4.9 bearing thump
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HT4100 4.1, 4.5, 4.9 Discussion, 4.9 bearing thump in Cadillac Engine Technical Discussion; Please excuse me but I have more to ramble on about....... The 4.9 Cadillac engine has a peculiarity in that ...
  1. #1
    kcnewell is offline Cadillac Owners Connoisseur
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    4.9 bearing thump

    Please excuse me but I have more to ramble on about.......
    The 4.9 Cadillac engine has a peculiarity in that the front main bearing can make a distressing noise which can cause knowledgeable owners to think it needs a bearing job. Those familiar with the sound of a bad bearing will be certain they hear a main bearing knock which requires fixing before it becomes worse.

    The good news here is that the bearing need not be fixed and that it will lead a long and happy life making noise all the way. If your engine fails it will not be the front main bearing that causes it. Those in the know refer to the noise as a "thump" to distinguish it from a knock. A knock is bad but a thump is benign.

    The engine uses a serpentine belt to drive the accessories and has an automatic tensioner to keep belt pressure correct. This particular engine design has the serpentine belt lifting almost straight up on the front damper which would be no problem except that whenever cylinder 1 or 2 fires they push downward. The lifting of the belt and the firing of the cylinders act against each. This causes the slightest oscillation of the crankshaft in the bearing due to the accumulation of normal clearances. The belt lifts the damper and crankshaft upward followed by the firing cylinder pushing it downward and a thump occurs.

    People studying the factory service manual for late night reading will notice that the specified clearance for replacement of the No.1 bearing is "greater than .002 inch" while for all other main bearings it is "greater than .004 inch." You may draw your own conclusion as to why this is true. Just for comparison the service clearance is normally .0008 to .0015 inch on the main bearings.

    Extensive testing and reports from the field show that the No 1 main bearing thump has no ill effect on durability or longevity. It will simply make noise. The noise may vary with the temperature of the engine, viscosity of the oil, and speed of the engine. Some people have reported that the thinner nature of synthetic oils tends to make the noise louder in which case using the factory recommended 10w30 mineral oil would make good sense.

    Should your engine thump predictably at idle it is easy to confirm the diagnosis. Take a breaker bar and as the engine idles thumping away unload the belt tensioner allowing the belt to stop turning or at least slip easily. The thump should magically disappear. Engines that thump only when under a slight load can also be tested but to do so is dangerous to careless persons so safety is an important concern. A second person will be needed to sit in the driver's seat and step on the brake while also slightly speeding up the engine until the thump is heard. The belt can then be unloaded as above. Care should be taken not to stand in front of the car or have a foot in front of the tire. Anytime a person reaches inside the engine compartment of a running engine care should be taken to avoid moving parts.

    There are of course people who cannot abide by this noise no matter how harmless it may be. For them the Cadillac dealer has the correct oversize replacement bearing in stock. Reportedly the replacement bearing is "purple" as GM designates its bearing sizes by color.


    That's all I've got to say about that....( Forest Gump )

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  3. #2
    SoundAdvantage's Avatar
    SoundAdvantage is offline Cadillac Owners Enthusiast
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    Interesting, So are you planning on replacing the bearing? Does it leak any oil?

  4. #3
    kcnewell is offline Cadillac Owners Connoisseur
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    Re: 4.9 bearing thump

    Quote Originally Posted by SoundAdvantage
    Interesting, So are you planning on replacing the bearing? Does it leak any oil?

    No, and............................................... ..............No!

  5. #4
    B. Fitzgerald is offline Cadillac Owners Member
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    Re: 4.9 bearing thump

    Quote Originally Posted by P.P.
    No, and............................................... ..............No!

    Thanks, Mine has had that noise since I got it with 75K on it. I haven't lost any sleep yet, but now I never will. Good info. bf

  6. #5
    ga_caddy is offline Cadillac Owners Member
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    Re: 4.9 bearing thump

    Quote Originally Posted by B. Fitzgerald
    Thanks, Mine has had that noise since I got it with 75K on it. I haven't lost any sleep yet, but now I never will. Good info. bf
    Mine has been doing that for some time too. It just rolled to 115k tonight. I'm not worried about it. If and when she goes I'll put a new engine in.

