Engine Bay Harness, Split Loom And Harness Tape Technique

Hi everyone — new member here. I work for an automotive wire harness tape manufacturer, so I spend my days thinking about how wire bundles survive under the hood. I noticed the old thread "how to properly wire along main harness?" is closed, so per forum rules I'm continuing the discussion here rather than digging it up. Adding accessory wiring along the factory engine harness comes up a lot, and the two most common mistakes I see in the field are: (1) bare wires zip-tied to a hot harness, and (2) wrapping everything in plain PVC electrical tape and calling it done. Both fail in the engine bay. Here's what actually lasts: 1. Conduit first, tape second. Split corrugated loom (PP is fine for most of the bay; PA6/nylon if you're near the exhaust or turbo) is the mechanical armor. The tape's job is to keep the loom's split closed, keep sections from walking, and seal the ends — not to be the primary protection. A loom with its split gaping open just collects water and grit. 2. Choose the tape for the environment, not the drawer. Plain PVC electrical tape softens and its adhesive oozes above ~80C continuous — fine inside the cabin, not great six inches from a manifold. For engine-bay harness work, the OEM approach is a cloth harness tape (fleece or PET): it handles heat better, doesn't leave the gooey residue PVC does when it cooks, and the fabric damps vibration so the bundle doesn't saw through against brackets. If you only have PVC tape, use it to tack the loom closed at intervals rather than spiral-wrapping the whole run. 3. Technique: spot-tape the loom every 4-6 inches and at both ends, or spiral-wrap with ~50% overlap if the run sees water splash. Always extend the tape onto the wire insulation past the loom ends so the entry is sealed — that's where moisture wicks in. Route like the factory did: clipped to existing hard points, away from sharp edges and heat sources, with a drip loop where a run drops down. 4. Heat standoff matters more than the tape spec. No tape survives sitting on an exhaust manifold. If a run must pass near extreme heat, add a standoff or heat-shield sleeve for that section rather than hoping the tape will take it. 5. Mind drainage. A fully spiral-wrapped bundle in a dry area is fine; in a splash zone, make sure water has a way out at the low point instead of pooling inside the loom. Quick check on your own work: after a hot drive, the wrap should be snug with no sticky ooze or flagging ends. If the tape has gone gummy, it was the wrong tape for that location — replace it with a heat-rated harness tape. Happy to answer questions about tape selection for specific spots in the bay. And to be transparent: I work for a harness tape manufacturer, so I know this stuff from the production side — but I'm here to talk technique, not to sell anything.