Hey y'all, I'm hoping y'all can help me out.
I have a 97 4.3 that i had rebuilt last year develop a knock after running at higher than normal rpms for an extended period of time. (Towing with the overdrive off)

The thing is that the knock goes away after I allow it to idle for a few minutes. If anything my oil pressure is high and it never dips under throttle. (40-60 psi)

There is absolutely no knock on initial startup or if I keep my rpms below 2k.

I used some Sealed Power lifters when I reassembled the engine long block. Could that be the issue?

 

And what is the normal than higher rpm's you were reving too. And I have run sealed power lifters with never a problem,

 

Right around the 3k ball park. I did take half of the sealed power lifters out and replaced them with good leftovers I had from before the rebuild and it lasted a good 30 minutes longer before the knock came back.

 

what type of oil are you running? If the oil you are running is too thick, depending on the type of knock you are hearing, it could cause a knocking noise at wierd times. I run Mobil 1 Full synthetic 5W-30 oil in my blazer.

 

My truck knocks at start up also, althought not rebuilt. What mine is is piston slap. Pretty common on theses trucks

 

I have been running 5w-30 dino oil just because of cost but it doesn't seem to matter what I put in it.

on startup this thing hums like a sewing machine.

 

shouldnt have any concerns with that oil then. Hmmm. Does it change or not do it depending on the engine temp? Does it get louder once its warmed up?

By the way x2 on what newguy said, these engines are common for piston slap.

 

Hi Jprevat. Hopefully your knock is just a concidence after your tow and not damage caused because of it. I had a similar knock on both 1993 GMC Sonoma 4.3 that I owned. Mine would only knock after a cold start and then go away after a few seconds of idling.

My solution was to change the oil as soon as I would hear it and add a quart of "Slick 50" oil treatment. The sound would go away for a year or so after using the additive.

An oil change would be the easiest place to start!

_______________________

1999 Blazer LT 4dr, Auto-4WD, AT
Bilstein 4600 Series Shocks
Axle: 3.42:1 w/ Eaton Detroit Truetrac Locker
Tires: 235/75-15 Michelin LTX A/T2
Gibson cat-back exhaust
K&N CAI
Hypertech Power Programmer Tune

 

It actually doesn't knock at all until directly after continuous higher rpm operation. It doesn't knock on start up and not even after it warms up. The oil has less than 100 miles on it so I'll pick up some slick 50 and see what happens.

thank you all for all the suggestions so far guys