1/2 ton Chevy Pick-up Engine knock

I have a 2001 Chevy 2wheel drive 4.3 V6 Pickup with a long lasting engine knock. I use it only to haul trash and building materials, maybe once every other week.

The knock is sporadic. If I take a long trip and drive on an interstate highway the following morning upon start-up the truck will knock heavily and continue to do so as long as it is running. If I take the truck for a short local trip, the following morning it will start with no knock and only start knocking after driving maybe 10 miles. If I drive only short trips, the truck will almost always start without knocking and only knock once it is fully warmed up.

I purchased this vehicle from my brother who drove it this way ever since he purchased it at a salvage auction (full of dents and some damage) who put over 20,000 miles on it with this same engine knock.

Other than the dents, the truck is in great shape, no rust, and has 118,000 miles on it. I would like to keep this thing because it was very cheap and I only use it sporadically. So, I’d like to know if I’m going to eventually have to put a motor in it or if there is some small fix I could make to stop this knocking sound.

The sound: It is heavy. It sounds bad. It knocks along with the rpm of the motor. My brother, who is also a great mechanic, thinks it may be piston slap or something in the valve system. I’ve also had a mechanic listen to it. He thinks it’s a rod bearing or wrist pin.

What I don’t understand is how could it be any of those things when it makes a difference how I drive it the last time it was used? Could it be something simple like a vacuum issue, knock sensor or crank sensor?

Please advise, thanks.

I should also add that while driving, the truck mainly knocks at a speed that requires only slight gas pedal pressure to maintain that speed. Under heavy load, it does not knock. It seems that when the internal parts are free to slack, that is when the play in the parts becomes apparent, like at the top of piston revolution, without lots of downward force being placed on it.

My WAG is that the difference depending on how you’ve driven it recently doesn’t change the underlying story. There is likely something very important in the low end (like bearing or wrist pin) that is out of spec/worn/damaged/whatever. The variation likely comes from how hot - and thus thin - the oil gets. After a really long drive with engine at full temp the oil will be thin and runny and most will run back down to the oil pan. On shorter drives the oil is thicker and more is left clinging to engine parts. That’s my WAG anyway.

Its not a vacuum issue since there isn’t one that will get the engine to thumping. The knock sensor is actually for a completely different kind of knock - the preignition kinds that is also called pinging. That’s the sound of something like marbles or BBs vibrating in a can while the engine is under load. Piston slap is sort of in the same family of sounds. If you had piston slap it would show up only when cold and it is basically a rattly-like sound that reminds one of what a diesel engine sounds like. The crank sensor is only about spark & ignition timing so I can’t see that figuring in.

Anyway, my guess is that you have something in the low end that is out of order. Since this is apparently not a daily driver and you just use it for utility purposes the first thing I might do is just go to the next higher oil weight (e.g. if you’re using 5-30 go to 10-30 or 10-40). Depending on my own plans for life and the truck I might even try one of those crazy additives such as Restore.

Also based on the fact that you just want this vehicle for occasional utility purposes, I would actually think about pulling the oil pan and having a look at the crank shaft, wrist pins, etc. I’d check the compression first just to be sure it isn’t already a total disaster in there. But I’d also be doing this myself - so it wouldn’t be costing me much.

Ok - well now (even though I said a bunch of stuff below) just clarify. Are you talking about a thump thump thump noise? Sort of deep and low from deep inside the engine. Or are you talking about the rattling-type of noise that sounds sort of like marbles vibrating in a can?

An excellent post, Cig. The only point I’d add is that unless the OP will be willing to do a rebuild should the bearings show a problem, there’s no sense pulling the caps.

Should the OP decide to do so, I found this nice website with photos of what to look for.

Thanks MB - excellent score on that pdf file on bearing failure. I’m keeping that one in my “auto info” collection.