Dumas posted a 3m28.900s at the wheel of the #708 Glickenhaus 007 LMH to beat the best of the Toyota GR010 HYBRIDs, the #8 machine of Brendon Hartley, by 0.348 seconds.
The best time in the two-hour night session was around 1.7s slower than the time Kamui Kobayashi set in the #7 Toyota in first qualifying.
Mike Conway put the #7 car third in the order with a best effort of 3m29.748s, exactly half a second down on the sister Toyota.
Glickenhaus' #709 entry was fourth-fastest in the hands of Ryan Briscoe, albeit 2.5s down on Dumas' benchmark, followed by the solo Alpine A480-Gibson LMP1 driven by Matthieu Vaxiviere.
LMP2 was topped by Rene Rast at the wheel of the best of the three WRT-run ORECA-Gibson 07s.
Rast posted a 3m33.119s to eclipse the early benchmark set by United Autosports man Filipe Albuquerque to the tune of a little over two tenths.
IMSA SportsCar Championship star Ricky Taylor was third-fastest in the Cool Racing ORECA, ahead of Robert Kubica in the Prema car and Richard Mille Racing's Charles Milesi.
Williams Formula 1 reserve Jack Aitken was sixth-fastest for Algarve Pro Racing and best of the Pro/Am runners.
Corvette Racing led the way in the GTE Pro ranks as Nicky Catsburg set a best effort of 3m53.492s early on in the quicker of the American marque's pair of C8.Rs, the #63 car.
Next up was the best of the factory Porsche 911 RSR-19s, the #91 example of Frederic Makowiecki, with the #64 Corvette of Tommy Milner a mere three hundredths further back in third.
The quickest Ferrari 488 GTE Evo was the privateer Riley Motorsports entry in fourth with Felipe Fraga at the wheel, ahead of the #92 Porsche of Kevin Estre and the two works AF Corse-entered Ferraris.
Danish racer Mikkel Pedersen set the time to beat in the GTE Am ranks at the wheel of the Team Project 1 Porsche.
Two cars involved in earlier accidents did not set a time during the session, the #45 Algarve Pro Racing that had earlier been crashed by Steven Thomas and the #93 Proton Competition Porsche in which Michael Fassbender suffered a shunt.
In a relatively incident-free session, Inter Europol Competition's #43 entry stopped along the start/finish straight with a suspension problem, while the #21 AF Corse Ferrari of Christoph Ulrich needed rescuing from the gravel following an off at Arnage.
