Heater will not heat up except on acceleration

check that the control module is actually turning on the heater control to allow coolant to flow through the heater core. Check that you do not have an air lock into the heater.

the ac core only blows cold air but You must be talking about the heater core.. In which case I would suggest that you bleed the air out of the coolant system as there is an air lock in the heater core.

During accelerations the engine needs all the vacuum that it can develop. If you have a vacuum powered heater/ Ac unit you also have a vacuum reservoir that maintains vacuum to your heater unit during accelerations. This bottle has a leak. This should cause your unit to go to defrost though.

does it ever overheat? (waterpump or thermostat)

Possible clog in heater core or pinched heater hose under hood.

If coolant/water is at proper level, and you don't over heat, the later will most likely be the issue.

Heater core is small radiator deep inside under the front passenger dash and is rather hard to access.

If you smell something like coolant inside, this is for sure the issue.

Hope this helps.

Sounds like a heater core issue. The heater core my be clogged so water will only flow in it when you are accelerating. Easy way to check is to turn the heat on while the truck is running, find the two lines running to your heater core and see is both are the same temp. If the heater core is clogged then only one line will be warm/hot.

Check that the blend door covering the heater core is opening completely. It's vacuum actuated and the vacuum is heavier when you accelerate.

With the engine cold check the coolant level. Add as necessary. If the coolant gets low enough the first thing to stop working is the heat. Accelerating will push coolant into the void and provide heat until you slow down and then the coolant drains back into the motor.

If you have lost enough coolant to loose heat there is a leak. Some common areas are the rear heater lines under the passenger side of the van, the water pump, and the radiator.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

heater core is glogging up ...needs replaced

heater core or thermostat issue or both, but most likely your heater core.

have you checked coolant? if it is low will do this. if full then probably heater core restricted need to back flush core