Feel the lower radiator hose when the engine is hot. If that hose is cold, coolant isn't flowing. You might consider removing the thermostat to see if that makes a difference. If there is still no evidence of circulation, suspect a problem with the water pump. One possibility is the impeller can be worn away or it can be cracked and spinning on the shaft. Also check the tension on the timing belt since it runs the water pump. A loose belt will slip over the pump's pulley.
If you repeatedly find air coming from the bleeder screw, suspect the head gaskets are leaking. When those gaskets are replaced, the heads must be checked for cracks and for warpage. Warpage must be checked at an engine machine shop or by a mechanic who has a straightedge. The maximum allowable warpage with aluminum heads is .002" in any direction.
To check for leakage, first look for the presence of bubbles in the reservoir when the engine is running. If you see any before the engine has warmed up, suspect the head gasket. The clue is the engine isn't hot yet and there's no steam with those bubbles. Most mechanics can perform a test for you that involves a glass cylinder with two chambers partially filled with a special dark blue liquid. Air from the reservoir is drawn through that cylinder. If combustion gases are present, the liquid will turn bright yellow. Those gases will create that air pocket again that prevents the thermostat from opening.
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there is a bypass passage. That gives the coolant someplace to go while the thermostat is closed.
When the overheating occurs, feel the lower radiator hose. If it is real hot, the heat is not leaving the radiator. That can be due to air flow blockage, a fan that's not turning on, a fan that's wired wrong and blowing the wrong way, a missing shroud that lets the hot air go back around and through the radiator rather than drawing cool air in. Related to that, look for a missing rubber strip under the front edge of the hood. That is a seal that forces air through the radiator.
If the lower hose is not hot, the coolant may be not circulating properly. Check the water pump impeller and be sure the timing belt is tight and not slipping. You may have to flush the cooling system and try removing the thermostat to verify it is not sticking closed. If the coolant is not circulating, the air from the heater will not be as hot as normal.
A clue to a fan-related problem is the overheating will not occur while driving at highway speeds.--------
When the overheating occurs, feel the lower radiator hose. If it is real hot, the heat is not leaving the radiator. That can be due to air flow blockage, a fan that's not turning on, a fan that's wired wrong and blowing the wrong way, a missing shroud that lets the hot air go back around and through the radiator rather than drawing cool air in. Related to that, look for a missing rubber strip under the front edge of the hood. That is a seal that forces air through the radiator.
If the lower hose is not hot, the coolant may be not circulating properly. Check the water pump impeller and be sure the timing belt is tight and not slipping. You may have to flush the cooling system and try removing the thermostat to verify it is not sticking closed. If the coolant is not circulating, the air from the heater will not be as hot as normal.
A clue to a fan-related problem is the overheating will not occur while driving at highway speeds.--------
Double-check that the fan is turning on and blowing the right way. Also check the the radiator. check if the cooling fins are rotted away. That will reduce its ability to give off the heat to the air.
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Also a bad or clogged heater core will cause over heating:---
Start by checking the coolant. As we already said coolant is what warms the heater core. So if the coolant is low, there may not be a sufficient amount of warmth to transfer heat to the air in the passenger compartment.Once the coolant is full, feel the heater hoses that go through the firewall. With the engine at normal operating temperature, BOTH of the heater hoses should be hot to the touch. If only one is hot, this indicates there is is a blockage in the heater core or there is air trapped in the heater core.One hose hot, one cold, points to a plugged heater core.If this is the case,try flushing the heater core.There are no valves in the heater core; it is free flow at all times.Also check the head gasket. just because there is no water coming out of the exhaust dos,nt mean the head gasket is ok.How much pressure is in the system?.Are the water hoses very hard when the engine is hot?.......air in the system will stop the heater from working.------------Also try this procedure to clean the block in the heater core.If there is any:--- Put in a cleaner, Prestone, and drive car few hours.Then check if nNo success. Then get cleaner made by Gunk.You can get it from any local auto part store. Drain heater core, leave both heater hoses attached to the heater. Remove other ends of both hoses from the tubes on the thermostat housing and raise these ends above the heater core. Pour Gunk cleaner in one hose and add water until cleaner started to come out other hose. Leave this sit 4 hours and back flush heater core in both directions. Next back-flush block and radiator in both directions. Water should come out of tube on the thermostat housing that has a hose attached to the outlet of the heater core.If not coming then Flush water directly into this tube about 15 to 20 times and you will hear a thud of water coming out.. Back flush block and radiator again and this time water should out the tube previously blocked. But if problem is same and no difference.Then is faulty heater core.Its causing the problem and it needs to be replaced.Also check the radiator,if its blocked then get the radiator back flushed too..------------
