4.9L Main Bearing Thump
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HT4100 4.1, 4.5, 4.9 Discussion, 4.9L Main Bearing Thump in Cadillac Engine Technical Discussion; So, I have this very nice 1991 Seville with the 4.9L and of course, it has the infamous Main Bearing ...
  1. #1
    bcpape is offline Cadillac Owners Member
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    4.9L Main Bearing Thump

    So, I have this very nice 1991 Seville with the 4.9L and of course, it has the infamous Main Bearing Thump. I have read so many differing opinions on this, so I stopped in at the local Cadillac stealership and talked with their 30+ year veteran service manager. He was more than familiar with the problem and basically told me to save my money. The so-called "fix" bearing that Cadillac came up with really does not correct the problem. He said that if I'm not going to change out the crank as well, I will be wasting my time and money. Does anyone have any first hand experience with this and should I just turn up the stereo and live with it? The car is nearly pristine and the engine runs very good, so I wouldn't mind putting some money toward this if it would indeed correct the problem.
    Thanks.

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    noahsdad's Avatar
    noahsdad is offline Cadillac Owners Connoisseur
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    Re: 4.9L Main Bearing Thump

    You're right - opinions vary widely. I stand firmly with one foot in both camps. I've seen 4.9's thump along well past 200k, and I've seen them crap out at 60k, but the latter is much less frequent. It's probably more annoyance than issue. My favorite Caddy tech told me that a siightly thicker oil (if you live in a moderate to warm climate) like 15W-X Delvac, can quiet the thump a bit. Oops, just noticed you're in Michigan, same as me. Forget that advice.
    If it were my car, I'd bank the money, drive it, then use the money saved to buy another Caddy if it craps out. There are plenty of other near pristine examples available cheap.

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    steelybill is offline Cadillac Owners Enthusiast
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    Re: 4.9L Main Bearing Thump

    My 4.1 has "thumped" since I've had it. It's annoying sometimes, but I no longer worry about it.

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    bcpape is offline Cadillac Owners Member
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    Re: 4.9L Main Bearing Thump

    Thanks for the opinions. I did add some Lucas Oil Stabilizer which did calm it down a little. I also have a 90 Seville with the 4.5L and it's totally silent. Strange.

  6. #5
    creeker is offline Cadillac Owners Connoisseur
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    Re: 4.9L Main Bearing Thump

    Quote Originally Posted by bcpape View Post
    Thanks for the opinions. I did add some Lucas Oil Stabilizer which did calm it down a little. I also have a 90 Seville with the 4.5L and it's totally silent. Strange.

    What was the mileage when you first noticed the thump,whats your mikeage now?.

  7. #6
    90BeigeElDorado is offline Cadillac Owners Member
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    Re: 4.9L Main Bearing Thump

    I had the same main bearing thump on my 91 Coupe DeVille. I owned that car for a little over a year and put 20,000 miles on it and it was never an issue, just persistant. I remember is being especially noticeable when at idle then accelerating up a hill. I have a 90 Eldorado with a 4.5 and it is totally silent as well. I don't think the noise is as prevelant in the 4.5's as it is in the 4.9's. May have something to do with the displacement increase from 90-91. Just so you know, my 91 drove perfect for that year. It is what really won me over on the 90+ 4.x engines.

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    creeker is offline Cadillac Owners Connoisseur
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    Re: 4.9L Main Bearing Thump

    Quote Originally Posted by 90BeigeElDorado View Post
    I had the same main bearing thump on my 91 Coupe DeVille. I owned that car for a little over a year and put 20,000 miles on it and it was never an issue, just persistant. I remember is being especially noticeable when at idle then accelerating up a hill. I have a 90 Eldorado with a 4.5 and it is totally silent as well. I don't think the noise is as prevelant in the 4.5's as it is in the 4.9's. May have something to do with the displacement increase from 90-91. Just so you know, my 91 drove perfect for that year. It is what really won me over on the 90+ 4.x engines.
    How do the 4.5 and 4.9 compare for power,off the line and over all power?.

