First confirm that the engine is actually overheating - steam, no heat to the interior etc. Sometimes a temperature sensor will fail and give a misleading 'overheat' reading on gauge. For that matter sometimes the gauge can fail and give the same misleading reading. Check that the cooling fan comes on when the engine temperature starts to rise above 'normal'. If not check (in order) fan fuse, fan relay, fan sensor, fan. Real overheating is caused by a lack of coolant AND/OR lack of coolant flow. Modern cooling systems are 'closed'. In other words there should be little or no loss of coolant. If the level is significantly low you have a leak....either external or internal. With the coolant topped up to the correct level and the radiator cap on, run the engine and look for visible leaks...hoses, valves, radiator etc. Internal leaks, eg. head gasket will usually be accompanied by white exhaust smoke (steam), hard starting, rough running and bubbles in the expansion tank. Sometimes, though NOT always, you will fiand the engine oil has a milky look and/or engine oil will be mixed in with the coolant.
Lack of coolant flow can be caused by anything in the system that causes an obstruction......in order of common occurance......thermostat stuck closed, radiator blockage, heater core blockage, water pump failure. One last point. because of design considerations many vehicles have the radiator mounted at a different level than the uppermost coolant passages in the engine thus trapping air in the coolant passages. In short the system must be bled of air in order to properly fill with coolant. Such systems will incorporate bleed screws usually in either the top/side of the radiator, the top hose, the thermostat housing and sometimes in all of these points. To bleed out the air do NOT run the engine as you fill the radiator. Instead, open all bleed screws. As you fill the radiator coolant will run out of one or more bleed points. Close this bleed point(s). Continue to fill until coolant escapes from the next bleed point then close that one. Proceed until coolant has escaped from ALL bleed points then continue to fill to the correct level. The expansion tank will have minimum and maximum level markings. Fill only to the minimum level mark with the engine cold.
Nov 10, 2011 |
Cars & Trucks