Three contributions in this article by Bob in Oz, K-Zed and 90TTZ (Please read all)
What can I check to reduce excessive fuel consumption? - Bob in OZ
Excessive fuel consumption most likely has a cause due to an external sensor:
Things to check are:
1. Coolant Temperature Sensor If the coolant temperature sensor connections are corroded, or the sensor is defective, the ECU will believe that the engine temperature is lower than normal and will increase enrichment. May not necessarily error code.
3. Oxygen Sensors Are the oxygen sensors operating correctly ? A defective oxygen sensor will cause excessive fuel consumption...a slow sensor will not necessarily set a code. This is one of the most common causes of excessive fuel consumption.
4. Fuel Pressure regulator Check the operation of the fuel pressure regulator....ensure that it is receiving vacuum...sometimes the solenoid controlling vacuum fails, resulting in richer mixtures during light load.
5. Fuel leak (even minor) Check all hoses for any seapage and tighten all hose clamps
- Bob in Oz
Extra ideas on high fuel consumption & power loss - K-Zed
Recent Tech Day's have highlighted some problems common to many Zed's, namely Vacuum or Boost leaks & failed [or sluggish, failing] O2 sensors.
Vacuum & Boost leaks not only cause high fuel consumption & power loss but can show O2 sensors to be possibly faulty.
If you're not getting circa 450 - 500k's [around town] 500 - 650k's [Highway] refer to 'how to blow up your engine' and test for Vac/boost leaks. Do this first, before replacing O2's.
After Vac/boost leaks have been eliminated and O2's do prove to be faulty replace BOTH. At circa $86 each [trade], they will save you precious $'s in improved fuel consumption very quickly.
O2 sensor generic replacement parts (considerably cheaper than Nissan) - NGK NTK part no. OZA446-E47-Stock No. 0185. (Or part no. 351-0039 thru Global Auto Spares).
Extra ideas on high fuel consumption & power loss - 90TTZ
Detonation Sensor CircuitIf the ECU has detected detonation (pinging) or a fault with the sensor, it will trigger a safety boost map condition.
Not only does this reduce turbo boost pressure to 7PSI but also enriches fuel mixture and retards ignition timing.
It is not unusual to see Zedâ€™s with full tanks fetching 200-250KMâ€™s with this condition.