Cooling fans and A/C settings

[Craig 94 TA GT]

I'm not sure if my '94 has high/low speed fans, but I think it's an earlier model of 1994. The fans are programmed to go on at 187/187 in LT1 edit and the stat is a 160 SLP. Here is the behavior:
- When driving on the freeway, the coolant temp will stay pretty low, roughly 3/4 between the 160 and first hash.
- When driving in the city without A/C, the coolant will hit around 191 but sometimes dip to 187.
- When driving in the city with the A/C going, the coolant will hit over 210 after a while. I'm too afraid to let it continue sitting though. I usually turn the A/C on during hot days, so I'm not sure if it's because the A/C changes the fan settings or turns off the secondary fan, or if it's just the sheer heat outside and it needing a flush.

Does anyone know if turning the A/C on changes the fans, if both already going full blast? Thanks in advance.

[Jeff_in_Atl]

Turning on the A/C will start the compressor. When the pressure hits a certain high side pressure the fans will come on. (some cars turn on the fan when it turns on the compressor but I think GM does it with the high side pressure sensor) It is required for the A/C to work properly for the fans to pull air across the coils to remove the heat out of the A/C. If they didn't the high side pressure will shoot up and turn off the compressor and the A/C won't be very cold. Turning on the A/C does not alter the programmed fan settings but it will override them as the A/C demands it.

Keep in mind that the way the A/C works is it compresses the Freon back to a liquid which causes it to heat up so it is run through the condenser coils in front of the radiator to cool it before it gets to the evaporator to cool the inside of the car. So that heats up the air before it gets to the radiator. Since the radiator gets hotter air it doesn't cool the engine coolant as well. Also keep in mind that GM put the fan temp at 226 so 210 isn't that hot to GM. Yea I don't like it that hot but it isn't anything to worry about.

[Craig 94 TA GT]

Thanks Jeff, that explains it really well! I hope a coolant flush will cool it down a tiny bit more, but now I realize this is definitely within range.

[Jeff_in_Atl]

Flush the heater core both ways. The Dexcool tends to clog them up.

[Craig 94 TA GT]

Thanks again Jeff - I will definitely try to flush it well. What's weird is that I first got the car in '99 and flushed the coolant REALLY well. I hadn't had a problem with the heater core at all in about 5 years. Back in mid to early 2000s (when I was still leary to work on some things on the car), I had a shop do the opti. After that, I had my first heater core problem - heat wouldn't work. I just flushed the core and it worked again. Well, after the motor rebuild, I already had the heater core clog 3 times! I think there's crap in the coolant, probably causing it to clog? Which means, for this flush, I should probably pull the engine knock sensors too, to make sure all the crap is drained.

[3800_Firebird]

Not to horn in, but how do you specifically flush the heater core?

[Craig 94 TA GT]

Shoebox has a writeup here: http://shbox.com/1/4th_gen_tech2.html#heaterflush

To find the hoses, just trace them to the core. I removed both ends, took a funnel and hose, and sprayed one end, then the other. The hardest part is just to make sure the water is pointed away from the electronics.