The first item to check is your coolant level. Heat that comes and goes like that is common if the coolant gets low and allows air into the system as the heater cores get air locked very easily due to the way the hoses are run and how the core is designed. So, check that first.
Next would be if the thermostat wasn't operating properly. In this case that would normally set a code and turn on the CEL as the computer calculates how long it takes the engine to warm up under a certain load. Is the check engine light on? If yes then see about getting the code(s) scanned at one of the parts stores that do it, write down whatever codes they tell you. A common one for a bad thermostat is P0128, it basically means the thermostat is stuck open and keeps the engine running cold. In your case though it sounds more like low coolant. One item that can really help on the vans and others that have a propensity of air locking is to use a vacuum filling device on the cooling system such as the attached image. However normal purge can work if it's low. The other issue would be that if it is low where did the coolant go? That could be a leak. To find that you would use a pressure test kit. Sort of the opposite of the vacuum fill.
Now if you look at the coolant level and it is full, and the CEL is off and you still have odd heat it could be a faulty blend door actuator, however that will usually show up as heat on the driver's side but not the passengers or reversed or no rear heat but still front heat.
So, a few questions, Is the coolant full? Do you get heat on one side but not the other? Is the CEL on? Check those things first and we have a starting point.
Tuesday, January 24th, 2023 AT 6:51 AM