  7. #6
    double0bondo is offline Cadillac Owners Member
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    ok then??

    is this a problem with the 4.5 also?? mine has what i would call a newly developed thud...or is it likely a rod or main bearing?

  8. #7
    chameleon is offline Cadillac Owners Member
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    Re: 4.9 bearing thump

    Thanks so much for this info! I just bought a 93 with 143k on it. It has this thump you are talking about and just wanted to make sure its nothing serious. Any ballpark figure on how much this would cost to replace? Thanks in advance.

  9. #8
    carnut is offline Cadillac Owners Master
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    My Experience With Cadillac Engines, 4.1, 4.5 And 4.9 Verifies That They All Will Have This Thud . I Agree, I've Never Seen A Problem Related To Durability.

  10. #9
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    caddypimp93 is offline Cadillac Owners Member
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    Lightbulb Re: 4.9 bearing thump

    the famous "bearing knock" is not a bearing at all, my HT4100 does it but neither of my 4.9s do.. its actually worn out wrist pins, my uncle had an 89 SDV with that noise, he replaced the wrist pins, and never heard it again..

  11. #10
    Anthony Cipriano is offline Cadillac Owners Fanatic
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    Re: 4.9 bearing thump

    Quote Originally Posted by caddypimp93
    the famous "bearing knock" is not a bearing at all, my HT4100 does it but neither of my 4.9s do.. its actually worn out wrist pins, my uncle had an 89 SDV with that noise, he replaced the wrist pins, and never heard it again..
    The bearing "thump" that's typically associated with any of the TFWD 4.1/4.5/4.9 engines is indeed the front main bearing. The original post is correct. Relaxing the load on the accessory drive belt tensioner and causing the noise to go away (a sure sign of the front main bearing thump) has nothing to do with the wrist pins.

    I have never, since the HT4100 started production heard of an engine having "worn out" wrist pins. The wrist pins in the 4.1/4.5/4.9 engines are pressed into the con rod and float only in the pistons - so what is to wear? The aluminum piston pin bore is not going to "wear" out a wrist pin. If your uncle replaced the wrist pins and the noise went away then he must have replaced the main bearings at the same time he did the job and the new mains cured the noise - not the wrist pins. Besides, wrist pins click when worn/too much clearance, click like a valve lifter with air in it. They don't make a "bearing thump" type of noise. The Northstar has full floating pins and those can make the ticking or clicking noise on coldstarts on occasion as I've mentioned. Your wrist pin information is either incorrect or there's a misunderstanding.

  12. #11
    Night Wolf's Avatar
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    Re: 4.9 bearing thump

    I have 2 or 3 noises coming form my engine/car.. not bad, and not noticable to the average perosn, but i know it is there...

    It has been almost 5 months since i drove the car though (soon to change in a few days) so I will have to see.... 1 of the nosies (and worst IMO) is then the engine is being raced in park or neutral.... another is a chassis nosie around 60mph and gets worse the faster you go (sicne I went that speed and put the tranny in neutral and shut the engine off and still got the squeek) and the third, which may be being, or *MAY* have something to do with the nasty thing i get when rascing the engine in park, which I *think* is due to the engine running lean.... once I get it on the road, my father said he'll take a look at things, hopefully we can get these little problems fixed....

  13. #12
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    Re: 4.9 bearing thump

    I get a nasty noise too, it comes from around the belt--alternator---and Power steering pump. We had thought it to be the bearing on the belt or the alternator, but with the hood open and moving the steering wheel, it starts to change frequency. When accelerating on full throttle it seems to get worse too, like its accelerating too. Is this the same problem?

  14. #13
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    Re: 4.9 bearing thump

    there appreas to be a groan type noise in all 4.9s that i have seen , after wensdays waterpump job i decided to change out the powersteering pump to see if i can locate it (i just got the powerster pump and will do it today (friday apr 9th))

    waterpumps make interesting noises for about a month before it decideds to shear its shaft in 2 peices and cut ball berings in half , the noise sounds alot like takeing a die grinder on a slow speed to the inside of a valve cover

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