How would I know if I have a bad valve. Currently, my Single Cab runs terrible, has no low end power, bogs down at low rpm. Eventually, if I feather the gas pedal I will take off. Gets a little better when totally warm, but still not right. Felt like a fuel problem, so I replaced the cheapest items, fuel filter and fuel pump. Made no difference. Motor's are not my strength for repairs. If it is a valve, is it better to buy a rebuilt long block? Please advise.