ADias

What wrench do you need to set it?

Edgy01

some time back I cleaned all the cosmoline off the engine and bottom body parts so I knew that wasn't what I was smelling. The burning rubber smell has been something I have smelled after any drive,--not spirited or anything. Been that way for several weeks. (The service manager didn't react to my repeated comments about this). There is little chance of a rodent affecting my belts. The garage is well buttoned up with devices to control that sort of thing, and no evidence of them anywhere around (droppings). The only thing that has been done to the engine in that department is the installation of a new water pump about 18 months ago. Will keep you in the loop.

ADias

There you go Dan...

GSIRM3

For me it was normal, easy driving.

Coochas

I'm glad you didn't get stranded Dan.
Three years in and a very well driven car,....I don't think it's unreasonable to lose a belt. Then again I haven't heard of it happening to anyone else.

ADias

How easy/difficult is it to replace the belt on the road?

Edgy01

What an interesting lesson this has been on the serpentine belt. The car was picked up this AM and flatbedded into the local dealership. The mechanic began to disassemble things,--which is simply to remove the air filter box. (Easier done by taking off the rubber boots). This action exposes the entire serpentine belt area. Yes, these things are quite readily serviceable along the side of the road, if needed. The only tool (after the airfilter box--standard screwdriver--is removed) needed to reduce the tension on the belt tensioning pulley is a 24mm open ended wrench.

I'm including a few diagrams to better clarify what things are in there. I have included a couple of extracts from the service manual but have added color coding for clarification.

As you can see, the GREEN pulley is set up to receive the 24mm wrench. The two yellow pulleys are additional idler pulleys that simply run along.

The accessories are the RED generator, the BLUE water pump, the PURPLE power steering pump, the LIGHT BLUE for the A/C compressor and finally, all this is driven off the GREY central crankshaft drive.

The actual photo is also color coded to match the diagram.

At this point we don't know the ultimate CAUSE of this premature belt failure. But we do know that the two (YELLOW) idler pulleys and the single (GREEN) tensioning pulley are all worn significantly. Their surfaces should be flat, without any sign of a rim or edge on either side of the belt's path. Once the mechanic got into this area he discovered that all three were worn enough to feel with your finger, significant wear on these plastic rollers. Not only to mine have these raised ridges along the edges, but there is a pronounced lack of flatness on these rollers. Perhaps the wearing of these pulleys ultimately worked against the belt sufficiently to cause it to fail. Porsche is replacing all three (2 YELLOW and the 1 GREEN) pulleys because they are causing them some worry.

But the question ultimately is, WHY are these pulley rollers WEARING like this with so little mileage (34,600) on the engine? An interesting additional data point is provided. Another Rennlister was in there at the same time and disovered his 3.8 engine (06) needed a water pump. Another PCA member we knew also was there for something else. We felt the pulleys on those other engines (all 06 3.8 engines) and the Rennlister's felt fine,--flat, like they should be at 17,000 miles. The PCA member's with 16,000 miles was already showing a bit of wear and a pronounced lip or rim on the most accessible idler pulley (the upper yellow one).

For all Rennlister with 997 cars I highly recommend that you check out your idler pulleys for this wear. I would suspect this problem may be present in Caymans and Boxsters, as well, since all these engines use serpentine belts for accessory drive.

Carry a 24mm open ended wrench and a spare belt just in case (just like we used to carry in our air cooled 911s). It can be fixed at the side of the road and can save a trip! BUT THEN, get your service department to figure out WHY these things are wearing. Be sure and print out a copy of the belt routing in the car so that you don't inadvertently misdirect the belt's path.

My brother in law is in the business and attributes serpentine belt failures to three possibilities:

(1) out of alignment pulley - can cause the belt to wear on one edge or the other.

(2) an accessory is dragging, putting an additional load upon the belt, which should be essentially free-wheeling with the engine.

(3) tensioner tension may be too tight. If the tensioner (GREEN) is placing too much tension on the entire system it may be causing the premature wear of itself and the other (YELLOW) pulleys. (All the other pulleys are metal).

Hope this helps!

P.S. Having not dug this far into a 997.2 engine, I can't say for sure if you will have similar issues but I would venture to say that you may.

