We have similar problems starting our 700 NL generator:
1. Replaced the glow plugs and the glow
plug relay. That helped the most despite being in the tropics.
2. Replaced all batteries-starters-switches, upgraded
wiring size and terminals, improved engine ground connection and even brought a jumper directly to a starter bolt on the bell housing. Did not help us. Still the generator sometimes only starts when the main engine is running and
charging the
batteries (to 13.7V) and when the house and starting
batteries are switch-combined.
3. a. Replaced all of the fuel lines, with 3/8" flared copper tubing, leading to a primary filter system for just the generator. Installed a check valve isolating the main engine fuel supply from the gen fuel supply but also re-piped the main engine fuel lines. Added a (Kubota tractor) fuel
lift pump powered via the fuel rack circuit. This may have helped some and was recommended by a mechanic and NL
service engineer. If you have a leak, each cool start is also a fuel bleed.
3. b. It is recommended to install the primary filter before the fuel-lift
pump. But that leaves no good way to remove air from the filter housing after a
water drain or filter change. Added a bypass valve after the fuel
lift pump to send air from the primary filter directly into the return line.
4. Cleaned the exhaust/mixing elbow. There was a lot of carbon and rust build-up that was constricting the exhaust
water injection.
5. Ran fuel return line with a high loop at the engine before it drained back to the tank. If the injector check valve, between the
injector pump and the injectors,
leaks when the engine is off and there is negative pressure in the fuel supply line, air might make it's way back to the
injector pump. At first it seemed to help. It didn't hurt.
6. Will install new injectors next season.