2000 Boxster Exhaust Mount failure - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Pbastian · 02-11-2016, 05:37 PM

My 2000 Boxster 5-speed has a sport exhaust whose mounting system has failed twice recently. Years ago, when I bought the car, I had to have the mounts to the bracket repaired. The exhaust itself has a couple of welded "caps" that each contain a bolt coming up thru it. These bolts attach to a pair of mounts that are on a bracket. The exhaust is kinda suspended from this bracket. Recently, the bolts that attach to the mounts failed. I took it to be repaired, and after one weekend at the track, these bolts have failed again. Sheared apart.To do the repair, you have to cut the welds to remove the old bolt and insert a new one.

Does anyone know what type of bolt I should be using to make this repair? Apparently, the bolts that were used in the most recent repair couldn't stand up to the stress. Mechanic told me they used strongest thing they could find, since they understood how it broke in the first place...

JFP in PA · 02-12-2016, 04:51 AM

I'd have to agree with your wrench, we see a lot of cars, and I have never seen these bolts fail. Something is wrong.

Pbastian · 03-23-2016, 05:56 AM

So, the final diagnosis on this was a failed lower exhaust mount. We now believe, as others have pointed out, that the "something else is wrong" was the failed lower exhaust mount. With that lower mount failure, we are speculating that the bolts on the upper mount couldn't handle all of the loads and stresses. Curiously, I inspected my repair records, and found that the last time i had this happen back in 2006 just after i bought the car, one of the upper mounting bolts had failed, but the mechanic also installed a missing lower mount bracket. So, that correlates. Exhaust is back in with repaired upper mounts and new lower mount bracket.

Pbastian · 05-26-2016, 08:09 AM

So I am back to square one.

Went back out on the track yesterday. Spun once, stayed on the track. Went off track on another lap, but only slightly so, and certainly not in rough terrain. Got home before I noticed that this damned thing has broken again. Bolts are sheared off. Wondering now if the spin causes enough lateral movement in the exhaust system to shear those bolts, or if I hit something when I went off the track on a subsequent lap. Unfortunately, I did not pay enough attention at the track to correlate a specific event to the mount failure. BIG problem here is that the repair requires removal and welding. I'm looking at replacing this whole behemoth of a muffler with something lighter, and easier to mount. Any suggestions would be appreciated.

911monty · 05-26-2016, 08:41 AM

Man I'm no expert but I stayed at a Holiday Inn once. LOL. I have replaced my entire muffler and hanger system to run an "S"......moment of reverence... and it is an extremely robust system. As you probably know the entire engine, trans and muffler are designed and move together as one unit. with the muffler being supported between the upper and lower supports which attach to the mount bolted to the tail of the trans. With that being said IMHO either the bolts are now weak from welding or the engine is moving too much and the muffler is hitting the body somewhere? How are your engine and trans mounts, any evidence of the muffler tip contacting the body? Is it possible to take some more pictures of the hanger system? Good Luck

Pbastian · 05-26-2016, 09:21 AM

911Monty - To answer your questions, engine mount is < 3 yrs old, trans mounts replaced with semi-solid Wevo's less than 1 year ago. Suspension upgrade with last 2 years. Bolts were replaced during last two repairs of this system. This meant cutting the welds on those caps to insert new bolts. Welds on caps don't fail, the bolts do. I get what you are saying...fact is, this failure correlates to either spins or hitting something off track in the dirt. My bad that I didn't check this during this last track day. Could have determined what happened. If going off track meant the muffler hit something, well, maybe it's not meant to withstand that, but me going off yesterday was what I would call "mild". Dry surface, dirt...no ditches or divots or holes.

I'll take a pic of the lower mounts this weekend. This will be an important diagnosis.

Pbastian · 05-27-2016, 09:22 AM

So, here is the latest diagnosis, FBO everyone who might have this experience. I am learning...so bear with me.

I spoke to a very good Porsche mechanic and Spec Boxster driver. After listening to my story, his conclusion is that the semi-solid Wevo's have affected the vibration dampening of the overall system. Without a lighter exhaust system, the Wevo's are not absorbing vibrations like a stock system would. His recommendation was to go back to the stock, OEM transmission mounts if I am keeping the stock muffler. So, that's my plan.

Per our discussion, the whole engine/transmission/exhaust system is coupled together. OK, that makes sense - motor mount, transmission mounts, and exhaust mounts all work together. Changing to the Wevo semi-solid mounts, without ALSO changing the exhaust to a lighter exhaust system, upset the overall balance of the system, leading to the exhaust mount failure.

I can't be 100% sure, but I have had at least two of these exhaust mount failures since putting in the Wevo's. We will see what happens when the new, stock OEM transmission mounts go in.

911monty · 05-27-2016, 12:10 PM

Yeah I saw the Wevo's mentioned and was thinking along that line but was first wanting to eliminate possible binding in the system. If you can take some high resolution Macro photos of the bolt failure point and post them for analysis. Tension, shear and hysteresis fractures have some very obvious failure images.

911monty · 05-29-2016, 08:29 AM

I have some additional thoughts on this and will explain in basics for thought and the collective input.
As earlier stated the engine, trans and exhaust are designed as a unit or structure. This structure is suspended on elastomeric mounts to isolate vibration (harmonics) from the vehicle body and the driver. You have probably noticed an increase in vibration to yourself and I suspect your mirrors after the WEVOs were installed. The engine package however was designed to deal with these harmonics. Rotating equipment creates harmonics, Most engines deal with these crank harmonics with a viscous or elastomeric damper mounted on the crankshaft end called the harmonic damper. Porsche uses the dual mass flywheel to control these harmonics. An interesting thing about harmonic vibration is that you can beef up one point i.e. the Wevo mounts, but the harmonics are transferred to some other point, such as your exhaust bolts. The harmonics to fatigue metal at the rate you seem to be experiencing would have to be in the order of >16k Hz. This should create some rather uncomfortable buzzing in your car. My question to you is by any chance are you running a LW flywheel?

Pbastian · 06-03-2016, 01:21 PM

I agree with you, now, after I have learned all this over the course of several weeks. I won't have any pics of the failed bolts at the moment, but I have a few things to add to this conversation. Technically, the jury is still out, because I don't have the car back, and won't find out for some time if those bolts will fail again. But, the shop where the car is now to be repaired said that the failed bolts were welded into their carrier, which is a mistake made by the previous mechanic. I think the stock installation had the bolts loose in the carrier. The carrier itself is welded to the muffler, but the bolts are just encapsulated in the carrier, and loose, and then connected to the upper mounting bracket. So, if the bolts were welded to their carrier on top of the muffler, then I can easily see where this would create issues, perhaps even with the Wevo's in place. Still, because of the "balance" created by the original engineering design, I am going back to that. Stock trans mounts are going in, Wevo's are coming out and muffler is being repaired and reinstalled.

We shall see...