Hot air - not so hot

I own a 1999 GMC Safari van. Within 6-months,mechanics replaced radiator & hoses, thermostat, heater core flush, water pump, heater core, heater core vales & assembly, radiator cap, vacuum checks and radiator flush.

The heater still blows luke warm air when I come to a stop sign and never heats the air like it used to. What is my problem?

Very possibly you have a stuck blend door. The blend door moves to mix hot air with cold. If something’s jamming it, it can let too much cold air into the mix and cause the symptom you described.

Auto repair mechanics have checked the blend door.
I can hear the blend door opening and closing, so I don’t believe the blend door is the problem.
Someone said the cam shaft may be worn, causing the problem.
The mileage on my engine reads 228,000
The mechanic has basically given-up, since they are stumped.
The problem appears to be associated to VACUUM somewhere, but nobody can figure it out.

Are you sure the blower fan is running? I had a similar issue and finally realized the blower fan was not running at speeds one and two out of 5. A new variable resistor, $35 and easy to replace solved my problem. There was enough natural air flow while moving to keep comfy, but minimal or no air flow at slow speeds or stop.

The air does come out of the vents, but it’s not hot.
Even when I’m driving, the air is not hot, but only warm.
When stopped and idling, any heat I had while driving turns cool.
Fan does continue to blow from my vents if that is what you are asking.
Fan stops blowing from vents when there is a strain on the engine when I’m going up hills.
If I then let my foot off of the accelerator when going up hills, air begind blowing from my vents again … but not exactly the hot heat I’m looking for or used to.