Fuel Heater: Testing and Inspection

The fuel heater is used to prevent diesel fuel from waxing during cold weather operation.

NOTE: The fuel heater element, fuel heater relay and fuel heater temperature sensor are not controlled by the Powertrain Control Module (PCM).

A malfunctioning fuel heater can cause a wax build-up in the fuel filter/water separator. Wax build-up in the filter/separator can cause engine starting problems and prevent the engine from revving up. It can also cause blue or white fog-like exhaust. If the heater is not operating in cold temperatures, the engine may not operate due to fuel waxing.

Fig.19 Fuel Heater Location:

The fuel heater assembly is located on the side of the fuel filter housing (Fig. 19).

The heater assembly is equipped with a built-in fuel temperature sensor (thermostat) that senses fuel temperature. When fuel temperature drops below 45 degrees ± 8 degrees F, the sensor allows current to flow to the built-in heater element to warm the fuel. When fuel temperature rises above 75 degrees ± 8 degrees F, the sensor stops current flow to the heater element (circuit is open).

Voltage to operate the fuel heater element is supplied from the ignition switch, through the fuel heater relay (also refer to Fuel Heater Relay), to the fuel temperature sensor and on to the fuel heater element.

The heater element operates on 12 volts, 300 watts at 0 degrees F. As temperature increases, power requirements decrease.

A minimum of 7 volts is required to operate the fuel heater. The resistance value of the heater element is less than 1 ohm (cold) and up to 1000 ohms warm.

Testing
1. Disconnect electrical connector at sensor (Fig. 19).

Turn key to ON position. 12 volts should be present at red wire. If not, check fuel heater relay and related wiring. Refer to Relay Test-Fuel Heater. If OK, proceed.

Turn key OFF. Check black wire in connector for ground continuity with an ohmmeter. If continuity is not present, correct open ground circuit. This test can also be performed with a voltmeter by backprobing black wire with it connected to sensor. Reconnect electrical connector and turn key ON. Voltage drop should not exceed 2 volts (2 volts lower than checked at 12V+ connector). If voltage is lower, check for dirty or corroded ground connection and repair. If OK, proceed.
2. With electrical connector disconnected at sensor and key OFF, check electrical/mechanical operation of fuel temperature sensor. Proceed to next step:
3. Using an ohmmeter, check for continuity across two terminals in electrical connector at side of sensor. Sensor circuit should be open if fuel temperature has risen above 75 degrees +/- 8 degrees F. Sensor circuit should be closed if fuel temperature has dropped below 45 degrees +/- 8 degrees F. If not, replace fuel heater assembly. This same test can also be performed using a voltmeter, with key ON, and by back- probing connector.