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 L05 350TBI random knock

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Posts : 7
Join date : 2020-12-02

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PostSubject: L05 350TBI random knock   L05 350TBI random knock Icon_minitimeTue Jan 26, 2021 8:31 pm

Ive already made a post about this, but I am wondering if anyone else has any thoughts.

Back in June I bought a 1992 Buick roadmaster wagon that needed it’s 700r4 rebuilt. I was told it has 200,000km on it and the cluster which reads 300,000 was swapped out of a different car. So I’m not really sure the mileage on it. Anyway, the oil was pretty black when I bought it so I changed the oil to 5w30 conventional with a fram filter. Oil pressure was good. 50 pounds on startup idle and about 25 when warm and about 15 in drive at 600rpm. It had been warm basically every day so I always had the windows down and music playing. One day it was raining maybe a month and a half after I bought it, and I had the windows up. I noticed that about 1500 to 1550 there was a knocking sound. Louder when the car was cold and more quiet as the car warmed up. Sometimes completely gone, sometimes barely there. NOTE THAT IT ONLY HAPPENS IN DRIVE, NOT IN PARK IF I REV IT. I am also ABLE to put my foot on the brake and gas and get it to make the noise while not moving. If I hold it there for a couple seconds the noise seems to slowly fade away and the be gone for the rest of the drive SOMETIMES. And if you get up to around 1600rpm the noise went away. I thought that it was probably timing related because my four cylinder mustang sounded the same until I reset the timing, so I just drove like that for another couple months because the hold down bolt wouldn’t move. Maybe 2 months ago I checked the timing and saw it was set to 0 degrees. That’s when I became a little concerned. I was able to get the hold down loose and adjusted the timing to 4 degrees atdc. That did not help or change anything. I was thinking piston slap but I’m not sure sure as it’s still there a bit when the car is warm sometimes. The car runs really well and seems strong. Burns no oil. I am so fed up with this stupid car. I bought a tbi because they are supposed to be more reliable than the LT1. Not sure bout that one in my case. I tried 10w30, and that made no difference. Tried 5w30 with half a bottle of Lucas. No help. The current oil in the car is 5w30 full synthetic high mileage with an ac delco filter.
Any thoughts are appreciated.
This is my first Chevy, my other two cars are Foxbody’s

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I have made a video about the sound. If you dont feel like listening to me ramble here are some important time stamps:
Sound can be heard at:7:45
Sound can not be heard in neutral:8:12
Noise is there, but much quieter as car is warmer:9:40
Car is warm, sound can not be heard:12:05

All I can think is cracked flexplate or something else dumb. I can't imagine it being a rod knock since the noise goes away when warm instead of getting worse. Ive put 4000-5000Km on this, but I am afraid to put my foot to the floor to merge onto highways if its the engine Sad . And I really need to since this has 2:73's
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Fix Until Broke

Posts : 162
Join date : 2019-04-05
Location : Southeast Wisconsin

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PostSubject: Re: L05 350TBI random knock   L05 350TBI random knock Icon_minitimeTue Jan 26, 2021 9:31 pm

Will it do it in reverse?

What you describe is likely due to engine movement. Best bet to find it is to get under it (safely) while having someone load the driveline - in drive, on the brakes - to recreate the noise. Once you're under the car, it's usually easier to find/isolate. Have a prybar handy to push/pull things to make it better/worse.

Worn motor mounts can make the engine move just a little more than it used to and something just barely contacts and will make a knocking noise. It sounds horrible like you describe, but it's usually something simple.

Body mounts also sag and reduce clearance between the driveline and the body and can do the same thing.

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