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I have a 95 Ls400 & I just replaced a new tensioner pulley assembly & got a new serpentine belt. I made sure everything went on properly & everything was in it’s place. But once i started the car I noticed that the automatic  tensioner pulley was bouncing & causing the serpentine belt was slipping off the crankshaft & tensioner pulley. I don’t know what to do? Or what i’m doing wrong. 

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Ouch. I have changed this belt on LS400 a few times. I have done also changed the tensioner on one LS400 due to that I did not realize that there was a left hand screw attaching the tensioner wheel. The tensioner seem to be a spring only device. No damping of any kind.  On A SAAB 9000 I did the tensioner had a serious oil damper and after experencing the belt jumping off several times I checked what was going on and saw that at some high rpm the belt started to vibrate violently and jumped off. Was fixed good with a new tensioner with a new good damper was fitted. On the LS400 the cam belt tesnioner has a oild damped tensioner

It is typically a matter of actual tension and length of free belt between pulleys what happens if you consider the design of the car. It seems the LS400 normally does not have this problem.   What could be different here is reasoanbly lower or higher belt tension or some of the pulleys having a defect or a serious bearing problem or a drvien device with a serious pulsating problem.  I would take off the belt ( takes 5 minutes to take off and put back ) and feel all the pulleys for anomalies . 
You could also look with a  flashligth at what the belt is doing if you slowly rev the motor.  A guess is that if it starts to vibrate violently at say 5000 rpm the belt tenions is too low.  This could happen due to a long belt ( there are indications on the tensioner were it should be at concerning spring tension) , there are variants out there of different length OR a out of spec tensioner.  

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Firstly check the old belt and the new belt are the same circumference and width and the same number of grooves.Check the belt tensioner is the same as the old one and is installed correctly.The nut that you connect to to pull back on the pulley whilst placing the belt is a left hand thread make sure you have'nt slackened this nut.

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On 12/3/2020 at 1:23 PM, Dianaxxx said:

I have a 95 Ls400 & I just replaced a new tensioner pulley assembly & got a new serpentine belt. I made sure everything went on properly & everything was in it’s place. But once i started the car I noticed that the automatic  tensioner pulley was bouncing & causing the serpentine belt was slipping off the crankshaft & tensioner pulley. I don’t know what to do? Or what i’m doing wrong. 

Update

the automatic tensioner pulley assembly no longer bounces, but the serpentine belt still slips off. I’ve made sure it’s the right belt as well as made sure the automatic tensioner was the correct one & still nothing. The belt still slips off the tensioner. I’m running out of things to do. 

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 Is something wrong with the tensioner, lift the belt from the tensioner and try to pull back the tensioner with your hand if you can do this and your not the incredible hulk it is the tensioner that is faulty.

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If it was just a noisy tensioner bearing it takes 10 minutes and a few quid to replace just the bearing, same with the idler pulley.

Check from above to see if the new tensioner lines up with the rest of the pulleys.

The tensioner should sit midway in the adjustment range with the correct belt, I have noticed variations in belt length quoted for the same model. A standard test for correct tension is to see if you can twist the belt 90 degrees on the longest run between pulleys.

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The printout supplied of the manual should give clear indication if the belt and the tensioner are as should be . The 10 Kg test can easily be done with a luggage or a fishing scale for 10£.
If that goes well I seriously suspect some of the rollers or the driven stuff are having a  serious problem. See tip how to check  in earlier post. 

To be clear. MOST of the stuff in front of the engine here is using

rubber lip sealed high grade japanese ball bearings

that will last a very long time unless in case of very bad luck. Hovever 26 years is already a Long time and then some. I have taken apart some of these bearings from 20 year old rollers and the grease looks more like black semisoft chunky cheese and lubrication is running on overtime for sure. I have exchanged two of these ballbearings in question on my 1995 with 120kmiles beacuese they were easy to exchange and I am listening carefully once in a while for the hissing sound from a badly lubricated bearing. Only takes a few seconds. The typical way of failing is making more and more noise over some time. .  When really bad it might have larger defects and run very badly and may cause a belt jumping off.  I would though expect very bad noise and running lumpy when rotated by hand before that happens. 

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In my case the bearings caused a squealing sound when the car was first started from cold. As the bearings warmed up this noise disappeared until the next cold start. Good quality Japanese replacements as mentioned by Mikael  sorted the problem with little time and money.

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On 12/5/2020 at 6:47 AM, Dianaxxx said:

Update

the automatic tensioner pulley assembly no longer bounces, but the serpentine belt still slips off. I’ve made sure it’s the right belt as well as made sure the automatic tensioner was the correct one & still nothing. The belt still slips off the tensioner. I’m running out of things to do. 

Questions are....was this happening before you changed the belt and tensioner and why did you replace the belt and tensioner initially?

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