I have a 1991 GMC C1500 with a 350 5.7 and it only knocks after it get up to tempeture and ran a while. It's isn't low on oil and I use 20W-50 GTX. whats could the problem be.

What is the oil pressure when the knocking starts?

//2000CAYukon

the oil preasure drops to about 15 when it starts and i noticed today if i turn the A/C on it stops

My foreman told me that it is my rod bearing going bad, How serious is that and how hard is it to fix? Is it something I can do in a day in the driveway or do I need to take it to a shop and let them pull it.

Hi

If you are asking that question , then definately take it to a garage. This is major ,lower engine work. Try another mechanic to make sure this is really the problem. I watched a show on car repairs and a garage said a particular car needed a carb kit. The vehicle was fuel injected????
Get the picture! good luck.

Kevin