Why does my 94 e250 heat fine at first, then not at all?

Transmission - reverse gear is shot or solenoid pack needs to be replaced. First step is check trans fluid. If the vehicle does not move at all in reverse, will need a transmission shop or dealership to look at it.

Thermostat? Water pump? Need more info

this should be it.

Well, it can be a sending unit issue, they are pretty inexpensive, so worth while to just replace it. Do be aware that the temprature DOES normally do what you describe. The condition is called heat soak, where the temp goes pretty high, due to no water circulation, after shutdown. This is normal and some vehicles run fans after shutdown to help. Usually not an issue with properly functioning cooling system that can hold pressure.

Remember the coolant recovery tanks? Their purpose was to recover teh small amount of coolant pushed out during heat soak.

get it hot and leave the hood up,hopefully the heat will dry it out.
NEVER pressure wash a modern car its full of electronics(its like putting a t.v. in the sink.....)

fuse box diagram for 94 econoline 250 van

Follow the wires from the blower motor back to the blower motor resistor, normally only a few inches away and held in by a 7 mm screw. Replace it

Check coolant level, if good, thermostat may be sticking. This is provided you are not referring to a fan issue.

it sounds like the pick up coil in the distributor is heating up and shutting down you have to pull the distributer and take it apart to replace it

warm your car up to temperture. Grab the upper rediator hose. Is it warm or cool. If its cool your thermosat is stuck closed and thats why you have no heat. If that works fine I would look at your heater core next.