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
- Slap a 1k linear potentiometer in series with the coolant temperature sensor. This will allow you to increase the dial the resistance from 325 ohms to 1325 ohms at operating temperature and simulate a colder engine of ~ +40°C. The ECU will increase the injector's duty cycle to add more fuel.
- Tune by the seat of the pants, watching the plugs, air-fuel meter and mileage.
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
- Tune EFI system to stock and work out bugs (especially AFM)
- Accurately measure injector duty cycle at normal operating temperature with ambient air temperature > 20°C at 3000 rpm
- Also measure coolant temperature and coolant temperature sensor resistance with ambient air temperature > 20°C
- Slap a 1k linear potentiometer in series with the coolant temperature sensor.
- Set the pot to 1/2 way
- Accurately measure injector duty cycle at normal operating temperature with ambient air temperature > 20°C at 3000 rpm and adjust the AFM spring so that the duty cycle matches the same as above.
- This calibration allows the pot to add or remove fuel
- Tune to adjust