Engine has a lack of power?

We did last night. My buddy who helped me said it was an 02 sensor or whatever he meant. He thinks it could be an exaust problem now. The plugs were really trashed and the wires worse. We put new ones on. It just keeps losing power after u drive it for a little while then its like it barley wants to go uphill no matter how much gas youy try to give it.

Hello, I just had the serpentine belt in my van replaced and all of a sudden I was on the highway and my van would no longer accelerate, instead the speed started decreasing. The car started to shake violently so I pulled off the road and waited about an hour and tried to start the car again all it did was shake violently when placed in reverse it struggled to move but did slightly and when placed in drive it just shook then stalled when you put your foot on the gas. Do you know what this could be from. Also my car has been loosing water out of the thing that you put water in although it never runs hot as long as we fill it every few weeks. Thank you so much have a great day.

The head gasket failed between 1&2 cyl. I had both heads shaved and I installed them. The engine runs poorly but does not seem to be missing. It acts like the timing is retarded, in drive ( front wheels off the ground) it will upshift normally. But when a load (brakes) is applied 1500-2000 rpm is all that is possible. When loaded as described an occasional popping (backfire?) Can be heard through the air intake. No check Engine lite, nor any OBD II codes show. Any ideas would be welcome.
Thanks Dick E.

Hi, You were very close, turned out to be a plugged muffler! Thanks Dick E.