My car Nissan 2.4 SE d/c year 2006, petrol, fuel injection has been giving me problems lately. a couple of years ago it started by smoking (black smoke), fuel consumption was too much, revs were inconsistent sometimes to a point of stalling. I took it to Nissan dealers and they replace the computer box and MAF sensors. It was okay for a while it has now started the same problem again. I am an offroad 25% of the time. It concerns me what could be the underlying cause or if at all that was the real problem. Please help

Possible faults:-

Coolant sensor - engine is signaled as being 'cold' all the time, not just at start up, causing ECU to set longer injection cycle

? Low coolant level can prevent sensor being able to detect coolant temperature

? Faulty 'open' thermostat allows coolant to circulate without regulation and perhaps prevents the engine from achieving normal running temperatures.

To check put a voltmeter across the coolant sensor connections and there should be a difference between the engine being cold and hot. If not the sensor is faulty. Check sensor connections and wiring.

FPR - broken diaphragm allows fuel to enter the vacuum line and then into the inlet manifold. Additional source of fuel makes the fuel air mix richer causing a faster and lumpy idle. To check the FPR have the engine at idle and pull the vacuum line off it. If the FPR is healthy the engine will pick up in revs for a few seconds and then settle back to a very lumpy idle. If fuel dribbles out of the vacuum line then replace the FPR.

Vacuum leak - 'high oxygen' signal from O2 sensor causes ECU to set longer injection cycle. Check all hoses and connections with the inlet manifold. Check the inlet manifold to block junction and around injector ports. A small squirt of WD40 on suspect areas will cause the engine to momentarily pick up in revs if there is a leak.

O2 sensor - 'high oxygen' sensor error causes ECU to set longer fuel injection cycle. A voltmeter across the sensor pins should show above 200mV (note this is AC voltage). Check again when hot as the sensor can fail progressively and lose voltage as it gets hotter.

MAF - 'over reads' in error the amount of air entering causing ECU to set longer fuel injection cycle. Located in the collar next to the air filter box on the air ducting. Make sure the MAF is clean, do not touch, just spray it with electrical cleaner when it is off the car. The MAF has three wires (0volts; 5 volts supply; signal). Check that the signal voltage varies according to engine speed (air being sucked in). If not change it

IAT - 'under reads' in error the temperature of incoming air causing ECU to set longer fuel injection cycle. often co-located with the MAF but can be nearer the throttle on the air ducting. Has two wires. Unplug the IAT from the ducting and check that its resistance changes when you blow on it. Give it clean with electrical cleaner spray. If not change it

Hope the above check list helps.