I replaced it, along with the fuel filter, plugs, plug wires, alternator, belt and battery. Then starting around February of this year the questionable behavior began to appear. The engine has begun to run extremely rough, especially when I am not accelerating or putting a load on it. If it is idling, you can hear an occassional miss, but it does not become obvious until I am at cruising speed, around 1200 to 2500 RPM, then the rough, choppy operation commences. If I drop down into drive from overdrive and bring the RPMs up to above 2500 or accelerate aggressively, the operation is as smooth as ever. It only runs rough when cruising. In addition, there is a very strong smell of fuel coming from the exhaust when it is idling, as if the fuel is not being burned, but passing through to the exhaust or too much is being sent to the engine to begin with. I thought it might be electrical, hence the new plugs, battery, wires, etc. That had no effect. I even tried a new MAP Sensor, but that did not help either. I have noticed one other thing. Before I replaced the fuel pump, whenever I would go to fill up on gas, there would be a vacuum on the gas cap when I removed it resulting in a hissing sound. Now, no matter if I have spent a full tank, or just topping off, there is no vacuum pressure whatsoever on the fuel cap. Its as if there might be a compromise in the integrity of the tank area, allowing the sealed fuel system to vent otherwise somehow. When I removed the old fuel pump, I also had to remove a lot of gummy material that seemed to assist in the sealing of the tank. When I installed the new pump, I did not replace the gummy material, relying on the provided o-ring seal to do the job. Could air somehow getting into the system be causing the problem? To me, it would seem to be having the same effect as leaving the gas cap off. Please, help with any input you could. I am trying to clear this up on the cheap, but unfortunately may have to fork over a lot of cash if its the heads, etc. THanks!