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Pontiac Bonneville Engine Knock

dhillgerdhillger Posts: 7
edited April 2014 in Pontiac
I've got a 1996 Pontiac Bonneville with the 3.8L V6, not supercharged. Its got this knock in the engine, and has for a while but has gotten more prominent in the past year. Seems like its more of in the middle of the engine rather than in a valve cover, but not to sure about that. It still runs fine and has great gas mileage but I don't like this sound. It seems to be the loudest when at idle and when I rev the engine it goes away briefly. Seems like it goes away when the rpm are rising quickly but comes back as the rpm fall. And when accelerated slowly, the knock is there the whole time.

What could I do to get rid of this knock?
I'm afraid to do a 0-60 test for fear of this knock destroying my engine.
Could it be a valve lifter?
Could it be a stuck valve?

Any help would greatly be appreciated!

More info:
136,000 miles
New head and water pump put in in 2005
Sparkplugs, wires, pcv valve, airfilter all replaced less than 300 miles ago.


  • azuazu Posts: 84
    I would make an educated guess that your harmonic balancer is shot. You would mostly hear this noise at idle and goes away as RPMs rise if it the HB. If you have it during higher RPMs I would suspect an oil pressure problem. How is your OP?

    You mention new head. Are you refering to the UIM(Upper intake manifold) or LIM (Lower intake manifold)?
  • Oil Pressure is fine, maybe a little low but the knock is gone at high rpms.

    I went to O'Rielys and bought an oil additive. (the kind thats says it will quiet noisy engines.) Put it in ad let it run for a few minutes. It has made the knock less noticeable at idle but is still there when revved. It now doesn't knock as much when coming down from high rpm, BUT it is still there. I'm not much of a mechanic so do you think replacing a push rod or rocker arm or race pin is hard? It it as easy as taking the valve cover off and doing it there? One of the valve covers is going to be hard to get to if I do.

    Here is my conclusion. This engine has been making this sound for at least a few years and has been driven about 10k miles per year. If it was a major problem, i think something would have happened by now. I think its just a slightly loose push rod or rocker. (thinking its a push rod where it touches the cam because the sound comes from deep in the engine) So... if that is all, I think this engine will be fine and will give another 70k to 80k miles no problem. What do you think?
  • yardhutyardhut Posts: 2
    I have a 1993 Pontiac Bonneville SSEI (supercharger) with a horrible noise at idle. We at first thought it was the bearing in the nose cone of the supercharger, had that fixed. Start it up and it's still there. Then we thought the tensioner unit caused the problem (supercharger) It is out and the noise is still there. It appears it's now in the crankshaft pulley. We need a diagram of that area to see how it is put together and further diagnose the problem. How is the pulley attached? Does it have a shear pin or what?
    Please answer as soon as possible.
  • imidazol97imidazol97 Crossroads of America I70 & I75 Posts: 21,512
    The crankshaft pulley also carries a block of metal that's called the balancer. They are separated by rubber. The rubber may deteriorate letting the inner and outer parts bang against the other as the crankshaft speeds up and slows down with the pulses of energy from the cylinders firing. See if you can wiggle this inner block of the crankshaft wheel.

    The crankshaft pulley comes off with a big nut. It requires an air gun that will turn it quickly against the inertia of the whole crankshaft and all or you block the crankshaft in some way to loosen it yourself with a long handle on the socket.

    Sounds like the people doing the superchargeer and idle pulley didn't fully verify that those were the source of the noises before they replaced them.

    2015 Cruze 2LT, 2014 Malibu 2LT, 2008 Cobalt 2LT

  • yardhutyardhut Posts: 2
    Thank you for the quick reply. We will check out that area and let you know how it turns out.
  • imidazol97imidazol97 Crossroads of America I70 & I75 Posts: 21,512
    I found a link about removing the harmonic balancer and crankshaft pulley.
    link title


    2015 Cruze 2LT, 2014 Malibu 2LT, 2008 Cobalt 2LT

This discussion has been closed.