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Old 07-16-2017, 09:09 PM   #1
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Noticed Some Wobble on Crankshaft Serpentine Belt Pulley

Hey folks,

In trying to diagnose some metallic rattling sounds of my 2001 Boxster base/2.7, I noticed my crankshaft serpentine belt pulley has some noticeable axial runout to it! The following video is on cold-start-up:
trim.262D9F95-2809-46F9-ADB5-1734EEA01D45.MOV - Video - VideoSprout

This next video is after it has been warmed up:
trim.A3E7C7EE-9CCB-4EEB-A065-AC4A1F58314E.MOV - Video - VideoSprout

There seems to be a metallic noise coming from behind the crankshaft pulley, but it's much easier to hear when driving the car, so I'm not sure this is the origin of this noise or not. I'm thinking because pulley bolt seems to be not wobbling, perhaps this is a bent pulley, or a somewhat loose pulley mounting bolt? (If it were loose, it would explain the metallic rattling sound.)

I found a YouTube video that shows someone else with a remarkably similar symptom, shown at 0:09. It seems when replacing the pulley, he found the key way was damaged. Seems like evidence it seems to me of an inadequately tightened pulley bolt!

Have anyone else experienced this? Cheers!
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Last edited by jakeru; 07-17-2017 at 08:19 AM.
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Old 07-17-2017, 04:45 AM   #2
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Seems like a good opportunity to switch to an under drive pulley. It doesn't take but five minutes to remove the old pulley to take a look but I'm betting your pulley is in the beginning stages of what happen to the guy in the video.
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Old 07-17-2017, 06:40 AM   #3
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And check all the idler+tension pulley bearings for noise/wear
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Old 07-17-2017, 11:50 PM   #4
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Thanks for the comments and suggestions. I wonder if anyone else has experienced this experienced this problem, or have any good theories on its cause?

Woody - An underdrive pulley is not for this car, as I have thoughts of one day autocrossing it in SCCA stock/street class. I've now got a new (used) stock crank pulley on the way. (I tried texting you for it first! )

Gelbster - On the accessory pulleys, yes, At 100k+ miles, it's time to preventively replace these sorts of things! And a long time ago, I have some new 5202-2RS bearings for those (same specification as dual-row IMS bearing calls for, I hear!) I read that pressing them in/out of the stamped sheetmetal idler/tensioner pulleys may be a pain, but maybe I can figure it out. I noticed at 3000 rpm, the tensioner pulley had a noticeable resonance. Maybe it's related to the wobbly crank pulley.

Water pump seems fine as far as I can tell, but I'll inspect all the pulleys much more closely when I get the serpentine belt off. I have a new serpentine belt sourced for whenever I need it, but haven't noticed a single crack or other damage on the current belt, so haven't been quick to replace it.

When I got the car, there was evidence of a careless previous mechanic's work in this area. A forgotten large pry tool was retrieved from underneath. (Maybe previous mechanic carelessly pried against the crank pulley, and bent it?) A remnant of a nitrile glove was retrieved from under a bolt head, things in the knock sensor area weren't routed quite right. Maybe a previous mechanic thought this noise was engine knock? Previous owner thought the metallic rattling was exhaust noise, but I think I've mostly ruled that out. Maybe this noise is why he sold me the car.

It's been a good, reliable driver for the last 3 or so years! If I can solve this metallic rattling noise, it's going to be awesome/icing on the cake. Kind of excited to find this wobbling crank pulley!
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Last edited by jakeru; 07-17-2017 at 11:52 PM.
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Old 07-18-2017, 01:22 AM   #5
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Check your crank shaft for end play.
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Old 07-18-2017, 07:23 AM   #6
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Thanks for the suggestion, JFP. I read in another thread your comment that it's possible the thrust bearing can fall out, so good idea to double check things are OK here, especially if it's convenient to do so.

Do you think I could get a good measurement on the axial crank play if I just removed the serpentine belt and gave the pulley a good push/pull? Will the timing chains being installed invalidate the reading? How about the clutch assembly? I'm pretty sure the rods and pistons being installed shouldn't invalidate it; probably a good amount of clearance at the piston pins.

This is a 5-speed / manual transmission car, FWIW. If the noise may be influenced by the axial/thrust crank motion, I think I may try as an experiment simply applying a bit of pressure to the clutch pedal under a condition where the noise is audible, and see if that affects the noise.

On another car I had which was known for thrust bearing issues, I recall a very wise mechanic suggested that a good preventive measure was to get in the habit of always starting the engine without depressing the clutch pedal! If you think about it, it makes perfect sense - loading the thrust bearing with full pressure plate force when you go to crank the engine over, when there is zero oil pressure, must be hard (to say the least!) on the thrust bearing. On that car, (it was a volkswagen) the factory crescent moon style thrust bearings were the best / most reliable. The ones that were most problematic were the integrated thrust/main bearing, especially if the top and bottom halves weren't properly "set" so there was no skew on the thrust surface. It's a bit dangerous to get in the habit of not pressing the clutch pedal, however, if you forget to put it in neutral! (And in the 986, the starter-clutch interlock would need to be "defeated", anyway, so it's definitely not Porsche's intention to start the engine this way). Aftermarket high spring force pressure plates are another factor which can also exhasorbate crankshaft thrust bearing problems...
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Old 07-18-2017, 07:40 AM   #7
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You are only worried about forward and reward motion; while the cam and IMS chains will inhibit the movement somewhat, what you are looking for is substantial movement with everything in place. Normal range for this is 0.05-0.24 mm, if you exceed 28mm, the engine needs to come apart.
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