My 1986 Grand Am, 4-cyl, has a problem groing up hills. It runs normally on flat road, and I get about 27 mpg on highway; which is more than I got in 1986 (about 23). But there is a long prominent hill on the freeway here where I seem to lose horsepower and the engine begins a fast "knock" (more like a rattle or clatter), I am forced to downshift to preserve speed and decrease the rattle (but still can't get over 50 mph on the uphill side); once I reach the top of the hill, the rattle stops and feels like I have power back. I've run some Redline Fuel System Cleaner thru it without much effect; it seems to run normally on flat road. I haven't tried 93 octane in this little engine. Do you think that would be worth trying? Thanks for any suggestions.
GS