A D5 is more than enough for that loop. The only resistance it's really facing will be from the GPU and CPU block, and with only one of each, it's more than capable. Your best bet right now will be to boot your system normally with the PWM pump cable connected to some fan header on your mobo (could be fan, w_pump, cpu_fan, etc). Depending on your motherboard, you might still be able to view the RPM's in the bios itself without booting into an OS. Max rpm of an EK D5 should be around 4800rpm (at least my pump is). Crank the pump all the way up to 100%, make sure RPM's are somewhat around there, and monitor cpu/gpu temps.
If your pump is significant low, you likely have a defective pump or defective power source, whether that is the molex cable or psu itself.