350TBI 1500RPM engine knock

[Andrew Barker]

Okay, to start I would like to say that this is not a truck, but infact a 92 Buick roadmaster wagon. This forum is my last resort as I’ve had no success anywhere else.

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 unable to put foot on brake and gas and get it to make the noise in drive while not moving. 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
The current oil in the car is 5w30 full synthetic high mileage with an ac delco filter.
Any thoughts will be appreciated.
This is my first Chevy, my other two cars are Foxbody’s

 

[df2x4]

The fact that you only hear the noise when it's in gear and moving makes me think something driveshaft related, like U-joints. Or maybe rear end.

 

[Andrew Barker]

The fact that you only hear the noise when it's in gear and moving makes me think something driveshaft related, like U-joints. Or maybe rear end.

It sounds like it’s the drivers side of the engine bay to me. My dad cant hear it outside of the car either. Could be u joints though.

 

[df2x4]

It sounds like it’s the drivers side of the engine bay to me.

Maybe the driver's side engine mount rubber is deteriorating and it's bouncing around a little? Just a complete shot in the dark guess.

 

[Andrew Barker]

Maybe the driver's side engine mount rubber is deteriorating and it's bouncing around a little? Just a complete shot in the dark guess.

Could be. I’m just so confused because this thing runs really good. I’ve floored it like once when I first got it and spun the tires all through first and chirp second. Haven’t even made it half throttle since becusee I’m concerned about the noise.

 

[thinger2]

What is the timing supposed to be set at?
Every TBI ive ever seen is supposed to be at 0 TDC with the reference wire disconnected.
How and why did you change the timing?
Make sure the timing is set correctly.
Make you arent hearing noise from a bad exhaust gasket or a cracked or split exhaust manifold.
A pop or backfire miss through a bad exhaust manifold can sound just like a rod knock but it will go away and comeback again as the casting swells and shrinks while its running.
Look for exhaust leaks.
Check it for codes even if the check engine light isnt on.
A bad knock sensor will pull timing because of an exhaust leak making it think its detonation, It will set a code but not turn on the CEL.
And, FYI folks.. this little kink about knock sensors also applies to OBD2 systems.

A cracked flexplate is another possibilty too
Or a bad Ujoint, motor mount, trans mount heat shield...

 

[Schurkey]

I bought a short-block that was removed due to a "rod knock". Once it was out, and another engine in place, they discovered the flexplate was cracked all the way around the bolt-holes that hold it to the crank.

Broken flexplates are common.

 

[Andrew Barker]

What is the timing supposed to be set at?
Every TBI ive ever seen is supposed to be at 0 TDC with the reference wire disconnected.
How and why did you change the timing?
Make sure the timing is set correctly.
Make you arent hearing noise from a bad exhaust gasket or a cracked or split exhaust manifold.
A pop or backfire miss through a bad exhaust manifold can sound just like a rod knock but it will go away and comeback again as the casting swells and shrinks while its running.
Look for exhaust leaks.
Check it for codes even if the check engine light isnt on.
A bad knock sensor will pull timing because of an exhaust leak making it think its detonation, It will set a code but not turn on the CEL.
And, FYI folks.. this little kink about knock sensors also applies to OBD2 systems.

A cracked flexplate is another possibilty too
Or a bad Ujoint, motor mount, trans mount heat shield...

I know the timing is supposed to be 0, I was retarded it to see if there was lots of carbon build up or something. It’s currently set a 0 again. I will look into heat shield, u joints and flex plate

 

[Andrew Barker]

I bought a short-block that was removed due to a "rod knock". Once it was out, and another engine in place, they discovered the flexplate was cracked all the way around the bolt-holes that hold it to the crank.

Broken flexplates are common.

Okay I’ll check it

 

[Andrew Barker]

I know the timing is supposed to be 0, I was retarded it to see if there was lots of carbon build up or something. It’s currently set a 0 again. I will look into heat shield, u joints and flex plate

Is there a way to check the flex plate without removing the transmission. Meaning is it possible to see through the inspection cover