I just had the timing belt an water pump replaced, the PVC valve unclogged, the crankcase breathing tube replaced, and cam and crankshaft front oil seals replaced in my 1989 Isuzu pickup. It now barely has enough power to get up a very slight incline and backfires a lot. I was told by the mechanic that one of the valve springs is broken and my truck needs a valve job; however, it had power in the low gears before I took it in to the mechanic. Any idea what the cause may be?