If engine heat is below normal on temp gauge then thermostat needs to replaced. Probably staying a little open when supposed to be fully closed. Also if it was extremely cold zero or below then I would try putting something in front of radiator (just part of it) so as to slow the airflow and let the engine build some heat. I've had to do this with my 93 a few times.

 

well...its 16 degrees outside right now, and it never got above 30 all day today. it took a good long while...like 15-20 miles before there was nice heat from my heater vents, before that it just barely lukewarm. my '90 f150 would cook you out of the cab, and my parents '86 bronco would too, they claimed it had a "Northland Package"? with a bigger heater and engine block heater. does anyone know if this was an option in '91? and if so, can i get this heater to go in my bronco??

 

Replacing my heater core was much, much and another much easier than I anticipated. Start by removing the glove box.

 

Like I said in another post, it took me about 40 mins to replace mine with my 6yr old helping......

 

Heres a thought. Check under the cowl and see if there ia any air getting through to the heater core. Either here, or in the heater box you can get leaves and crap plugging up the airflow. Second, see if the air doors in the heater box are moving. It they are not directing the air correctly, then the air will not go through the heater core and will not heat. Check the little skinny plastic hose coming out the passengers side firewall to see if it is broke. This line leads to the heater controls and breaks often.

 

Also check that vaccum motor that line gos to. On my truck there was some dirt and rust stopping the full travel of the vac. motor. This caused the door to stay open. Ihad the same conditions. Not to cold of a day seamed to have heat veary cold day no heat.

 

All those things are ok, changed the heater core and not much of a differance.

 

Check for water flow coming out of the engine. Pull off the heater hoses from the heater core and start the truck. Do this when the engine is warm. Put the hoses into a pail and see if water is pouring out of one of the hoses. If so, reconnect this hose to the heater core and restart engine. Water should pour out of the heater core. If not, then water is not circulating for some reason.

Also, is there a thermostat in the engine? You may want to replace it. Pretty cheap and easy check.