Tip & How-To about Nissan Sentra

Cooling system fault finding..

Your cooling system works by pumping coolant(water and antifreeze) through the engine where it absorbs heat, through the radiator (where it loses that heat) and back into the engine.
Main components of the cooling system.

  1. Coolant. It can be plain water. Adding Glycol based antifreeze is definately recommended as it not only raises the boiling point of the water and lowers the freezing point but also contains very important corrosion inhibitors. Adding too much antifreeze lowers the coolant's effectiveness so don't go more than a 50/50 mix unless stated otherwise by the manufacturer.
  2. Radiator. This acts as a heat exchanger between the coolant and outside air. It is actually a whole bunch of small pipes that have thin pieces of metal conncted to them to increase the effective surface area.
  3. Waterpump. Well that's what it is. A pump to circulate the coolant around the engine.
  4. Thermostat. This unit regulates the flow of coolant around the system. You need to get the engine to operating temperature as quick as possible, then you need to keep it there as this aids in lowering fuel consumption and emissions and this is the job of the thermostat. It is installed in the system in such a way that it can block the flow of coolant between the radiator and the engine.
  5. Cooling fan. Cooling fans help airflow through the radiator at low speed or when the vehicle is stationary This may be electrical - powered by a thermostatic switch and a relay, directly driven by a belt off the crankshaft or have a viscous coupling. The viscous coupling is also directly driven but contains a fluid which couples the fan to the pulley only when needed.

Reasons for overheating :
  1. Insufficient coolant
  2. Insufficient circulation of the coolant
  3. Insufficient airflow
  4. Excess heat generation
Do's and dont's
When topping up it is better to use antifreeze only, as you will not be diluting the mixture. When refilling use the correct mix of water and antifreeze or fill with pre-mixed coolant.
Replace coolant according to service schedule.
Never open a hot cooling system.

The first and most obvious thing to check is coolant level.
No reservoir : coolant is added directly to the radiator. Fill to the top. You may find a little fluid missing everytime you open the radiator. This is due to water expanding when heated which escapes through the overflow pipe at the radiator cap.
Non pressurised reservoir. This coolant tank is not pressurised and can be opened at any time. Usually has a cap that just clips on. Fill to the max mark when the engine is hot or at least to the min mark when the engine is cold. If this reservoir is filled it does not nescesarily mean the system is full since it depends on expansion and contraction of air and coolant within the system to get coolant back into the radiator. The radiator should be filled as well.
Pressurised reservoir : Most modern-day vehicles are fitted with this type of system. It is pressurised and should only be opened when the engine has cooled down. Do not overfill, always keep coolant levels between the minimum and maximum marks.
If coolant levels keep dropping with no visible signs of leaking have the engine tested for a blown head gasket.

The internal passageways that allow for coolant flow through the engine are metal and constantly exposed to coolant which causes rust which is transported throughout the system. The radiators' internal pipes have very small diameters and easily become blocked by this rust. This decreases the effective area of the radiator and the amount of water that can pass through it. Regular replacement of the coolant will prevent this build-up.
Waterpump impellers may break and decrease the efficiency of the pump although this is rare.
A slipping fan belt will also cause decreased coolant flow but is usually quite noticeable as a high pitched shrieking noise especially on startup and when revving the engine.
The thermostat is in effect a valve which opens and closes with temperature. They go faulty mostly because of rust and scale buildup and can get stuck in a closed or half-open position, thereby blocking waterflow to the radiator.

The radiator fins are spaced closely together so can easily catch debris such as bugs and grass which will impede airflow through the radiator. The fan assists with airflow so if not working will see the engine overheat when stationary or in stop-start traffic. If the engine cools down while driving above 35mph this is the obvious starting point.

Incorrect fuel/air mixture and incorrect ignition timing may cause the engine to generate heat in exess of the coolings systems' capabilities.

How to test and repair :
  • Check the fluid level. Also check for leaks. They will usually show up as brown or green streaks of dried coolant.
  • Check your fan belt/serpentine belt - is it still intact? Check the operation of the fan. On a direct coupling you should not be able to move the fan. A viscous coupling should not spin freely by more than a quarter turn. See below for the electrical fan.
  • Flush the system. Remove the upper and lower radiator hoses and if possible, remove the radiator. Stick a hosepipe into the outlet side of the radiator and let the water run until it comes out clear on the other side. Then repeat the process with the hosepipe in the inlet side. Continue with this until the water runs absolutely clear.Now remove the thermostat from the engine and do the same to the engine. Clear the radiator fins of bugs and debris.
  • Check the thermostat. It should be in the closed position. Drop it in a cup of boiling water and you will notice it opening. Since it is already out you may as well replace it since it is relatively inexpensive
Electric fan
On an electric radiator fan, let the vehicle heat up to just before overheating. If by this time the fan has not come on switch off the engine.Locate the cooling fan fuse and check that it is intact. Using a multimeter, measure between each of its' contacts and earth. See diagram. Measure between 1 and earth(bare metal point on vehicle body or on battery negative), and then between 2 and earth. Both readings should be 12V or more with the ignition on. If 1 reading is found only, the fuse is faulty. If no reading is found the wiring between the ignition and fuse is faulty.