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    90BeigeElDorado is offline Cadillac Owners Member
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    Re: 4.9L Main Bearing Thump

    In 1990 the 4.5 got the same PFI that the 4.9 has. The 4.5 for 90 is rated at 180hp and 245tq. The 91 4.9 is 200hp and 275tq. It seems that in day to day driving they both feel the same to me so far, with my 90 ElDorado feeling a little faster(seat of the pants) than my 91 CoupeDeVille. Of course, I think that my 91 weighed more than my Eldo. If that is the case then that would explain why the Eldo feels a little faster. It definately doesn't feel like it is giving anything up to the 91 Coupe that I had. I am sure that a 91 Eldo with the 4.9 would feel quite a bit more lively than my 90 Eldo. Someone else who knows more than me could probably weigh in with curb weights and such.

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    steelybill is offline Cadillac Owners Enthusiast
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    Re: 4.9L Main Bearing Thump

    I've wondered why the front main bearing would develop the thump in the first place. My 4.1 has about 123K on it, and the thump does seem to be getting gradually louder. These engines are not Cadillac's best venture into engineering.
    Too bad a higher volume/pressure oil pump isn't available. That would maybe help.
    Hopefully mine will hang together. I have way too much money in this car, but I love the way it drives and handles.

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    creeker is offline Cadillac Owners Connoisseur
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    Re: 4.9L Main Bearing Thump

    Quote Originally Posted by 90BeigeElDorado View Post
    In 1990 the 4.5 got the same PFI that the 4.9 has. The 4.5 for 90 is rated at 180hp and 245tq. The 91 4.9 is 200hp and 275tq. It seems that in day to day driving they both feel the same to me so far, with my 90 ElDorado feeling a little faster(seat of the pants) than my 91 CoupeDeVille. Of course, I think that my 91 weighed more than my Eldo. If that is the case then that would explain why the Eldo feels a little faster. It definately doesn't feel like it is giving anything up to the 91 Coupe that I had. I am sure that a 91 Eldo with the 4.9 would feel quite a bit more lively than my 90 Eldo. Someone else who knows more than me could probably weigh in with curb weights and such.
    your 1990 weighs 3426 lbs. and your 91 weighs 3545 lbs.

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    90BeigeElDorado is offline Cadillac Owners Member
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    Re: 4.9L Main Bearing Thump

    I am guessing that maybe the main bearings start to loosen up when these engines start to get a few miles on them. My 91 had 91K on it when I got it and had 111K when I sold it. The main bearing thump seemed to be the same the entire time that I owned it. From what I have read on this forum, most 4.9's will develop the thump sooner or later. The only possible fix seems to be the oversized main bearings available from GM for awhile. It seems that this may be a design characteristic for this series of engines. Most people agree that even with the thump you can drive these cars 200K+ miles. From an engineering standpoint, I agree that the early HT4100's definately had major issues. However, towards the '88 model year I think that GM had most of the quirks worked out with these engines. The engines got better year after year until GM stopped production in '95. You still have to keep an eye on intake gaskets, but in my experience every GM will develop leaking intake gaskets sooner or later. I have had both Northstar's and 4.x engines, and I would take a 90+ 4.x engine over a Northstar any day of the week. HaHa. Seriously, I think that these are really good engines(88's and up) as long as they are properly maintained. Heck, I think that their are people on this forum with later model 4.1's that have well over 150K on them.

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    90BeigeElDorado is offline Cadillac Owners Member
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    Re: 4.9L Main Bearing Thump

    Quote Originally Posted by creeker View Post
    your 1990 weighs 3426 lbs. and your 91 weighs 3545 lbs.
    See, definately seat of the pants!!

  14. #13
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    Re: 4.9L Main Bearing Thump

    My '92 had 125K and never a peep from the bearing. The Guru said the thump was caused by belt pressure pulling in the same direction and putting a constant load on the bearing or something like that. He said it was nothing more than an annoyance. He also said that the service bearing would solve it, for what it's worth.

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    steelybill is offline Cadillac Owners Enthusiast
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    Re: 4.9L Main Bearing Thump

    Not sure if the front main could be changed with the engine in the car. Next time I do an oil change I'm gonna look it over. The pan comes off easy enough, so maybe the front main cap is accessable on mine.

    Meanwhile, it just thumps happily along

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    Re: 4.9L Main Bearing Thump

    If you can get the pan off it should be a piece of cake.

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