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Did you put the thermostat in backwards? If you did, it wont open when hot. Thermostats look pretty much the same - with some minor variations. It's important to note the the side with the spring should be facing the (hot) engine and the other side facing the (cool) radiator as shown in the image below. If the spring side doesn't get hot - it won't open to let hot water from the engine flow to the radiator for proper cooling.
Holts Wondarweld Flush engine. EXTERNAL CRACKS 1 Run engine until hot,drain off enough coolant to remove thermostat. 2 Remove thermostat. 3 Mix contents of can thoughoughly! and pour contents into thermostat opening. 4 Replace thermostat cover.& fill radiator with hot water,replace radiator cap. 5 Run engine at a fast idle for 15 mins,dont allow to boil.Top up if necessary. 6 Leave Wondarweld in system for 2-3 days then drain off,flush system replace thermostat and refill. HAIRLINE CRACKS 1 MIX thoroughly and pour into cold engine (radiator if it has a cap) 2 Run engine until hot. 3 Allow engine to cool then run again until hot. 4 Follow procedure 6 above. INTERNAL CRACKS 1 Follow external crack instructions except after pouring Wondarweld into engine remove spark plug from affected cylinder and run engine for 20 mins. If more than one cylinder affected run for 5 minute intervals. 2 Allow engine to cool then drain cooling system and refill with clean water. 3 Drain sump & flush,refill with fresh oil. Note:- 1/2 pint of Wondarweld for every 6 pints of water,DO NOT add antifreeze for at least 3 days after process.
An explanation of the cooling system.
You start your car, it is cold at first. The thermostat is shut. Car warms up and raises the temperature of the coolant in the engine block. Thermostat opens up. Water from the radiator (cool) is exchanged for water from the engine block (hot) The cool water causes the thermostat to shut. This traps the hot water in the radiator so that it can be cooled down. The cool water that was exchanged for the hot water, now becomes the hot water because it is in the engine block. It heats up until it's temperature causes the thermostat to open again, exchanging cool water for hot water. If you have no thermostat, then you will never hold water inside your radiator. You have to hold it there to allow it to cool off, then it becomes useful to cool your engine. So, yes put a thermostat back in it, and please vote as to whether this was helpful or not.
CHECK ENGINE COOLANT LEVEL.IF OKAY.CHECK BOTH RADIATOR HOSES BOTH SHOULD BE HOT. IF THERMOSTAT TOP HOSE NOT HOT YOU HAVE A FAULTY THERMOSTAT AND RADIATOR PRESSURE CAP IF RADIATOR HOSES HOT AND THERMOSTAT OKAY.IF ENGINE OVERHEATING WHILE CAR SITTING IN A LONG TRAFFIC LINE COOLANT FANS NOT RUNNING.THE COOLANT TEMPERATURE SENSOR IS BAD OR YOU HAVE FAULTY FUSE OR RELAY.THE FAN MOTOR DEFECTIVE, WIRES AT MOTOR HAS A SHORT OR PCM FAILURE DEAD DRIVE CIRCUIT TO FAN.
I've had problems with Datsun (later Nissan) engines hanging the closed thermostat until it boiled, then the thermostat released and operated normally. If you increase the small vent hole in the thermostat valve plate a small amount, the problem disappears. It seems that steam from engine operation gathers behind the thermostat sensing bulb and delays opening. The increase in vent hole size lets the steam through and allows the hot water to open the valve. Hope this helps!
There are sometimes different thermostats available, but not that much different (180 or 190 deg) This is more likely a radiator problem than the thermostat. Especially if it is getting worse. You could have it flushed, but in my experience, that rarely, if ever, helps. Check the guage, though. If it isn't steaming or boiling over, maybe it isn't really running so hot after all. If it is boiling over, have a mechanic make sure there isn't a bad head gasket. Also make sure the fins aren't full of dead butterflies. Look inside the radiator and see if there are crusty bits around the circulation holes (if you can see inside). If so, there are probably some plugged tubes. Simplest solution is a new or rebuilt one.
There are two primary symptoms of a faulty thermostat (assuming you mean the coolant thermostat in the engine not the A/C Heater Thermostat). The thermostat can fail in basically two ways:
1) Stuck open - Engine runs colder, heater takes forever to heat up.
2) Stuck closed - Engine runs very hot, then will suddenly drop in temperature, then run very hot again. Or engine overheats constantly. When the engine is hot, feel the upper radiator hose. If it is not hot, you can bet the thermostat is stuck closed. If it is hot, the thermostat may not be stuck close, but not opening at the proper temperature.
Stuck open is annoying, but not dangerous. Stuck close is dangerous and can cause overheating-related damage to your engine.
If you are handy, you can replace it yourself. Otherwise it is an easy job for a mechanic.