Temp Sensor Tweak

The amount of fuel injected in a Datsun 280z Bosch L-Jetronic system is determined by the ECU when it reads the coolant temperature sensor, air flow meter, air temperature sensor and throttle position switch.

By fooling the ECU, one can get it to increase or decrease the amount of fuel injected.

The easiest way to trick the ECU is by increasing or decreasing it's reading of the coolant temperature sensor. To do this, you simply need to change the resistance of the coolant temperature sensor.

Here is a chart showing the resistance of the coolant temperature sensor vs. coolant temperature:

C   F Resistance
-30 -22 20,300 Ω to 33, 000 Ω
-10 -14  7,600 Ω to 10,800 Ω
+10 50 3,250 Ω to 4,150 Ω
+20 68 2,250 Ω to 2,750 Ω
+50 122  740 Ω to 940 Ω
+80 176 290 Ω to 360Ω

It should be easy to see that the resistance decreases as the engine warms up.  It is also worth noting that the FSM says the amount of fuel injected decreases as the engine warms up. So if you want to trick the ECU to add more fuel, you simply add more resistance to the sensor to simulate a colder engine.

At normal engine operating temperature, the resistance of the sensor is 325 ohms.

Simple Crude Tweak


            Simplified stock 280z connections  between ECU and Coolant Temp. Sensor. 
              There are bullet connectors ~ 12" downstream from the sensor. This is the easiest area to connect a pot.


       Here are the same connections but with the 1k pot connected to the bullet connectors. 
        Note the orientation of the pot and turning center to  right or left to add resistance or to "short it out". 
        This detail is important when you connect yours to the ECU harness if you want to add more fuel by turning clockwise or counter clockwise.


            Here is a detail for wiring a 1k linear pot. 
               Note:  the center and shorted pins on the pot are connected to the sensor and the other pin is connected to the ECU if you want to add more fuel by turning the pot clockwise.

Proper Tweak

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