Knucklehead97 Posted October 25, 2016 Share Posted October 25, 2016 Hey guys. Been hearing a knocking noise coming from the engine/tranny for quite a while (week after I got it). Have researched into it some and according to the almighty internet it's either my flex plate is cracked or it's a bearing going out. Just stumping me a little bit because it's coming from the rear of ghe engine/front of the tranny (I think) and it sometimes will do it as soon as it starts up and other times waits till it's warm. It gets faster and slightly louder when I rev the engine but ovviously dissapears once I get moving at a good speed (more than "parking lot" speed). Changed my oil and didn't find any chunks of metal and didn't have a glittery look. I've put 4k miles on it with this sound and haven't had a single problem other than it worries me a little bit. Engine runs good other than a little rich. I would have expected a bearing to disable the engine by now. Link to comment Share on other sites More sharing options...
sinkrun Posted October 25, 2016 Share Posted October 25, 2016 If its a bearing you should see a drop in oil pressure. Broken piston skirt will knock quite bad with good oil pressure. Going away above idle can be piston slap, theres lots of you tube videos with the sounds. Link to comment Share on other sites More sharing options...
Kalal92 Posted October 25, 2016 Share Posted October 25, 2016 Sounds like a rocker arm to me Sent from my iPhone using Tapatalk Link to comment Share on other sites More sharing options...
gogmorgo Posted October 25, 2016 Share Posted October 25, 2016 Get a stethoscope and poke around. I had a flexplate crack on me a few years ago, and the sound was definitely coming from the bellhousing. It was also visible as a hairline crack once I pulled the inspection cover off, right up around where it bolts to the crackshaft. By the time I got around to changing it out, about four months after I found the source of the noise, six months after I started noticing it, it was in two pieces. Still running and driving, but causing timing problems, stalling out and such, since the outer part where the CPS reads wasn't attached to the crankshaft. Although the crack wasn't perfectly circular and everything is held pretty tight together in there, so it didn't stop me from yanking a second-gen Avalanche out of deep snow in a ditch, or really anything less severe. IIRC it made the most noise at low load, i.e. parking lot speeds, a bit at idle, and not as much under heavy loads. Link to comment Share on other sites More sharing options...
Knucklehead97 Posted October 25, 2016 Author Share Posted October 25, 2016 If its a bearing you should see a drop in oil pressure. Broken piston skirt will knock quite bad with good oil pressure. Going away above idle can be piston slap, theres lots of you tube videos with the sounds. I still have a dummy light cluster so I can't tell if oil pressure drops unless it gets really low. Gotta get around to finding a full cluster and swapping it in haha. Link to comment Share on other sites More sharing options...
Knucklehead97 Posted October 25, 2016 Author Share Posted October 25, 2016 Get a stethoscope and poke around. I had a flexplate crack on me a few years ago, and the sound was definitely coming from the bellhousing. It was also visible as a hairline crack once I pulled the inspection cover off, right up around where it bolts to the crackshaft. By the time I got around to changing it out, about four months after I found the source of the noise, six months after I started noticing it, it was in two pieces. Still running and driving, but causing timing problems, stalling out and such, since the outer part where the CPS reads wasn't attached to the crankshaft. Although the crack wasn't perfectly circular and everything is held pretty tight together in there, so it didn't stop me from yanking a second-gen Avalanche out of deep snow in a ditch, or really anything less severe. IIRC it made the most noise at low load, i.e. parking lot speeds, a bit at idle, and not as much under heavy loads. I've had some stalling out trouble sometimes at low speeds. And going up a mountain back when I was always in "Comfort" mode the truck would jolt every once in awhile like it was skipping. Might be due to a flex plate crack, who knows. I'm honestly hoping it's the flex plate since it's a bit cheaper to fix from what I've heard. I'll have to get around to pulling off the inspection plate sometime. Link to comment Share on other sites More sharing options...
gogmorgo Posted October 25, 2016 Share Posted October 25, 2016 Or just hook up a mechanical gauge to test oil pressure. That sounds kinda similar to what I experienced, but probably no two flexplates crack the same way. Like I said, I had no problems pulling a pretty heavy load up a steep ditch out of a pretty stuck place. Driving around a parking lot or stop and go traffic caused problems. Something else worth checking, are all the bellhousing bolts tight? I had a pretty rough go of it when mine weren't. Strange tapping noise from the back of the engine bay, stalling out under load, etc. Turns out that two of the bolts between the engine and bellhousing had sheared, one had gone completely missing, and the last one (a tiny top one) was hanging on by only a couple threads. Pretty certain that was what lead to my flexplate cracking, given it was really the only thing holding the engine and trans together at that point. Link to comment Share on other sites More sharing options...
Knucklehead97 Posted October 25, 2016 Author Share Posted October 25, 2016 Sounds like a rocker arm to me Sent from my iPhone using Tapatalk how can I look into this? Link to comment Share on other sites More sharing options...
Kalal92 Posted October 25, 2016 Share Posted October 25, 2016 Sounds like a rocker arm to me Sent from my iPhone using Tapatalk how can I look into this? take your valve cover off and check your rocker arms to see if they're straight Sent from my iPhone using Tapatalk Link to comment Share on other sites More sharing options...
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