My heater blows cold air. I have had it checked. The water is hot and is running properly through. Mechanic had no explanation as to why only cold air was blowing in and not hot.

check the heater core hoses after the engine is hot both should be hot but if i am right one will be hot the other cold and if so heater core is plugged and is an easy fix with some compressed air and CLR. you will have to remove the hoses to the core and blow air into the cold side to get junk out of it then put CLR in the core for a bit to clean it then blow out again and do not use alot of air just low pressure then rinse out with water and hook back up hoses and run engine and top back up fluids, I have have to do this 20 times to diff vehicles in the past few months

How hot is the engine running in degrees f? Does the radiator fan run?
With hvac turned on, check voltage and ground at the blower motor. If good voltage and ground, suspect the blower motor is faulty.

Sounds like you have water in the oil which means you could have a blown head gasket. Can you tell what got in the oil. Is it antifreeze or gas. If gas you probably have a injector sticking open.

Explain how you had a mechanic work on the problem
& not resolve

If the hose from the water pump is hot & the return from
the heater core is not ,then there is a control valve somewhere

Some mechanic that is

We don't have a data base to look up that ,we are at home like
you,social web site

To troubleshoot this types of problem,click the link below :----http://howtobyme.blogspot.in/2012/02/heater-blows-only-from-defrost.html
--------Go through the procedure to confirm the issue.

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Engine Overheats and heater blowing cold?

http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

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Heat blows cold on drivers side and hot on passenger side?

http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html

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This will help.Thanks.

If the car is hot and the heater blows cold air your thermostat is stuck shut or your radiator is really low on water/antifreeze check when engine is cold hasn't been ran like overnight take off the radiator cap and if it is low add so antifreeze/water if heater still blows cold air or water level is ok your thermostat is stuck shut and needs to be replaced.

that is entirely possible yes, could also be a broken cable to change the temp from hot to cold, could be a problem with certain ac components under the dash

First you have to identify what causes the problem.
To do this you have to:
1) When engine is cold check the level of coolant. If it is low fill it up.
2) Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced.
3) If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is worm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core.
4) If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator.
5) If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest to flush the heater core.

First you have to identify what causes the problem.
To do this you have to:
1) When engine is cold check the level of coolant. If it is low fill it up.
2) Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced.
3) If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is warm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core.
4) If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator.
5) If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest flushing the heater core.

Check both heater hoses under hood to see if they get hot, if they do the heater is fine. need to check to see if the selection doors inside the unit are opening and closing.