Hi, I have a 1991 Full Size Blazer with a 350 TBI engine. The engine is original to the truck and it is all stock. The engine has 109,000 miles on it.
It recently started knocking when it warms up, and the knock is quite loud. The knock will get faster/slower depending on engine RPM. I will post a video of this tomorrow so whoever tries to help me can get a better idea of the knock. I am not sure if it is the engine itself of a pulley or water pump or something like that. The engine has great oil pressure and has lots of power, no overheating or anything like that. I just changed the oil a few weeks ago on the truck using Castrol GTX 10W40 and an ACDelco oil filter. There are no oil leaks and the oil level is right where it is supposed to be. The oil looks nice and clean, no metal particles or anything like that.

 

The noise seems to be the loudest at/near the crank pulley.
Here is a video:

 

If it has good oil pressure then it likely is a rod knock. Does it go away once everything warms up or stick around? If it sticks around, then it definitely is a rod knock. Have you tried some Lucas in there yet?

You could narrow it down to which rod possibly by pulling off plug wires one at a time seeing if you can get it to change pitch. Be careful, the spark is HOT!

 

Swartlkk, I always appreciate a response from you, thanks.

The engine has really good oil pressure and it runs good, even drives good, never overheats or anything like that. Starts right up. The knock is almost non existent when the engine is cold, but gets loud and stays loud when it is at operating emp. The engine was up to operating temperature in that video. I have not put any Lucas or anything like that in the engine. Just good old Castrol GTX 10W40.

 

Ok. I missed that it wasn't there when it's cold. Sure sounds like a rod bearing then. As the parts expand and clearances open up due to heat, the noise gets louder. Makes sense.

You may have enough room to drop the pan and replace the rod bearings with out doing much extra effort. If you can get the pan off, it should be readily apparent which one is bad by doing the wiggle test (very technical).

 

Yes, that test is very technical indeed
As much as I hate to ask this, do you believe the noise could be coming from a pulley or a bearing? Water pump or power steering pump? I noticed the noise is the loudest at/near the crank pulley underneath the engine and the pulley seems to be wobbling a little?

 

It is very doubtful, but easy enough to check for. Pop off the accessory drive belt. The only thing that would be rotating then is the crank pulley & balancer. You can unbolt the crank pulley and remove that as well as a test if you wanted to, but I really doubt that a bolted on pulley is causing that kind of a knock.

 

What is involved in removing the crank pulley? I strongly believe that is the source of the noise. It wobbles and the noise sounds the loudest at the crank pulley, like I have previously mentioned. I drove the Blazer all day today and I still heard the knocking but it was more faint than it has previously been.

 

Verified that the problem is, in fact, the crankshaft pulley.

 

Hello. First post here.

I have a similar knocking in my 91 K5 and was just told it is the piston pin. Noise is very apparent when going up hills and goes away briefly when RPM's go up then comes back and only knocks when accelerating for right now.

Any suggestions on where to get a good replacement engine, I wanting to put somewhat of a performance engine in it with more HP than the stock GM 350 or is it better to stay with the stock GM engine.