Temprature is running high and heater works on and off. when heater does come on, temp. decreases. is it the thermostat?

Or the thermostat might be faulty which raises the temprature at which the thermostat starts to open. Giving a high reading on the gauge. if it gets too hot you might be at risk of blowing a pype or head gasket. If so replace thermostat. Check for a loose wire to the temp sensor as well. See if the radiator fan turns on when temp rises. If not check a fuse or the radiator fan temprature switch might be faulty.

I am very doubtful you have air in your hoses causing the problem. Feel the upper and lower heater core hoses at running temp and verify they are both hot, and should be same temp.(make sure heat is on high in car) Some newer cars lose some heat to the cab just because they are low on fluid. More likely though, you may have a bad radiator cap. If your cap is good, watch your hoses and make sure they aren't collapsing( the hoses will look like they are breathing in and out with car idling). You could even have a bad thermostat, or your heater core could be plugged. You can flush your heater core and see if that don't fix it, but since you have heat then it decreases, your core is probably ok.

Mid way is ok, but u need check thermostat, cheap to replace, also check rad is not blocked with insects, oil ok? Water cap tight? Turn on heat full in car see the temp drop, water pump can be on way out

your coolant temp sender could be bad. locate the temp sender. this is a 1 wire sensor. the other sensor in the block has 2 wires and starts the cooling fan.

Possible faulty thermostat

Sounds like a lazy thermostat or a temp sender that is failing. Replace the stat and see if it helps. Thermostats are inexpensive so don't even bother testing it. Make sure when re-filling, you put the heater on high heat so it does not get air bound.

you didn't say that you replaced the thermostat....sounds as if it is sticking. You can always remove it and put the housing and such back together (w/o the thermostat), you won't have heat; but this will tell you if the thermostat is the problem or not. Also there is a temp sending unit....usually around the thermostat housing - indicated with a elec. plug running to it. It would be a good idea to remove it from the housing and clean/replace it, the sending unit can also make the engine run terribly if not working properly, and decrease gas mileage significantly.

is there an air pocket in the system,make sure the front of the car is pointing upwards,like parking up a steep driveway to bleed it,by just running it with the heater on,does the cooling fan work,maybe blown head gasket

I don't think the 2 have been synchronized, i.e. the thermostat and the temp gauge as if they for different manufacturers products. As a result your fan is always running no matter what the engine temp is, so I guess the thermostat is either faulty or it has not been installed properly.....sodeep

Hello;
EVen with the temps as they have been you should be able to get to "operating temperature". Yes the thermostat is probably stuck open. Follow the top radiator hose to the water neck on the engine. There you will find 2 bolts holding the water neck in place. With the engine off and cold, release the radiator cap slowly. Then take the two bolts out. Remove the hose out of the way and there you will find a poppet thermostst. Pull it up and replace it with the new one. ( I always test my new ones in a pot of boiling water to ensure they work properly) Clean the mating surfaces, andf reinstall in the reverse order.
Hope this helps you and rate me highly please.
Thanks;
Rich
RPM Northwest