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I've got all of the factory loom and tape removed from my 2006 GTO harness. The loom/tape on the knock and crank sensor wires was notably brittle; clearly affected by the exhaust heat.
I bought F6 Techflex split loom and Elliott ETN1000E PVC harness tape (search eBay). Other than attempting to route the wires as far from the heat source as possible, is there any other precaution/wrap that's good for protecting those particular wires?
In general, do you warp the harness with tape in its entirety prior to inserting it into the split loom? Or use just enough tape to keep it organized?
I use 3m high heat electrical tape on all my harnesses. Holds up pretty good even on the oxygen sensors and starter wire. I put enough tape to keep it organize and take the loom ever 5 inches or so to keep it tidy and I tape the ends liberally. On some harnesses that I know people are going to use stock manifolds, I put heat shield tape on the ends to keep it protected.
Many OEM type products are "high tech" versions of simple products, developed in order to facilitate quick assembly in a factory. From experience, I have found that using simple stuff in ways which does not look purely professional gives good results.
IE, if I was doing what you are doing (and actually I am) I would just use some nylon convoluted split wire loom tube over the wire, wrap the outside of the loom tube with a couple layers of heavy gauge Reynolds Wrap aluminum foil (which my wife has in the kitchen) and retain it all in a few places with stainless steel safety wire wrapped around the foil and tube. Copper foil works too, but my wife doesn't cook with it, so none at home.
Put a cable tie on the O2 wire several inches from the O2 sensor, or somehow fix the tube/foil to the chassis, so the O2 sensor does not contact the plastic loom and melt it.
Try to leave both ends of the tube open so the heat can flow out the top, but don't position the bottom directly over the hot air rising off the exhaust tube.
The aluminum foil is an excellent thermal conductor and will spread the hot spots from the exhaust over a very large surface area. Forced convection cooling airflow from driving the car or running the electric fan will keep the foil a relatively cool overall average temp.
Regarding wrapping before inserting in the loom, it is your preference. However, if you wrap the whole thing, undoing the wrap to change anything is a pain in the butt.
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Harness Heat Protection? 
