I have just had the engine in my Nissan Patrol ZD30 fully rebuilt. When I start the engine from cold it builds up temp as it should, but half a kilometre down the road the temp gauge drops off to zero. The top hose is reading about 45-50 degrees, bottom hose around 27. I cannot work out why it drops off to zero on the gauge, or why the bottom hose is so cool. I have checked the thermostat even though it was new and genuine, I have replaced both temp senders, have checked the water pump, and all looks to be as good as new. Any ideas?
The top hose temperature, if what you say is correct, is way too low, it should be up around 100 degrees, hence the low bottom hose temperature. I would suspect the thermostat is stuck open.