Re: Buick 3.8 V6 - strange knocking/rapping sound?

> I have a 1988 Buick Park Avenue with 65K miles on the 3.8 L (balance shaft) > engine that is(well was) in perfect condition. > > I have a rapping/knocking sound when the car is placed in gear, brake > applied, and reved to approximately 1200 rpms. The rapping is so loud that > is sounds like the rods are going to come through the block. The sound > radiates across the complete engine. Doesn't matter if the engine is cold > or hot. > > Now the strange part. If I remove the serpentine belt the noise disappears, > the engine runs perfect, smooth and no noise at any rpms, in gear or out. > > The power steering pulley, water pump, and alternator pulley are all very > smooth. Checked the damper pulley and bolt tightness...all OK! > > I have replaced the idler pulley and the tensioner spring assembly and also > replaced the A/C compressor pulley bearing in hopes of eliminating the > noise....no luck! > > I have rotated the belt, put a new one on....no help. > > Again the strange thing is the rapping noise is defening with the belt on > and gone with the belt off. > > What kind of load is the serpentine belt applying to the the crankshaft to > cause such a noise? How could this cause the crankshaft to move, if any, to > make noise? > > Any help as to what to look at next would be appreciated? >

With the engine running and the rapping noise present, get a long screwdriver and place the handle against your ear or other part of your head so that sound will conduct through the screwdriver, then start placing the screwdriver against various accessories until you hear the one making the most noise. It will be evident right away what the source is.

JazzMan

Reply to
JazzMan
Loading thread data ...

I have used a sethescope and still coulding find the problem. Noise radiates across the engine.

Reply to
Bob Petruska

Check that harmonic balancer again. It does tend to break in 3.8s of that age, and it causes just that kind of noise.

Reply to
Christopher Green

Chris,

Can you explain in more detail what I should be looking for in the harmonic balancer to be bad?

Reply to
Bob Petruska

Could this also apply to a 3800 engine in an Olds Silhouette? I've changed the oil and filter in mine religiously since new, and now I have a knock (just at 2000 rpm) that is activating my knock sensor (Electronic Spark Control) such that it's retarding the timing 15 degrees. Dealership says engine is dying and "they're sorry". It's their loss, since there won't be another GM in my future as a result! M.B.

Reply to
M.Burns

The others are right, a bad vibration damper (harmonic balancer) is very commone in the 3.8 V6. Look for seperation of the outer ring from the center section. If you wait too long, it may wipe out your crank sensor.

Reply to
Curly

The harmonic balancer consists of metal disks joined by a rubber damper. The purpose of the thing is to damp shock loads in the crankshaft, especially loads in torsion (twisting). A sudden twist applied to the crankshaft will be resisted by the inertia of the balancer and dissipated in the rubber member.

What most often happens is that the damper develops fatigue cracks or the rubber wears out. It's a well-known problem on the 3800 (one of the engine's few faults, and not a really bad one, because it's not difficult to replace). When it goes, it tends to bang around noticeably at idle. One way to see this is to look at it with a timing light.

Not a difficult job to replace if you have the proper puller. A garage wouldn't charge an arm and a leg for this, either.

Reply to
Christopher Green

Chris,

This definitely sounds like the problem, even though the balancer looks very good. I'm going to remove it this weekend. Yep it's on there by a bolt torqued to 210 ft-lbs and doesn't need a puller as it usually falls off after the bolt is removed. I removed it one time to replace the timing chain on the engine. I woill probably try a used one first just to see if the noise changes. I'll also use a timing light to watch for the balancer to dance.

I will post here my findings....

Thanks again,

Bob P.

Reply to
Bob Petruska

BINGO!

I was the harmonic balancer. I was not aware of it's construction as compared to other balancers that I'm familiar with. The rubber diaphram stops were against the Y yoke bars causing the noise. On the new balancer the stops are centered between the bars. There was major slippage or diaphram degradation.

Thanks again and thanks to all that suggested the balancer. I could have saved 2 weeks looking for this noise if I wasn't so lazy to borrow the 3/4" breaker bar to remove it. I would have also saved me the cost of the tensioner spring and pulley assembly and a used power steering pump!

Reply to
Bob Petruska

BINGO! It was the balancer!

Reply to
Bob Petruska

Thanks for posting the result. I have archived this for future reference since I just bought a buick 3.8 to eventually go into my 29 rod. "Bob Petruska" ( snipped-for-privacy@mail.microserve.net) writes: > BINGO! It was the balancer!

Reply to
Ken Hilson

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.