98 vw jetta gls Heater core

If both hoses are hot, the problem could be the inside...could be a blend door actuator

More than likely heater core leak, I assume you are losing coolant. I've changed a couple heater core in my life but nothing like changing the one in the video. I saw 3 videos, and all said you have to pull the dash.

How VW Parts Fail ~ Heater Cores - YouTube

VW Heater Core Replacement - 1993-1998 VW Jetta and Golf - YouTube

yes its a fairly common fault with all VW motors from this age including skoda and seat

Sounds like the heater core and it will cost anywhere from 200.00 to 400.00 unless you can replace it your self.

Many shops (and the manual also) say that the AC has to be disconnected and refilled to replace the heater core. I had it done by a VW specialist who left the AC filled, saving a lot of money. It can be done.

If it is necessary to remove the heater assembly, the cooling system must be drained before removing the heater core.
When a heater core leaks, a new heater core is installed or the old one repaired.
Heater Housing

• The heater housing is usually under the dash and must be removed to gain access to the heater core.
• Procedures for replacing the heater core vary with the year, make, and model of car. It threfore necessary to consult the manufacturer's repair manuals for the proper procedure for replacement.

The heater housing is disassembled to get to the heater core. Heater Core

• Remove the access panels(s) or the split heater/air conditioning case to gain acess to the heater core.
• Remove the heater coolant hoses.
• Remove the cable and/or vacuum control lines (if equipped).
• Remove the heater core securing brackets and/or clamps.
• Lift the core from the case. Do not use force. Take care not to damage the fins of the heater core when removing.
• To reinstall the heater core, reverse the removal steps.

When the heater core leaks and must be repaired or replaced, it is a very difficult and time-consuming job primarily because of the core's location deep within the firewall of the car. For this reason always leak test a replacement heater core before installation.

it is more than likely airlocked bleed air from system or open heater core pipe make sure heater core is full

Check the lines coming into the heater core. With the engine running, touch both lines, they should both be hot. If not, you have a blockage in the system. If they're both hot, you may need to replace the heater core.

ARE WE TALIKING ABOUT THE KEY CYLINDER OR THE ELECTRICAL SWITCH?

There are known problems with this vehicle's heater core not getting enough coolant flowing through it to blow hot air.
1) Even if the coolant looks good, there are problems with the heater core plugging up. One solution is to use a heavy duty cleaner (CLR pipe cleaner) to back flush the heater core. To know if the heater core is plugged up, disconnect the 2 heater hoses at the heater core and somehow connect a hose to one end of the core. Try to flow water through, it doesn't have to be high pressure, and there should be a steady stream. If not, try to get the cleaner directly into the heater core and let it soak. Then back flush it and a lot of scale should be seen coming out.

2) The other problem is that since the heater core is the highest point in the cooling system air tends to get trapped in it. This should only happen if the coolant goes low or someone has opened the system to service it. There are a number of ways to bleed the system (vacuum bleed, pressure bleed). One of the hoses should have a small hole in it for bleeding air, pull that hose back just enough so the hole allows coolant to flow out of it. Then accelerate the engine and get a good flow coming out, when no more air bubbles are present, there should be good heat.

3)I've heard of bad water pumps with low flow causing issues as well as an aux. water pump not fuctioning correctly leading to poor heat output from the heater core. Someone should confirm this as I cannot get much information on the aux water pump(electronic pump by the alternator that is supposed to run after the car is shut off).

Hope this helps.