knock/valve noise comes and goes
 

I have a 93 5.7 tbi with 80,000 miles. its been perfectly reliable and runs fine (maybe not perfect but sounds smooth and all) but recently on a 300 mile trip it started making a knocking or valve noise. It only happened when I was going uphill and it was under heavy load. It also seemed to loose a lot of power when it started doing it. it made the noise for 30 miles before I pulled over in the next town. It was still making the noise at idle so i shut it off and checked the oil. oil was maybe one quart over full. I turned the truck back on to try and find a mechanic and the noise was gone. then it ran fine for another 150 miles until i got to a steep pass. it started doing it again and would barely accelerate. I also noticed the oil pressure went down to 20-30 when its usually running at 40-60. after I crested the top of the pass and started down the other side, pressure went back up to 50-60. then the noise went away again.

Now I have to drive the trip again tomorrow. Has anybody experienced anything like this? does this seem like valve noise or something worse? Ive done a lot of googling and havent found any cases of people having a noise come and go like this. I don't want to try to drive it home if its a bearing or something that might trash my motor but i pretty much need to drive it home, even if that means going 20 mph up the hills... but i also don;t know that slowing down will help all that much on the long steep pass...

Also, I just did an oil change with 10-40 and I ran some lucas fuel system cleaner through on the last tank fill up.

Any ideas?

Just an update. Made it back, barely. The truck is fairly weak all the time now, but will go 65. Every time i come to a long hill, it will start knocking again. I have to go 25 up long hills, and even then it starts to knock eventually. I filled it with a fresh tank of 91 octane so i think bad gas is out. I did some reading and it sounds like some people have experienced spark knock due to a lean condition. This seemed like it could make sense since the knock only comes on after maybe 20 seconds of hard throttle, and it will go away if i coast for 30 seconds or so. I'm thinking maybe the combustion chamber is heating up due to the lean condition and causing the spark knock maybe?

Any thoughts anybody? I'm home now so hopefully tomorrow i'll be able to pull the plugs and see how they look and test the fuel pressure, and go from there. Any and all input is much appreciated

Just wanted to update in case anybody else ever has this issue. I had a mechanic ride along with me and he told me he was pretty sure the noise was from a valve lifter, and told me he thought it was probably collapsing (to me this didnt make sense because my oil pressure didnt drop but i went with the idea anyway). I bought a camshaft and set of lifters and was going to replace them and hope it fixed it, but while i was waiting for a good time to do it, i drove the truck. I ran a can of seafoam through the gas tank and then ran a gas tank with marvel mystery oil. nothing changed. I added a third of a can of seafoam to the crankcase (my oil change had about 1500 miles on it at this point) and drove it for another gas tank and was going to change the oil and then pull the cam and lifters. I changed the oil (this time using 5w-30 as recommended, last time i accidentally used 10w-40) and added one quart of rislone motor treatment to the oil.

The noise is now gone. I drove up a mountain that caused the noise multiple times before, and this time nothing happened. Not sure if it was the rislone, the seafoam, or just the fact that I switched back to a 5w-30 increasing oil flow but either way, saved me what probably would have been a 500 dollar job from a mechanic...

A collapsed lifter alone will not effect the oil pressure.

Yeah. Nobody is saying it would.

More like the oil pressure alone will affect a collapsed lifter...

Assuming the check valve is working.