Turning Archive 2005
>Hi Jen, I saw your post on the message board about your mom's car...since the problem existed before the mechanic worked on it, it's probably nothing he did while changing the water pump. However, he should have changed the thermostat as a precaution when he changed the water pump. I don't believe a nissan sentra uses vacuum to control heat, so that's not the problem.
I would check the hoses going into the firewall from the engine (heater hoses) They should both be HOT. If not, the heater core may be plugged; esp. if someone put stopleak into the cooling system. The cable that operates the heater valve may be defective, or the valve itself defective...Have someone operate the heat controls while you watch the valve move, and feel for heat on both sides of the valve.
Make sure the coolant level is full (while cold). If the level is low, that will cause the problem you described. The mechanic MAY have inadvertedly created an airpocket in the cooling system when he did the pump....but not likely since the problem was there previously.
Last, if all else checks out ok, suspect an internal engine problem. A bad head gasket will introduce combustion pressure into the cooling system, creating the problem you describe :(
Hope this helps...be glad it's not a Mercedes!