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1991 Cadillac Seville - 1998 Cadillac DeVille - 2013 Cadillac XTS4
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Discussion Starter #1
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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'87 SDV
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1,296 Posts
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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1991 Cadillac Seville - 1998 Cadillac DeVille - 2013 Cadillac XTS4
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22 Posts
Discussion Starter #4
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.
 

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1992 eldorado. 2007 dodge magnum hemi r/t
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3,013 Posts
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?.
 

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1990 Cadillac Eldorado 4.5 PFI
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63 Posts
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. :thumbsup:
 

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1992 eldorado. 2007 dodge magnum hemi r/t
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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. :thumbsup:
How do the 4.5 and 4.9 compare for power,off the line and over all power?.
 

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1990 Cadillac Eldorado 4.5 PFI
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63 Posts
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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'85 Fleetwood FWD
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728 Posts
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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1992 eldorado. 2007 dodge magnum hemi r/t
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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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1990 Cadillac Eldorado 4.5 PFI
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63 Posts
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.:stirpot: 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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White Diamond '03 DHS (with DTS floor shift)
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87,112 Posts
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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'85 Fleetwood FWD
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728 Posts
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:D
 

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White Diamond '03 DHS (with DTS floor shift)
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If you can get the pan off it should be a piece of cake.
 

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1984 coupe deville 4.1 67,000original/1991 sedan deville 4.9
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85 Posts
I got a 84'coupe deville with a 4.1 68,000 original miles on it, it has always been real sluggish, is this normal? Seems like the tranny slow going gear to gear, almost like its slipping. But no mechanical problems with the motor, x-cept a receipt I found for a rocker stud replacement back in like 1992.
 

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White Diamond '03 DHS (with DTS floor shift)
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I got a 84'coupe deville with a 4.1 68,000 original miles on it, it has always been real sluggish, is this normal? Seems like the tranny slow going gear to gear, almost like its slipping. But no mechanical problems with the motor, x-cept a receipt I found for a rocker stud replacement back in like 1992.
The 4.1 was never known for performance. Sluggish is being kind.
 

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1984 coupe deville 4.1 67,000original/1991 sedan deville 4.9
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85 Posts
The 4.1 was never known for performance. Sluggish is being kind.
Well, I feel better, i suppose. I definately know its a highway cruiser. Anything I can do to improve the performance? I already did a new fuel pump, filter, air filter, plugs, wires ,cap, rotor button.
 

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1990 Cadillac sedan devile
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616 Posts
I got a 84'coupe deville with a 4.1 68,000 original miles on it, it has always been real sluggish, is this normal? Seems like the tranny slow going gear to gear, almost like its slipping. But no mechanical problems with the motor, x-cept a receipt I found for a rocker stud replacement back in like 1992.
if it feals like its slipping or having soft shifts you might want to try replacing the vac modulator on the trans. it like under 20 if it is that then that 20 just saved you a trip to the shop
 

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White Diamond '03 DHS (with DTS floor shift)
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Well, I feel better, i suppose. I definately know its a highway cruiser. Anything I can do to improve the performance? I already did a new fuel pump, filter, air filter, plugs, wires ,cap, rotor button.
Nothing I know of. I improved mine by trading it for a '92 with a 4.9.
 
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