Nissan Repair: 2.3 serena desal, water in the fuel, fuel additive

Question
I have a strange problem thats just started. Car starts and seems to run effortlessly until I meet a small incline and in 5th gear, the engine starts to vibrate, its does seem to be the gear box, and no it is not that I am in a too higher gear for the hill. Its so hard to explained as it use to run so smooth, the engine seems to pull but like roars?????

Answer
Mick,

Yes, when the engine is loaded, as on a slight incline, it will buck.  As in slight vibration.  I know it did not do it before so something has changed.  You could address the problem by a tuneup with a new fuel filter and air filter.  Also, if the grade of engine oil is too thick for the cold weather that can cause a degrade in power.  Also, a fuel treatment could be an answer if there is water in the fuel.  Slight water can affect the fuel and cause less power.  That is where a fuel additive and a new fuel filter would come in.  Lastly, if all things being equal check out the car for brakes dragging and that the tires are inflated and in good order.  I have seen dragging brakes do things as well as bad tires.