350 Chevy knocking noise!

[76-77-78]

I have a 350 L31 carbed vortec engine in my 77/83 Chevy k20 that developed a very loud knock that sounds identical to the one in this vid

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I fired it up one afternoon to fill gas in town and shut the motor off at the pump and it hadn’t made a sound. but when I had finished filling gas and fired the motor back up it immediately started knocking really bad, but I was able to drive it the 3/4 mile home without it self destructing. the sound is definitely louder on the drivers side it sounds to be lower end as far as I can tell and it’s very loud. Some additional info engine has approx 180k oil pressure 40psi cruising and 25psi idle with engine warm and didn’t change after knock occurred. Just want some opinions and feedback from the people here with way more experience then me. it will be a few weeks before it’s warm enough out to tackle this project just want to know what I’m in for and maybe some tips for how to fix my problem right

 

[Snoots]

Pull the valve covers and check for a bad rocker arm.
I had one go bad many moons ago and the side of the arm that presses on the valve had worn thru.
Rocker arm from the boneyard and a new valve keeper fixed it.

It's worth checking out before you pull the motor apart.

 

[1987 GMC Jimmy]

Can you do a one at a time unplug of the spark plug wires at the plug and run it on seven to see if you can isolate a specific cylinder? Other than something in the valvetrain, a very serious exhaust leak, or a flexplate issue, that sounds kind of like a connecting rod bearing to me. Please exhaust all other avenues because no one wants to hear that, and I don’t like to be the bearer of bad news. That’s your worst case scenario.

 

[76-77-78]

Ok I’ll try all those suggestions tomorrow it’s supposed up in the 40s so I shouldn’t freeze my fingers off. Still won’t be fun though

 

[1987 GMC Jimmy]

I'll take that 1973-77 A Body in your profile pic if that helps you out. I'd like a '73 Chevelle Laguna with the rear window like yours. That big triangle looking thing throws me off.

 

[76-77-78]

I'll take that 1973-77 A Body in your profile pic if that helps you out. I'd like a '73 Chevelle Laguna with the rear window like yours. That big triangle looking thing throws me off.

Well you’re about the 50th person to ask to buy it so you’d have to wait in line. but I wouldn’t sell it if I was dying and needed surgery money. it has a long history in my family I’m the third generation owner of it and love it. I will probably start a build thread for it if I ever get some extra cash around to start work on it

 

[1987 GMC Jimmy]

Well you’re about the 50th person to ask to buy it so you’d have to wait in line. but I wouldn’t sell it if I was dying and needed surgery money. it has a long history in my family I’m the third generation owner of it and love it. I will probably start a build thread for it if I ever get some extra cash around to start work on it

Oh, I was just suggesting you give it to me. Does that move me up in the line? I’m kidding, of course. It’s nice that you’re keeping it in the family, which I’m the exact same way. As beneficiaries, many people would just rather sell it and go for the green. What exactly is it?

 

[75gmck25]

Since you don't remember any catastrophic event (over revving, etc), another simple item to check for the knocking is to tighten the bolts holding the engine and transmission together. If one or more have loosened up you can get a "knock" as the engine vibrates against the transmission housing. It sounds like its coming from inside the engine, but is really simple to fix.

Bruce

 

[HotRodPC]

I'm going to go with a cracked flexplate. Of course we don't get to really hear it on the camera. I'm also guessing that's that's not the original engine. Run the VIN, I have a feeling it was originally an Inline 6 truck.

 

[76-77-78]

Oh, I was just suggesting you give it to me. Does that move me up in the line? I’m kidding, of course. It’s nice that you’re keeping it in the family, which I’m the exact same way. As beneficiaries, many people would just rather sell it and go for the green. What exactly is it?

It is a 1976 chevy Malibu classic with 72k on it a 250 L6 engine th350 trans and is almost completely stock other than shocks wheels tires and audio system. The 250 isn’t much for power but the car cruises great and rides like a caddy so I just use it as a daily driver and don’t really try and hotrod it much. Oh it also still has the original bias ply spare tire in the trunk also.

 

[Frankenchevy]

Oh it also still has the original bias ply spare tire in the trunk also.

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[76-77-78]

Since you don't remember any catastrophic event (over revving, etc), another simple item to check for the knocking is to tighten the bolts holding the engine and transmission together. If one or more have loosened up you can get a "knock" as the engine vibrates against the transmission housing. It sounds like its coming from inside the engine, but is really simple to fix.

Bruce

Well a few days prior to the knock starting I did get stuck on the lake while ice fishing and did some revving for a bit but probably never went over 4000 rpms in first/second gear it has a sm465 so I was in the first gear that’s not granny gear so maybe that had something to with the knock I don’t know. But I pulled valve covers and didn’t see anything wrong so I put the covers back on and fired up the engine and it immediately started knocking like crazy and one time it even pulled the engine down enough to kill it so I’m guessing that it’s a bearing but what do you guys think?

 

[TravisB]

I had a truck that made a noise like that when the torque converter bolts loosened and were hitting the dust cover. Check that too if you have an auto. The dust cover was loose too and it made the sound sort of intermittent. It was probably the 3rd time replacing the 700r4 that came in the 84 and I guess we were getting lazy lol.

 

[76-77-78]

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Lol yeah it’s definitely not the best option for a spare but it been with the car since the beginning and I just can’t bring myself to separate the two but that 43 year old tire as far as I know nobody has put air in it and it still has like 45 pounds in there

 

[76-77-78]

I'm going to go with a cracked flexplate. Of course we don't get to really hear it on the camera. I'm also guessing that's that's not the original engine. Run the VIN, I have a feeling it was originally an Inline 6 truck.

The truck in the vid is not mine just a vid I found with the exact same sound my pickup is making my truck was an original 350 4bbl truck with a manual and is still a manual but I’ve had enough suggestions to check that so I guess I’ll man up and lay in the melting snow for awhile and look the clutch and flywheel and bellhousing bolts just to make sure they aren’t the problem