1995 Chevrolet G20 3/4 ton Van 4.3L V6. The van never runs any warmer than 1/4 on the instrument panel. I have replaced the thermostat twice. Same issues. I have replaced the temperature control sensor. Still same issue. Doesnt matter if the van runs for 20 mins or is driven for 4 hours straight. Never raises above the 1/4 mark on the gauge and no heat. Any suggestions as to why it never warms up? Heater core inlet and outlet hoses both get hot ones not colder then the other. No fluid leaks anywhere. Been over it a dozen times. Van has a clutch fan instead of electric and clutch appears to be fine.