Temp light flashes,, coolant leaking into the car above the gas pedal.. air in the cooling system

If the heater matrix is buried somewhere behind the glovebox, the noise could have been air or combustion gas being pumped through the matrix.

Cylinder head gaskets normally allow small amounts of combustion gas to escape and the cooling system is designed to allow it to rise rapidly above the coolant level, often with additional ports and vent hoses leading to the degassing tank.

If the degassing system becomes blocked due to the cooling system being dirty or corroded the gas bubbles can build up and displace coolant and disrupt the thermostat operation and eventually be forced into the heater matrix causing a vapour lock.

A failing head gasket will have a similar effect...

These things will need checking. Overheating is invariably caused by insufficient coolant circulation and/or insufficient cooling air flow through the radiator.

No your car will not overheat, but it is advisable to my 50% coolant on the water to prevent deposits of rust and other debris that may affect the effectiveness of the cooling system.

seen as there has been a leak in the coolant system in the heater core it is possible that the coolant system has a air lock in it and a few things can resolve this. fill the coolant system to the correct level just with water until the problem is rectified. most vehicles have a air bleed screw. with the engine run up to normal temp and the pressure cap on bleed the air out of the coolant system by undoing the screw a few turns and releasing the air from the coolant system, if fitted with a bleed valve it will be some where on the higher part of the coolant system. if not fitted with one it can also be done by raising the front of the vehicle in the air by a trolley jack and using a coolant pressure tester on the coolant system , compressing the air and moving it along to the radiator . then when the compression tester is moved off the system top up the rad and reserve to the correct level and see if this helps . also it is possible that the thermostat may have been damaged and isn't working properly. check the throttle cable for being kinked , jammed or faulty return spring , also check full throttle travel at the butterfly and that it is returning to the closed position. next up would be clean the throttle body as well as the iac (idle control circuit) these get gummed up from time to time and can cause a issue as well. other possibles are a vaccume leak . hope this is of some help mate.

Most likely causes are bad engine coolant temp sensor OR leaking fuel injectors. Bad engine coolant temp sensor will give computer false reading and computer provides wrong air fuel mixture. Leaking fuel injectors will load the engine with raw gas--just like flooding it. Try pushing the gas pedal to the floor and holding it there when cranking when hot. That forces the computer into "clear flood" mode and it shuts off the injectors. If it starts up with pedal down, that would be a sign of leaking injectors.

it is overheating. low on coolant/ thermostat stuck shut? see if both radiator hoses are hot.

The reason it did not read hot, on your temp. gauge, is cause your temp. sensor read's coolant or water temp only, not air, if cooling system is empty, coolant temp. sensor will not read, like an air temp. sensor well, more the likely there could be a coolant leak for cooling system to be empty, also could have damage your cylinder's head's, need to have a block test and cylinder compression test done to fine out if head's are damage.

Try checking the knock sensor and clean the throttle plate inside and out and also the back of the throttle plae, many times the plate sticks from back firing that you can't detect by ear or feel. Also could have a plugged cay.

One guess is there is air in the cooling system. The air pockets will cause temporary overheating (fluctuating guage).

It sounds like a bad head gasket is creating these air pockets.

It sounds to me like you're low on coolant. When you checked the coolant level, did you actually open the radiator cap and make sure the radiator is full? The plastic tank you generally add coolant too is actually an 'overflow' or 'reserve' tank. There is a hose that runs from that tank to the top of the radiator so if the radiator gets low on coolant, more coolant -should- run in from the overflow tank. However in practice this does not always work. I have a Jeep that leaks coolant and experiences this exact same problem. Coolant will not run from the reserve tank into the radiator because the hose is plugged up, and whenever the radiator gets low on coolant the temp gauge will go real high and then drop, and the heater doesn't work well. This is because the cooling system of your car is supposed to be a closed system, full of coolant and no air. When coolant leaks out, the space it used to occupy is now occupied by air, which does not transfer heat well. When 'air' is passing through your cooling system, no heat can be transferred from your engine to the heater and radiator, resulting in a hot engine and no heat at the heater. Then when a pocket of water passes through the system, the temperature gauge quickly falls as the water absorbs the heat from the engine. The hot water that cools the engine is where the heater gets it's heat from as well, so when water passes through the heater core, the heater works, but when it's filled with air, it doesn't.