Heater bows cold when idling

find out what's the trouble code:
http://forums.nicoclub.com/how-to-read-obd1-trouble-codes-for-1993-1995-j-s-t217489.html
then post back

Heater core may be clogged. Try flushing it to see if water flows through freely. Also check the heater water control valve for restrictions.

Possibly bad compressor, worn bearings in a pulley or lines TO compressor. Also, perhaps wrong refrigerant added. Sometimes it makes a difference as there are some 7 different kinds.

You are dealing with a ignition misfire,This can be from coil spark seeking ground,to a sparkplug cable seeking ground.

check ur water level

Check your coolant level. If it is low, what you describe will happen. Water pump works faster as rpm's increase, managing to get hot coolant to heater core (highway speeds). At idle, pump is not working fast enough to get the coolant to the heater core.

thermostat might be stuck open need to replace it ..just went through that with my car

Check the coolant level.If you are low it will stop flowing thru the heater core at low RPMs.

Hi!
It appears we have an Air lock scenario and you will need to perform a system Bleed.
Park the vehicle on level ground, when cold remove coolant filler cap, start engine and leave to idle, turn heater on full and blower to max. When engine reaches operating temperature watch and listen near coolant filler, keep clear as gurgling and hopefully a boil over should occur. Top up with very warm coolant and wait as it may do it again.
Check for heat inside vehicle if warm replace coolant cap but keep an eye on temperature gauge as the ~Air lock may have moved on from heater matrix/core so proceedure needs to be carried out again from COLD.

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Paul 'W' U.K

Replace your thermostat