What would cause a 3.5 engine knock?

[Jeepwalker]

Hey all,

I looked at and test drove an 06 today a guy's selling cheap. Starting up it had a pretty rough knock that died back after a few seconds ...but was still there when giving it some fuel. It sounded more like a popping sound than a hard rapping sound like one would normally attribute to a bad rod. On a test drive around town or up to 50mph, the knock was more prevelant during acceleration, not present during deceleration. Wasn't knocking during just sitting idling, but ya rev it up and it would knock. Doesn't sound like I would normally attribute to an exhaust manifold leak. Similar to a rod knock, but not quite as hard of a knock as I'm used to hearing ...then again I haven't heard a rod knock on one of these blocks.

Are the bottom ends pretty robust on these engines as a whole? Could there be any noise coming from a balance shaft? What are some of the common knocking sounds these engines can exhibit, if any?

Thanks!

 

[alrock]

What would cause a knock? Maybe someone at the door.

Sorry, don't know engines well enough. But a mis-used engine, or one starved of oil, could of course cause failure at any point regardless of miles or typical quality of the engines. Considering the number of used H3s available, I'd walk away from this one unless they were giving it away. New/reman I5s are unfortunately not cheap.

 

[Happy Hummer]

Improper oil change pattern, schedule. Perhaps a oil passage sludged up or a baffle broken in oil pan. The cam actuators screens clogged up or fell into engine. Rods or journals. Piston slap. Wrong spark plugs, as in there to long and hitting. Broken ring. Collapsed valve. Spun bearing... Etc.

 

[JPaul]

Use a mechanics stethoscope and listen to it. Could still be an exhaust leak. I had a rod bearing go at 85k on my 3.7, aftermarket warranty covered it since everything else checked out fine when the shop pulled the pan. Stuff happens.

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[jakesz28]

Sounds like it needs a new engine. Or a set of bearings crank and a connecting rod.

 

[Jeepwalker]

Thanks guys, I passed on the rig. The noise was coming from the lower passengers side, below the exh manifold. Pretty sure it was rod bearing noise, or something with the rotating lower end (that seemed to be the timing). Yeah, could be oiling, I was wondering that too. Also there was something going on with a head issue or head-gasket. I could deal with all those and the 20 other issues it had, but 1) I've got a lot going on this summer work and project-wise, 2) There are lots of other deals out there with decent running engines (albiet for more $$), 4) I really don't need a second one right now.

I was thinking I could put a snow plow on one and get rid of two Jeeps.

 

[harveyspecter]

Thanks guys, I passed on the rig. The noise was coming from the lower passengers side, below the exh manifold. Pretty sure it was rod bearing noise, or something with the rotating lower end (that seemed to be the timing). Yeah, could be oiling, I was wondering that too. Also there was something going on with a head issue or head-gasket. I could deal with all those and the 20 other issues it had, but 1) I've got a lot going on this summer work and project-wise, 2) There are lots of other deals out there with decent running engines (albiet for more $$), 4) I really don't need a second one right now.

I was thinking I could put a snow plow on one and get rid of two Jeeps.
Find the obd2 scanner in AutoGuysLand website

Good for you. If the noise came from the engine, you should use some tools to diagnostic.

 

[Mb30sdl]

Engine is gone
3.5/3.7 last ~120k before they have to be rebuilt cause long block is no longer made by GM
Best case scenario you looking 5/6 k by good shop with 3year/100k warranty

Yourself ~1/2k


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