Monday, October 12, 2015

Engine Noises and Surprise in a Porsche 911 -

How often do seeming disparate problems converge with symptoms that seem to go together?  It’s rare but it happens.



This car – a 2003 Porsche 911 C4s - came in with a nasty rattle in the engine, and a fault code for a camshaft position error.  Like most mechanics the owner took that to mean the tensioner or the intermediate shaft bearing had gone bad, and the engine was in imminent danger of self-destructing.

We saw no reason to disagree with that assessment, and we expected the diagnosis to be validated on teardown.  When we installed the holding tools to keep the cams and crank lined up we found one cam slightly off.

But when we removed the transaxle and looked at the IMS bearing it was tight. 



There were no signs of metal in the oil.



All we found was slight grab marks on one tensioner, indicating it may have slipped back a bit.  We therefore had an explanation for the slack. But we didn’t have an explanation for the nasty rattle.  We looked inside with a fiber optic camera, and saw nothing.



The technician decided to put the flywheel on the engine, fit new tensioners, a new IMS bearing, and start the motor to listen without the transaxle in the way.  When he did, the result was surprising.  The cam position fault was gone, but the noise was unchanged.  The clatter sounded just like it was coming from the timing chains.



But it wasn’t.  As you see in this short video the noise was emanating from the flywheel.



With a new flywheel and new tensioners, the cam faults were gone, and the rattle was fixed.


The dual mass flywheel was not visibly bad, but its slop was at the extreme limit of the acceptable range.  The noise was obvious, though, once it was twisted hard.   

What is the takeaway from this?  Sometimes two totally different problems will appear virtually at once, and by combining their symptoms you can imagine a very different diagnostic path.  Many times motorists come to us with a list of problems and the hope that there is one thing - the magic bullet failure - that will cure them all.  We have to explain that worn brakes and oil leaks have nothing to do with one another.  And rarely - as in this case - the opposite happens.  A car comes in with seemingly related symptoms, but in reality it has two totally independent failures.

Or are they independent?  Thinking more on this, we theorize that the failing flywheel may have set up a vibration pattern at the back of the engine that caused the timing chain tensioners to vibrate internally, and one to eventually stick.  

Many techs would have changed those tensioners and then reassembled everything. And from outside, the noise would have seemed like chain noise for sure.  The next step – an unnecessary engine teardown.


No matter how much you know, cars can always surprise you.  


(c) 2015 John Elder Robison
John Elder Robison is the general manager of J E Robison Service Company, celebrating 30 years of independent Porsche restoration and repair in Springfield, Massachusetts.  John is a longtime technical consultant to the Porsche clubs, and he’s owned and restored many fine specimens.  Find him online at www.robisonservice.com or in the real world at 413-785-1665

Reading this article will make you smarter, especially when it comes to car stuff.  So it's good for you.  But don't take that too far - printing and eating it will probably make you sick.

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