2003 525i bmw heater will only blow cold air

The car has to run awhile to heat up the water that cycles through the heater coil.

Check heater core.

These selections are run by Engine vacuum. sounds lie a diaphragm is NG to control the lower door

Replace the thermostat.

you need to 1 check your thermostat or 2 take it to a reputable Rad shop and have them check the rad cores on you car ,sometimes they clog up and they pump coolant under pressure and it clears the core

Check there is water in it first. A black cap on top of the radiator. If the level is ok then a wee red dipstick should float up.

my bmw 323i heaters blowing cold on passenger side anyone help

Sounds like a thermostat sticking. Some things to do. First check coolant level is up and moving. Second (most likely), change thermostat. Very easy fix for backyard mechanic. Make sure to put thermostat in so water flow is correct direction. Look for arrow on side of thermostat. Next, is belt to water pump loose. Another easy fix in most cars.

motor is going bad replace it

probably an airlock in the heater matrix---bleed radiator while engine running with the bleed screw located near the filler cap,when no air escapes should be ok but will need a water top up----good luck.