A number of people have experienced arcing between the Coil On Plug (COP) body and the metal tube in the valve cover of the 2ZZ-GE. This leads to misfire as well as permanent physical damage to the body of the COP assembly. In this post, I describe a solution that should reduce the likelihood of such an event.
A number of posts on LotusTalk have discussed damage to COP bodies due to unintended arcing. Coil Pack Arcing
is one example. A more recent post is Coil Pack Arcing and Spark Plug Questions
. These and many other posts recommend using dielectric grease to coat the spark plug contact and the exterior of the COP body. Coating the metal contact that touches the spark plug tip is an intended use of dielectric grease. Its function is to exclude oxygen and moisture to help insure a good electrical connection.
Using dielectric grease on the COP body puts it in the role of an insulator. But dielectric grease is not a great insulator. The data sheet for one example, Loctite LB8423
, list the dielectric strength as 19.8kV/mm. There is about 0.010"/ 0.25 mm space between the COP body and the metal tube in the valve cover. That means a 0.010" thick film of LB8423 could contribute 4,950 volts of insulation value before breaking down (19.8 * 1000 * .25 = 4950). That's just not very much insulation value and it would depend on packing the tube completely full of grease, something very unlikely to happen as the COP body is pushed into position and the grease is moved around.
A better solution is to use a material that is designed to be an insulator, will stay in position when the COP body is installed and will survive the heat of the environment. One such product is 3M 61 PTFE
tape. This material is 0.005" thick and has a dielectric breakdown strength of 15,000 volts. See the data on page 10 of this document
. The secondary winding of a typical COP iginition system outputs anywhere from 20,000 to 50,000 volts, so the tape's 15,000 volt strength is a substantial fraction of those values (3/4 to 3/10) and should make a substantial contribution to insulating the COP body.
Here is how I applied the tape to the COP bodies on my Elise. The plug body is just under 3" in circumference, so I used three pieces of 1" wide tape applied along the length of the body. I only applied tape to the smooth section of the body because the tape tends to roll up if it extends farther towards the spark plug boot. Here is a picture of applying the first piece of tape:
The tape has been cut to 3-1/16" long and applied just below the ridges that secure a rubber gasket on the body. The body was cleaned with denatured alcohol prior to tape application to ensure good adhesion and that no debris would be trapped under the tape. It is easiest and most accurate to cut the tape on a clean mat before application:
Here is a picture of a COP body completely wrapped in 3M 61 tape:
An ideal application of tape would be overlapping layers, but there is insufficient clearance between the body and the metal tube to allow very much overlap. I applied the first piece of tape flat, then butt-jointed the long edge of the second piece of tape. The third and final piece of tape was applied with about 1/32" overlap along each edge. This build up in thickness of tape still allows the body to be inserted in the metal tube . . . just barely. To make insertion easier and less likely to result in damaging the tape, I wiped a thin coat of dielectric grease on the tape, then wiped it off. The remaining microscopically-thin coat of grease is enough to allow easy insertion into the metal tube.
My COP bodies show no evidence of arcing. They are the OEM parts on a 2011 Elise SC with 29,000 miles on them. I'll continue to monitor them for arcing and report back over time.