I have a 98 Blazer and I just replaced the engine in it (4.3 ltr). The engine ran great and i even drove it about 10 miles testing it out. The thing purrs! The only problem is when I start the engine, there are a lot of loud knocks... I only hear them when it's turning over or only one loud knock when I turn the ignition off. Otherwise runs fine. It did get to the point where it got so bad it wouldn't start. We tried replacing the timing chain to no avail. Any thoughts as to what may be causing this? Thanks

 

Have you figured out what is knocking?

What oil and oil filter are you using? Make sure hte filter has a check valve.

 

No not yet. Was hoping someone here would point me in the right direction. Just got the motor from a junk yard an put it in. It only knocks during startup so i'm thinking it's not a rod or the other usual suspects. The oil is 5w 30 napa brand i think and the oil filter is a fram

 

The best tool for finding a knock is a long piece of metal rod. I use a hot dog stick for over a campfire, break off the double tong and use the rod/wood handle. Place the metal rod agains the engine block by the crank/cam, and heads by the valves. The noise should get obviously louder the closer you get to the source.