"I have got nothing left to give. I am absolutely spent," a delighted Reynolds said afterwards.
"It is massive. We moved race shops twice in the last year and a half, we have changed manufacturers and we have been getting better and better.
"I was struggling to hold in the tears the last couple of laps."
Coulthard, whose partner is due to give birth to twins shortly, will be stoked to leave Bathurst with the championship lead.
"A credit to all of my guys downstairs," Coulthard said. "Everyone at Shell V-Power did a fantastic job getting [co-drive Tony D'Alberto] and I up here.
"My other half at home Becs - hopefully she's all good with the babies."
Earlier pole-sitter McLaughlin's engine gave up and he did not finish along with co-driver Alex Premat. It was a cruel blow for the man who delivered the fast ever lap at the mountain in the shootout.
Reigning series champion van Gisbergen proved fast all day in the wet but he lost time with co-driver Matt Campbell really struggling to keep the car on the road. Van Gisbergen played the strategy game and was very aggressive to get back into a position where he could win but he went wide while leading on a safety car re-start late in the race. He fought back but just as he went for another aggressive pass to get on the podium he forced too hard and went off into the sand trap at the end of Conrod Straight, puncturing a tyre. He ended up fifth.
Sandown 500 winners Cam Waters and Stanaway led for big part of the race but their Prodrive Racing team got the strategy game wrong and he was caught up in an incident late the race that ruined all their good work. Stanaway proved superb in the trying conditions and further pushed his case for a fulltime drive next season.
Whincup managed to get his car out with a couple of laps remaining to at least be recorded as a finisher and he was able to collect a few points.
Coulthard takes 91-point lead in the championship to the Gold Coast in a fortnight. Whincup is second with McLaughlin third and six points further back.
Race results
1. David Reynolds/Luke Youlden (Holden)
2. Scott Pye/Warren Luff (Holden)
3. Fabian Coulthard/Tony D'Alberto (Ford)
4. Dale Wood/Chris Pither (Holden)
5. Shane van Gisbergen/Matt Campbell (Holden)
Championship Standings
Fabian Coulthard 2431
Jamie Whincup -91
Scott McLaughlin - 97
Chaz Mostert - 223
Shane van Gisbergen -289