ADias

Excellent report! Thanks.

lthatch

Thanks for the superb presentation, Dan.

gota911

Dan, great write up! Glad they found the problem, if not the actual cause. Point #3 from your brother-in-law seems to be the most plausible cause at this stage , but obviously, there is no definitive evidence to prove that.

Edgy01

I hope to look closer at the failed parts and at the engine as a whole when it is all together later tomorrow. Another friend of mine is a mech. eng. and he was telling me that just the slightest amount of misalignment on these wide serpentine belts can place a tremendous amount of pressure on a bearing,--making alignment of these pulleys absolutely critical,--far more so than the days of standard V belts (like found on the air-cooled 911). Given enough time (and enough CSI guys!) we may find that even the water pump failure at 9,000 miles two years ago may be related. Time will tell. Stand by this channel.

PAULSPEED

I removed the alternator and added a new belt
when I bolted it back together. The belt look
nothing more different then a go-kart belt
and I just made sure that all the pulleys were free
wheeling and they did not wiggle. These Porsche motors
are excellent products!!!
Sorry to hear about your belt Edgy.
Paul

Edgy01

UPDATE. The service folks have been replacing pulleys. They ordered up a set of idlers and the tensioner pulley as the original ones are worn considerably. It you look at the photos you can see how they have worn a wide groove into those pulleys. The new tensioner pulley can be seen to be very smooth, as can the newly installed idler pulleys.

Finally, there is the revised motor,--the Direct Injection motor (MY09 - ) As you can see, Porsche has re-engineered the entire serpentine belt routing and tensioning system. This is rather curious as the design prior to MY09 had been around since MY99,--why change a good thing? Note the single tensioner pulley with no idler pulleys. The tensioner pulley appears to have some sore of hydraulic devise keeping it under tension. (It looks a LOT like an old style chain tensioner (if you're old enough to remember), only larger). Now missing a 24mm head to receive a socket on the pulley itself, you can see that Porsche has put that 24mm head onto the assembly between the yellow and red arrows.

Stand by for further developments. At this time we can see the results of the serpentine belt shredding and how the plastic pulleys are deformed by wear, but we don't know WHY this has happened yet.

OCBen

Sorry to hear about this ordeal of yours, Edge. I've been following along intently as I have a squeak associated with my belt as it makes its circuitous route through the pulley system. Took it in to the dealer and the mechanic sprayed a little WD40 underneath the belt right about where it goes over the power steering pulley (the purple one in your diagram above) and it silenced the squeak temporarily, as it eventually came back, hence my concern that it might lead to a belt failure as you experienced. The squeaking clearly is an audible symptom of rubbing, and rubbing leads to wear, and in your case premature wear and failure of the belt.

Well clearly there was excessive tension in the belt to cause the wear as shown. Judging by the worn width-of-the-belt grooves, the belt load on the pulleys appear to be purely radial loads, indicating no misalignment issues. Under such loads I would except the pulley bearings to fail before the belt did - a testament to the integrity of the design of these idlers and the rest of the serpentine belt pulley system. I'm really surprised the belt lasted this long.

Do you know how many pounds of tension these belts need to be set for? Next time I go in I'll have the mechanic check the tension to see if it's way out of range, and therefore be the cause of the squeaking I'm hearing. Do you recall hearing the belt squeak at all, especially after start up? Even though I have much lower miles than you do (~5600) I'm fearing that I might have the same problem with mine as yours and that it will ultimately lead to belt failure as in your case.

To make a good thing even better by improving the design? I would imagine there have been reports of belt failures that may have led them to redesign and improve the belt system. Another reason why I would like to get a new 911 with the new engine. Though I'd rather wait until oh ten to buy one, as this new engine will likely get some of the bugs worked out that undoubtedly crept into the initial production release.

Yeah, please keep us posted.

ADias

I agree. Given the deep uniform indentation on the pulleys the belt seems to be under enormous tension. If that proves to be the case I wonder how that affected the integrity of the spindles of the various accessories...

Even though the M96/M97 engines have a circuitous serpentine belt, revised in the new 9A1 engine, I do not think the issue Dan experienced is epidemic in those engines. I bet this occurred because his belt was over tensioned.