1998 Pontiac Transport overheating

Hello marinevietvet
Is the fan coming on when it should?
If you're not sure, one way to quick check the fan circuit is to start the engine and turn on the A/C to max. If the fan runs, the fan motor, relay, fuse and wiring are all okay. But this test doesn't show if the temperature switch or temperature sensor and PCM are energizing the fan when coolant temperatures are high.
To check the temperature at which the fan comes on, turn the A/C off and keep the engine running until it reaches normal operating temperature. Most fans should come on when the coolant reaches about 200 to 230 degrees. If the fan does not come on, something in the control circuit is defective. Resistance checks should then be made on the temperature sensor or switch, and a voltage check on both sides of the relay (you will probably need a wiring diagram of the fan cooling circuit on your vehicle to identify the relay terminals and wiring connections).

The fan motor itself can be checked by using jumper wires. Unplug the wiring connector on the fan, and use the jumper wires from the battery to route power directly to the fan. If the fan motor is good, the fan should spin at normal speed when supplied with 12 volts

The problem should be either a blockage in the heater core or a defective heater control valve.
note the temperature control valve(129)

What we need to do is to compare the temperature of the heater inlet and outlet hoses where they enter the firewall. Both should be hot, and the inlet hose should be about 20 degrees warmer than the outlet hose. If the outlet hose is not hot, the core is clogged or the heater control
Valve is defective. If reverse flushing the core fails to open the blockage, you will have to replace the heater core.