Now check the operation of the fan itself. Disconnect the fan. Connect a lead wirefrom the battery negative to one of the fan connector pins and from thebattery positive to the other pin. See diagram: 1 - Fan motor
2 and 3 - Connect directly to battery

This is a DC motor so polarity doesnot matter. If the fan operates reconnect it to the harness. If not, replace the fan.
Locate thefan relay. You will need a multimeter to test. Remove the relay. Switch on the ignition. Measure voltage at allthe connections in the relay socket to earth. Two of them have to be at 12V or more. If not, the problem is between thefuse and the relay. Re-insert the relay.
Disconnect the thermoswitch. Usually located on the radiator. See photo : 1 - Thermostatic switch. 2- Leaked and dried coolant on a neglected radiator.

Shortcircuit the 2 wires from the harness that were a moment ago connected to the thermo switch.(With a single wire thermo switch the wire needs to be connected to earth or battery negative). Switch on the ignition. The fan should start running again. If not switch off the ignition and remove the relay again. You already know the connecting point with 12V. Measure the other 3 wires for resistance to earth. One of themshould read more than 5 ohm. Connect a lead between the "12V" connector and the "5ohm" connector. Switch ignition on. If the fan runs the relay is faulty. If not the wiring between the relay and fan is faulty.
Simplified wiring diagram:
A- Battery
B - Ignition Switch
C - Fuses
D - Relay
E - Thermostatic switch
F - Radiator fan

Leak detection
The most obvious tell-tale would be a puddle of coolant under the vehicle after a trip. Don't confuse this with condensation dripping from the aircon.
Inspect pipes for cracks and leaking at joints.
Inspect the radiator for dried coolant. (See photo above)
Inspect the engine block for coolant streaks. Leaks can start on the block itself through the welch plugs which are also constantly exposed to coolant.
Another leak point is the water pump shaft seal which cannot be repaired. Replace the whole pump.
Remember that the cabin heater is plumbed into the cooling system and can also leak. Wet carpeting in the footwell is indicative of this although such wet carpeting may also be the result of condensation from the A/C.

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Why is the temperature light keep coming on and the gauge keep going up and down?

Temperature Warning Light is on Inspection Service How this system works: With the exception of older, air-cooled vehicles, cars rely on a liquid called coolant (or antifreeze) to keep the engine at an optimal temperature. Coolant is a mixture of water and ethylene glycol, and it circulates around the engine block and absorbs excess heat, which keeps the engine from overheating. The coolant converts that heat to air in the radiator, and then the air is emitted, at which time the coolant is ready to absorb more heat. Without coolant, the engine would quickly ruin itself with its own heat production. Common reasons for this to happen:
  • Coolant is low or weak: The most common culprit when an engine overheats is low or weak coolant. If your car doesn't have enough coolant, then it can't absorb enough heat. While your car will naturally lose small amounts of coolant over the years, a leak is the most likely cause of low coolant levels.The proper ratio of coolant to water can also get distorted, resulting in a problem. Too little or even too much antifreeze can dramatically lower the boiling point of the coolant. A proper ratio of antifreeze to water is 50/50 to 60/40, depending on the vehicle.
  • Broken radiator fan shroud: The radiator fan shroud directs the airflow across the radiator so the air can absorb the coolant's heat. When the fan shroud breaks or becomes dislodged, air fails to enter the radiator, and the coolant will no longer have a place to direct the transfer of heat.
  • Broken or missing air dam: Along with the shroud, some vehicles have an air dam (or deflector) underneath the vehicle. If this is broken or missing then the air can pass underneath the vehicle but not also through the radiator, which will cause overheating. These air dams are essential in newer vehicles, as they force the air through the fan shroud.
  • Faulty coolant temperature sensor: The temperature sensor takes constant readings of the coolant temperature and sends that information to the engine control unit. Based on the temperature of the coolant, the engine control unit adjusts the ignition timing, the fuel injector pulse, and the operation of the electric cooling fan.
  • Bad water pump: The water pump is responsible for keeping the coolant cycling throughout the engine. After the coolant transfers its heat energy to the air, the water pump recirculates it around the engine so that it can absorb more heat. The most common water pump problems are a leaking pump, bad bearings, or an impeller that has rotted away due to a low coolant ratio.
  • Stuck thermostat: The thermostat acts as a dam for the coolant. When the engine first turns on, and it is still cold, the thermostat keeps the coolant from circulating, which allows the engine to warm up as quickly as possible. Once the engine has reached its operating temperature, the thermostat opens and allows the coolant to circulate. A stuck thermometer may stay permanently sealed and therefore keep the coolant from reaching the engine block.
  • The thermostat may also stick open. This will not usually result in overheating, but it will waste gas.
  • Broken engine cooling fan: The engine has a cooling fan that is deployed when the coolant needs some extra help. When the coolant temperature sensor notices that the coolant temperature is getting too high, the engine control unit (on newer vehicles) will initiate the cooling fan to reduce the temperature.
  • Broken thermostatic fan clutch: Older vehicles use a thermostatic fan clutch to engage the engine cooling fan, which is mounted to the fan blades. The fan clutch uses a bi-metallic spring that tightens when the temperature increases. This acts as a "high speed" option for the fan, and when engaged, it draws more air across the radiator.
  • Blown head gasket: The head gaskets sit between the engine block and the cylinder heads, and keep coolant from entering the engine's oil and combustion chamber. When a gasket blows and coolant seeps in, the issue is not only that the engine will overheat, but also that damage may be done to the catalytic converter and oxygen sensors due to contamination from the coolant. What to expect: A top-rated mobile mechanic will come to your home or office to determine the cause of the temperature warning light turning on and the source of the overheating, and will then provide a detailed inspection report that includes the scope and cost of the necessary repairs. How important is this service? An overheating engine is extremely dangerous. It is not safe to drive a vehicle with an overheating engine, or you may ruin the engine completely and put yourself at risk. As soon as you notice the light come on, pull over. If there is no place to safely pull over, turn off your radio and other electrical units, and turn your heat on high (this will funnel some of the hot engine air into the cabin). As soon as you can safely pull over, do so, and then book one of our mechanics to perform an inspection.

