Warm but not Hot

I have a 1991 S10 Blazer 4.3L TBI (no vortec here). Anyway, my problem is my heater takes a long time to get warm in cold weather. I can drive 20 miles in 30 degree weather (in drive, not overdrive, to increase the RPM's and get it warm) and the heater, vent and defrost only blow out warm air, not hot like it should be. Also, when I place my fan in the OFF position I can hear and air leaking sound coming from the dash where the "defrost" air comes out. Any other fan position is fine except off. I was thinking of trying to switch the thermostat for the heck of it to see if that would help. My temp gauge doesn't work because the wire harness pigtail is corroded and when it does work (sporadically) it shows it's overheating but it's not. I have a digital instrument panel which I hate. Anyway, any suggestions would be very helpful. Heater core plugged or needing replacement?

1. Replace your thermostat.
2. the air leak sound you hear when fan is off is most likely a vacuum leak inside the dash.

I had the same issue with mine. What the problem was is the hose that goes from the heater box to the intake manifold where the metal pipe connects to the intake has a smaller diameter hole and the end of the pipe begins to wash away. Small pcs gradually wear off and end up getting stuck into the end of this pipe. What you need to do is disconnect the pipe from the intake, clean the end of it and reinstall it.

Check out what IceFire is talking about, but my first instinct would be to replac the thermostat. If you havent done it, its only a couple bucks and a few minutes to change.

1.) Heater core should be checked and reverse flushed.
2.) Remove the control panel for the dash and reconnect or fix and reference where the vacuum leak is coming from.
3.) Check your dash to see if the engien is actually coming up to temperature. It should be 1 or two ticks under the half way point when warmed up.

You might