I have a 1993 GMC Sierra K2500 with a 7.4L Big Block V8, manual transmission. When my engine idles it has extremely low oil pressure, it nears the red line. While driving it goes up, but never over 30psi. The engine also runs hotter than normal (say at about 210). When I first start it, it surges really bad and then gradually gets better but never completely stops surging. Finally, the engine also makes a loud tapping noise which is slightly in sync with the rpms and seems to be coming from the back of the block, possibly near the bottom.

I recently rebuilt this engine and replaced the camshaft and checked the valves, lifters, rocker arms etc. and they all seemed fine. The MAP sensor has just been replaced as well. I'm also almost positive that it is not an exhaust leak. The noise has remained constant before and after the rebuild.

Someone told me that on these 5th Generation blocks, the oil bypass valves can sometimes make noise. Is that a possibility?

I am at a loss here!

 

Sounds like it could be a spun bearing. Same symptons I had when I spun mine. Not the kind of news you want to hear, I know.

The noise could just be a lifter.

 

sounds like a spun cam bearing?!?!?!?!? just my opinion...