OVERHEATING, CONSTANTLY FILLING RADIATOR WITH ANTIFREEZE AND NO VISIBLE EVIDENCE OF LEAK, CHECK ENGINE LIGHT ON, DIAGNOSTIC TEST SAYS 3RD CYLINDER IS MIS FIRING, HEATER QUIT WORKING

Hi Keith, Hopefully is only a faulty thermostat but the links below might have the answer.......Good luck..
How To Fix A Motor That Over Heats
How To Fix An Overheating Car
How To Test And Fix Head Gasket Leaks
How to Test Head Gasket
Symptoms Of Blown Head Gasket Radiators
Fixing Tough Head Gasket Leaks

If you are losing coolant then you have either an external or internal leak.
If you can find no indication of an external leak from the radiator, the radiator overflow tank, any of the hoses on the system or around the water pump fitting then you may well have an internal leak. Check the radiator overflow tank carefully and the hoses to it for any signs of external leakage.

An internal leak of coolant can be via a failed cylinder head gasket or a crack in the cylinder head or both. Coolant can then leak under pressure into the cylinders or into the engine's lubrication system and will then mix with the oil. Check the condition and colour of your oil for evidence of coolant contamination. Check your exhaust for a constant flow of white exhaust emissions which is evidence of coolant entering the combustion chamber.

If you have run the vehicle with the engine overheating this will generally cause failure of the cylinder head gasket and possibly a crack in the cylinder head.

There can be numerous reasons for coolant loss.

External Coolant loss
1. Coolant escaping from the radiator cap. Replace the cap.

2. There is an external leak in the system which could be from a hole or crack in the radiator(especially the plastic sections) or a hole in a radiator hose or there is a loose hose connection.

3. The water pump gasket has deteriorated and coolant is escaping from around the water pump fitting.

4. There is a crack in the coolant overflow reservoir and coolant is leaking out.

Internal Coolant Loss
1. Coolant is escaping into the engine's cylinders in which event you will see white smoke coming out of the exhaust and you will also have combustion gasses leaking into the coolant passages which will cause bubbles and overheating of the coolant. This problem will be due to a failed cylinder head gasket, a crack in the cylinder head or both.

2. Coolant is escaping into the engine lubrication system and in that event you will see evidence of coolant in the engine oil. Pull the oil dipstick and check the condition of the oil. If it looks a bit like chocolate milk and not just normal oil colouring you have coolant in there. Also check the radiator cap when the engine is cold. There may also be evidence of oil in the radiator coolant that is stuck to the radiator cap and around the neck of the radiator.
This problem will also be due to a failed cylinder head gasket or a crack in the cylinder head or both.

Whatever the cause in your case don't hesitate to have it checked out by a competent professional If you lose coolant and you allow the level to run significantly lower than normal the engine will overheat and you will cause very expensive engine damage.

Pretty much same answer as I gave for 87 Reliant see below. Shops have equipment to do cylinder leakage test. Do basic stuff below before doing last test ...cylinder leakage...which costs $$$.

Main point you are losing antifreeze/water. Check to see if you have water on the dipstick mixed in with oil. If so...probably a head gasket. If oil looks normal..... Before you do any test, tighten every hose connection. Check for loose connection into antifreeze reservoir. Sometimes there are small cracks on top which lets out pressure from cooling system. Look for leaks on water pump and radiator. Look at spot where you park the car...any liquid on floor? Not sure?..Put piece of cardboard under car to see if liquid is dripping from car. No leaks?...Start with a pressure test of the system. The pressure test is simply equipment that replace the radiator cap, a hand pump connected to it then air is pumped into cooling system. You watch the gauge to see if pressure is dropping. If pressure does not drop ..problem is most like a worn radiator pressure cap or stuck thermostat.. Do not buy the kind with button on cap to push down and release pressure. get original type and correct pressure. If overheating still, thermostat could be stuck, replace. Overheating of engine causes vapor lock, bubbles in fuel, which causes car to stall and kill. When cools off, fuel cools down and fuel will flow through fuel system again. Large amount of oil disappearing with overheating also sounds like head gasket or cracked head....Tighten up all bolts to gaskets, look for oil leaks on garage floor or driveway. If engine has developed a miss when running, pull plugs out and look for wet fouled plug. If antifreeze leaking into cylinder...plug tip will be whitish, if oil leaking into cylinder, grimey oil fouled. Do compression test on cylinder that is fouled and that will pinpoint if bad head gasket or cracked head. When doing compression test, radiator cap is removed and you listen to hear bubbles in radiator from the cylinder leaking into cooling system. 87 Reliant not worth pulling engine apart if head gasket or cracked head. Best oil treatment to reduce oil consumption is "Engine Restore" Used on my 58 Impala 348 tri power for 28 years. If I did not add it after oil changes it used oil and smoked....I swear by it for ANY engine after 100,000 miles.

you have most likely blown a head gasket. need to have it inspected and dye tested to make sure. if it is the head gaskets make sure the head is inspected for a crack. hope this helps

Check all around the intake manifold gasket for evidence of antifreeze. Also check your oil for evidence of coolant. The Dexcool antifreeze used in GM engines can cause the intake gaskets to deteriorate causing coolant leakage. Very difficult to detect.

Sounds like you have no coolant in the radiator and your engine is overheating. And without coolant the temperature gage would not show that the engine was getting hot. Let the engine cool off, then remove the radiator cap and see if there is any antifreeze in the radiator. If it's empty, then you must have a leak. And the missing engine makes me think the antifreeze may have leaked past the intake gasket into the engine. Check the oil and look for evidence of water or antifreeze contamination.

Replace the theremostat first.Fill system and top off after the themostat opens..check for a leaking reservoir, Then fill the radiator,if it is low,with engine running. then, pressure check the cap with a tester,or just replace with a new one,it may be cheaper just to buy a new cap. Look at the fill neck of the radiator down where the cap seals,look for any uneven sealing surfaces cracks ect.
You can also load the car in gear Automatic transmission application a bit, about 1/3 throttle, with the cap off and look for bubbles comming out of the radiator.fill the radiator while making the test with a helper,friend. If you see a bubble now and then it may be a bad head gasket or a cracked head. You can remove the plugs, and check them for anti freeze,they will have a smell of sweet antifreez,if the engine got over heated enough!

check under the car after it has sat for leaks if none found check you engine oil see if its milkey in colour also do you smell antfreeze as you drive? you might want to get it pressue tested for leaks

When the engine is cool, check the fluid level in the radiator. If low add 50/50 solution of water and antifreeze. Do not continue to drive your car until this is fixed or you could cause a lot of damage to the engine.