Sep 30, 2016 | 2008 Pontiac G6

2 Answers

my honda is over heatin why


Nov 08, 2009 | 1992 Honda Accord

2 Answers

94 cutlass supreme is overheating,all fluids are fine. I did have someone tell me i need 3 motor mounts replaced. could this be an issue of the car overheating?

Over heating can be caused by a faulty thermostat and or weak water pump and dirty radiator and old antifreeze or the radiator fan now working when at idle but should not over heat when driving. Service the cooling system if it has never been serviced before. Replace the thermostat, buy some radiator flush to clean out the radiator and heater core. Just follow the instruction and replace the antifreeze and make sure it's a 50/50 mix water/antifreeze. If you not sure how to mix it 50/50 you can buy them 50/50 premixed. Water boils at 100'C so you need antifreeze mixed with it but antifreeze does not have the same heat transfer as water so you cannot run 100% antifreeze cause it does not transfer heat fast enough and you over heat. Good luck and hope this helps. Keep me posted, Be glad to help, once you service the cooling system and your still over heating, then we look at the water pump.

Jun 16, 2009 | 1994 Oldsmobile Cutlass Supreme

1 Answer

my 2001 lincoin ls gauge goes up to the h and then come back down

you probably have air trapped in the system. Have dealer or radiator shop bleed system.

I assume your cooling fan is working.

what engine v6 or v8?

SECTION 303-03: Engine Cooling 2001 Lincoln LS Workshop Manual
DESCRIPTION AND OPERATION Engine Cooling The cooling system components include the:
  • block heater (optional)
  • cylinder head temperature sensor
  • fan blade, fan motor and fan shroud assembly
  • radiator
  • pressure relief cap
  • degas bottle
  • radiator draincock
  • water pump
  • oil cooler (optional)
  • water thermostat
The water thermostat:
  • controls the engine coolant temperature.
  • allows quicker engine warm-up.
The degas bottle:
  • provides a location for system fill.
  • contains coolant expansion and system pressurization.
  • provides air separation during operation.
  • replenishes the engine coolant to the system.
The fan blade draws air through the radiator to help cool the engine coolant.
The fan motor:
  • operates only when the engine is running.
  • will not operate when the engine is off.
The engine coolant flows:
  • from the lower radiator hose to the water pump.
  • from the water pump to the engine block and the cylinder heads.
A closed water thermostat returns the engine coolant to the water pump. An open water thermostat allows the engine coolant to flow to the radiator.
Unsatisfactory coolant materials:
  • Alcohol-type antifreeze does not provide adequate water pump lubrication.
    • has lower boiling point
    • reduced antifreeze protection
  • Alkaline brine solutions will cause serious engine cooling system damage.
The cylinder head temperature sensor provides a signal to the temperature gauge.
  • will invoke failsafe cooling.
The optional block heater:
  • electrical heating element is installed in the block cooling jacket.
    • uses a standard 110V (220V in Europe) electrical supply
  • keeps the engine coolant warm during cold weather.
The auxiliary water pump (3.9L only):
  • provides heater coolant flow boost.
  • has a secondary function of providing engine-off cooling.

May 01, 2009 | 2001 Lincoln LS

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