greetings to all. i have a 98 xk8. knock sounds like rod knock, but is intermitant. engine idles fine, drives fine. when it reaches operating temp, starts to loose oil pressure.i'm hoping primary timing chain tensioner?

 

I'd start by pulling the valve covers and look to see if the sencondary timing chain tensioners are original plastic. If so, change them first before going any further. I've heard weird noises that sounds like a knocking type noise end up being a tensioner that's so close to letting loose that oil pressure bypasses the oring inside the tensioner because it's so far up the bore. To some extent, let's hope this is what your engines problem is. It's so much easier to replace the secondary tensioners than a rod bearing.

 

Tensioner failure will result in a chain rattle, not a knock. Usually they still pressurize after startup unless total disintegration has occurred, and the rattle stops.

Do you know for certain that the secondary tensioners have been replaced? If not, remove the right side cam cover and inspect. You can also see the primary and guide to some extent.

When you say the oil pressure drops when hot do you mean the light comes on? The gauge is not really functional and should be at mid- scale. It's for appearance, not function.

A knock is usually a rod bearing failure, but a VVT can make a clicking sound that might be interpreted as a knock. You may want to get a stethoscope or just use a metal rod to better locate the sound.

 

thank you gentlemen for your posts. it definatly sounds like a rod knock. but comes and goes. from my experience, i never heard of a rod knock being intermentant. i am hoping its a tensioner. i once pulled a gm v8 and on tear down found thrust bearing worn out causing crankshaft end play allowing crank lobe to hit the block. yes oil pressure gauge needle ddrops a red light comes on. however i'm not hearing any tappet or valve train noise.engine does seam to loose power. i pulled valve covers and upper tensioners seam tight. will check composision of tensioners in morning.