The engine harnesses are all silicone-thermal insulation - cannot even brown the insulation with a hot soldering iron - best bet is to hit yer local boneyard and snag sections of the oem harness for any modifications\additions\upgrades within the engine bay - resin-core solder is required, plus a bare crimp (insulation removed): crimp the splice, then solder it - coat the finished splice with a dab of RTV, slide the shrink tube over the freshly RTV'ed splice, hit it with the heatgun, and you gotta waterproof\fuel-proof\antifreeze-proof splice, soldered for good electronic connection, crimped for good mechanical connection
I cut the copper splice in half, strip the wire lengths short to fit the half-splice section, making the resulting splice shorter - works well in a large harness where multiple wires require splicing, allowing the splices to be at various positions along the length so the resulting harness is nearly as compact as the un-spliced harness