LO5 350TBI knock

Take a seat this is gonna be a long one. 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. 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 two weeks 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. The car runs really well and seems strong. Burns no oil. My next thought is to see what temp thermostat is in the car because it has a new rad, so it is possible that they had an overheating issue and put in a 160f instead of a 195. I bought a 195, just haven’t installed yet, and now I might be waiting even longer to install it because now there is a giant oil leak from the top of the engine coming from either the intake manifold or distributor gasket. 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. 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

I can’t post a link yet, so if you type 10381047291047391 in YouTube the video should come up. Is slightly quieter than that when fully warm, sometimes gone, and significantly louder when cold.

You've probably a tired motor mount and/or something rubbing when the engine moves under load.

If you can - safely - be under the car while someone else replicates the conditions, it'll likely be easy to find as you watch the engine rock back/forth.

I chased a similar issue on another car for months - thought it was an engine issue (recently rebuilt), but it turned out to be an engine movement issue bumping into the frame - sounded exactly like engine knock.

Fix Until Broke wrote:

You've probably a tired motor mount and/or something rubbing when the engine moves under load.

If you can - safely - be under the car while someone else replicates the conditions, it'll likely be easy to find as you watch the engine rock back/forth.

I chased a similar issue on another car for months - thought it was an engine issue (recently rebuilt), but it turned out to be an engine movement issue bumping into the frame - sounded exactly like engine knock.

Alright, I will put it on the lift at my school the next time I get a chance to use it (damn covid screwing everything up)

Any update on this? I have the exact same issue. When I cold start I get this faint knocking from the drivers side of the engine which slowly fade away. Good oil pressure the whole time.

Pubtramp wrote:

Any update on this? I have the exact same issue. When I cold start I get this faint knocking from the drivers side of the engine which slowly fade away. Good oil pressure the whole time.

I cannot figure it out for the life of me. It’s not getting any worse so I’m not sure. I’m starting to drive it as if I couldn’t hear the noise to see if it gets worse. If it gets worse, I have another engine I can put in. If it’s the same, it’s probably fine.

I was reading somewhere that on b body’s (long roof or sedans) have exhaust manifold leak issues which can sound like a rod knock but very quiet. It could be that, but I don’t think so.

How long have you had the sound for?

I saw a TSB from GM a while ago that talked about that kind of noise that is often confused with piston slap.

The noise was said to come from crank movement on the The rear main bearing. Apparently, under certain condtions the the crank could rock up in the back and make a slapping sound on the bearing. The solution was to put in selective main bearings to reduce the clearance on the top side of the bearing. It said it was being implemented on on the new engines that way, also. When I disassembled my 1996 LT1 RMW engine recently it selected fit main bearings in several spots, varying from .0003 to .0007.

had a noised on my 86 dually that I was afraid might be an engine problem, it was always there though but the noise just started one day as the engine cranked and when it fired it ran rough before stalling out, the noised though turned out to just be the fan hitting the fan shroud, as after some trouble shooting found that if I pushed down slightly on the fan shroud the noise would go away.

but there has been a slight ticking from the engine both before the rebuild aswell as after the HIPO rebuild, but I know there is a crack in a weld on one of the stainless steel exhaust manifolds so figure it's valve noise, also this is a 454, as to oil leak from the top, having dealt with numerous 350 tbi's of my own with my other trucks, I have never had oil from the distributor mounting area, but the valve covers would actually be my first suspect, then the intake manifold gasket. if the valve covers ever got tightened down wrong with a cork gasket the gasket can easily get crushed causing it to start to leak. then there's always just age causing things to dry out and fail.

booster wrote:

I saw a TSB from GM a while ago that talked about that kind of noise that is often confused with piston slap.

The noise was said to come from crank movement on the The rear main bearing.  Apparently, under certain condtions the the crank could rock up in the back and make a slapping sound on the bearing.  The solution was to put in selective main bearings to reduce the clearance on the top side  of the bearing.  It said it was being implemented on on the new engines that way, also.  When I disassembled my 1996 LT1 RMW engine recently it selected fit main bearings in several spots, varying from .0003 to .0007.

Was that tsb for lt1 or tbi motors?

Okaywhatever wrote:

booster wrote:

I saw a TSB from GM a while ago that talked about that kind of noise that is often confused with piston slap.

The noise was said to come from crank movement on the The rear main bearing.  Apparently, under certain condtions the the crank could rock up in the back and make a slapping sound on the bearing.  The solution was to put in selective main bearings to reduce the clearance on the top side  of the bearing.  It said it was being implemented on on the new engines that way, also.  When I disassembled my 1996 LT1 RMW engine recently it selected fit main bearings in several spots, varying from .0003 to .0007.

Was that tsb for lt1 or tbi motors?

It was not listed for the LT1 engines, but my 96 had the changes listed as had other Lt1s per what I found on other forum.  Here is a link to a thread on it I started a while ago when I found out the reason I had such odd main bearings.  It includes text of the TSB.

[You must be registered and logged in to see this link.]

Okaywhatever wrote:

Pubtramp wrote:

Any update on this? I have the exact same issue. When I cold start I get this faint knocking from the drivers side of the engine which slowly fade away. Good oil pressure the whole time.

I cannot figure it out for the life of me. It’s not getting any worse so I’m not sure. I’m starting to drive it as if I couldn’t hear the noise to see if it gets worse. If it gets worse, I have another engine I can put in. If it’s the same, it’s probably fine.

I was reading somewhere that on b body’s (long roof or sedans) have exhaust manifold leak issues which can sound like a rod knock but very quiet. It could be that, but I don’t think so.

How long have you had the sound for?

I just picked my RMW up on Valentines day and it has had it the entire time. I have noticed that it will come and go. I am sorta working on the theory that it IS the exhaust manifold and once the metal (maybe) gaskets heat up and expand it closes off the leak???? It is a stretch since I have no idea what the gaskets are made of but it is what I am running with right now.

It will likely have the doubled up steel gaskets as ours that I have been rebuilding the engine. We also had a cold start noise, but it sounded more like a lifter tic than anything knocking. Both exhaust manifolds show leak tracks on them from the rear bolt straight back. This has been discussed a bit on another discussion on the forum about cutting the struts out the manifolds to prevent leaks better.

I’ve been driving the car fairly hard for the past week or so, and the noise hasn’t changed. I’m going to continues to drive it a little bit hard for the next bit and see if it gets worse. If not, it’s probably fine

Pubtramp wrote:

Okaywhatever wrote:

Pubtramp wrote:

Any update on this? I have the exact same issue. When I cold start I get this faint knocking from the drivers side of the engine which slowly fade away. Good oil pressure the whole time.

I cannot figure it out for the life of me. It’s not getting any worse so I’m not sure. I’m starting to drive it as if I couldn’t hear the noise to see if it gets worse. If it gets worse, I have another engine I can put in. If it’s the same, it’s probably fine.

I was reading somewhere that on b body’s (long roof or sedans) have exhaust manifold leak issues which can sound like a rod knock but very quiet. It could be that, but I don’t think so.

How long have you had the sound for?

I just picked my RMW up on Valentines day and it has had it the entire time. I have noticed that it will come and go. I am sorta working on the theory that it IS the exhaust manifold and once the metal (maybe) gaskets heat up and expand it closes off the leak???? It is a stretch since I have no idea what the gaskets are made of but it is what I am running with right now.

FYI I know what it is. I found a tsb for 91 and 92 roadmaster that have piston slap for like 10 minutes or more when cold. Apparently it’s fine, but the solution was to replace the engine/piston if the person cared enough. So I’m just gonna keep on sendin er