the Walnut Diary

I’ve driven many Toyotas and Volvos for extended mileage without changing an ignition coil, but for some reason my ’04 Porsche Cayenne has a real taste for them.  Perhaps it began when the first one failed.  I thought it was the driveshaft because the vibration in the car was so extreme.  So of course I fixed the driveshaft.  That didn’t help Ruby’s drivability.

Eventually the check engine light came on and informed me that cylinder #7 was misfiring.  A single coil cost about 80 USD plus shipping, and I found that I could buy a set of eight on Amazon.ca for 260 CDN.  A vendor on Vancouver Island sent them along in short order.  I replaced the #7 coil and Ruby was whole again.

For well over a year the remaining coils sat in the shop in their package until Ruby started to miss a little.  I bought a set of spark plugs and resolved to do a tune up.  It turned out that #7 is the easiest coil to change on the whole car.

The other bank of cylinders has less room to work.  #2 is the worst because of an engine mount which forms a sort of flying buttress from the right fender to keep the engine from twisting under load.  The coil fits inside the H-shaped mount, and the electrical lead must thread through it.  A critical ground wire anchors to this cast aluminum device, as well.

After her tune-up, Ruby ran very well.  Then came little hesitations when accelerating from 1600 rpm to 2000.  These occurred primarily in heavy traffic, so I learned to use the fingertip shifting paddles to drop a gear and use the higher revolutions to get over the rough spot on the engine’s torque band.

The big annoyance about a misfiring coil is that it is maddeningly difficult to tell which one of the eight is the problem, because a coil appears only to misfire after it is hot, and only when accelerating after a slight slowdown, and the check engine light only comes on if there is a 2% or higher interruption in the firing process.   The oxygen sensor is much more alert than the CEL, though, and leans out the fuel rail on that side of the engine almost instantly to protect itself from an overly-rich mixture.  So a single miss creates an engine firing on only four cylinders.

Last April the problem had gradually grown worse until I had trained myself to downshift in anticipation of the misses.  Then finally, blessedly, the CEL came on in city traffic.  I pulled into a service station parking lot, clapped on the tester, determined that the code was P0302, and scooted home to fix it.  Two hours later, Ruby was fine.

Then the miss returned.  I pleaded for advice online, but received only a blanket condemnation of Asian aftermarket coils from the moderator.

But then came one of those family emergencies where schedules required that I tow a borrowed covered trailer loaded with a sofa twenty miles down the highway.  The increased wind resistance was all it took to cause Ruby’s CEL to pop another Code P0302.  That was the  newest coil in the car, the one I had “just” replaced.  Once I had the information, the repair was simple and Ruby’s drivability was restored.

A Cayenne S is a marvellous car to drive when firing properly.

My take-away from Ruby’s coil troubles so far?

1.  Buy the best coils, whether or not you can afford them.  This is no place for cost-cutting.

2.  When in doubt, replace the coil you put in  most recently.

3.  Check the tightness of the spark plug when the coil is out.  Six months ago the plug had worked itself loose even though I had carefully torqued it.  This time it was fine.

UPDATE:  18 October, 2018

4.  Another coil failed spectacularly today and I just managed to limp home for repairs.  It was #7 and it smelled like a burned computer component when I took it out.  See #1 above.  I called the Porsche dealer and ordered 8 new OEM coils.  $611. with tax and cheap at the price.

UPDATE:  See the sequel to this rant at

Ruby’s ignition coils.