The dreaded piston slap.

Since I bought this Blazer it has been a pain in the butt. It isn't the fault of the Blazer it had an engine installed before I bought it & I didn't find out until after I bought it for cash.
Well, since just after I bought it I noticed a couple of different knocks. One I was sure was a lifter the other I thought could be a valve, piston, or carbon knock. The other thing I noticed was I felt the oil pressure was not as good as it should. 15 PSI at idle & 40 PSI at 1500+ steady RPMs.
So I tried going to synthetic oil, I added synthetic Lucas, I have used Techron, & Lucas injector cleaner. Nothing seemed to help. Once I was pulling a 5000# trailer up a steep grade in second gear at about 3000 RPMs when all of a sudden I heard a loud continuous knock, I thought I blew the engine. I backed out of it & around 2000 RPMs the knock quit. Well I continued & it did it once more. So I pulled the trailer the last 500 miles home with no trouble. Well these knocks have gotten worse but not bad, bad. So I decided to Seafoam this engine & see what happens. Well, it has given me the answers. I have a piston slapping if I take the RPMs up under load & there is a lifter or rocker that is wore. So I took it to the shop & it was agreed that the best thing I can do is buy another engine. So I'm going to buy a used engine & install it. The shop has a trusted yard to buy from. So $600-$800 for a guaranteed good engine will corect the issue. Plus labor. But the guy that owns the shop told me I can help so that should knock a little off.
I agree with those of you that believe as I do good oil & filter on a regular basis is the best. But I was pretty sure this engine was going SOUTH anyway.
So know I will drive it until we pick up the used engine or it blows. I really don't care either way. I prefer it doesn't blow but I'm not going to let it sit. I want as much of my money out of this as I can. I have tow insurance through my insurance company & they will tow to where I want it towed.
I really like the Blazer as a truck. It is a good 4x4, it is comfortable & get good mileage 16-23 MPG. It gets 19-23 regularly on the road @ 70 MPH.

Good Luck

What is the oil pressure you guys run on your 4.3L V-6?

I have a 95 blazer that had low oil pressure. Changed oil sending unit its under the back of the distributor that fixed everything. oil pressure is 40 idle hot and anywhere from 60 - 80 under load also have a 97 zr2 that has the same oil pressure with 160,000 miles on it. blazer knocks slightly on hills but thats fuel grade.

how hard was it to change the sender? i have a 95 with pressure readings around 20pis at hot idle and 40-42 depending on load. doesnt knock or tap. from what i've seen all 95's ran low pressure. not sure when they bumped it up.

Pulling a 5000lb trailer with a 4.3L V6 is a great way to kill the engine. You far exceeded the tow recommendations and directly caused the failure. If you plan to continue pulling such heavy trailers then it is time for a bigger truck with a few more cylinders.