I have owned this truck for about a month and just started having a problem. It is a 350 5.7l. When truck is cold there is a bad knock in the motor and oil pressure reads 15 to 30. As soon as truck warms up I have no oil pressure and the oil pressure goes up as long as i have my foot on the gas. Also have full power of the motor when cold but when at operating temp I lose about 50% of the power in motor. What could be causing all this????

 

Doesnt sound good. A knock,and no oil pressure would point to worn bearings. If the oil pressure was low the lifters could bleed down. Put a good mechanical gauge on it to verify the oil press. If it is low better start digging.

 

Yea, I would suspect you had a bad oil pump and now you may have a bad motor. This is seldom the case on rotary gear oil pumps, but depending on maker you could have one of them sliding vane pumps where the vanes are plastic and have to slide in the housing to create suction. All is speculation though. Check your oil level and externally mount a gauge to check pump pressure.