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Thread: Overheating Causes and Cures

  1. #1
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    Overheating Causes and Cures

    Internal combustion engines run on heat. Chemical energy in the fuel is transformed into thermal energy when the fuel burns, which produces mechanical energy to push the pistons, spin the crankshaft and drive the vehicle down the road.

    As efficient as today’s engines are, they still waste a lot of the heat energy they produce. The average gasoline engine is only about 22 to 28 percent efficient. That means over two-thirds of the heat produced by each gallon of fuel either goes out the tailpipe or is soaked up by the engine itself. Diesels squeeze a little more bang out of each buck’s worth of fuel with efficiency ratings of 32 to 38 percent, but even that leaves a lot of waste heat that must be managed and carried away by the cooling system.

    Ironically, the hotter an engine runs the more efficient it becomes. But there’s a limit because aluminum pistons and heads can only get so hot before they start to soften and melt. The same goes for cast iron. Engineers have been tinkering with exotic ceramic materials and metallic-ceramic alloys in an attempt to build high-temperature, super efficient engines. They’ve realized some significant gains but ceramics are still too expensive for everyday applications.

    How Hot Is Too Hot?
    Most engines today are designed to operate within a "normal" temperature range of about 195 to 220 degrees F. A relatively constant operating temperature is absolutely essential for proper emissions control, good fuel economy and performance.

    A 50/50 mixture of water and ethylene glycol antifreeze in the cooling system will boil at 225 degrees if the cap is open. But as long as the system is sealed and holds pressure, a radiator cap rated at 15 psi will increase the boiling temperature of a 50/50 coolant blend up to 265 degrees. If the concentration of antifreeze to water is upped to 70/30 (the maximum recommended), the boiling temperature under 15 psi of pressure goes up to 276 degrees.

    So does this mean a cooling system with a maximum concentration of antifreeze in the coolant (70 percent) can run as hot as 276 degrees without boiling over? Theoretically yes - but realistically no. The clearances in most of today’s engines are much, much closer than those in engines built in the 1970s and early 1980s. Piston-to-cylinder clearances are much tighter to reduce blowby for lower emissions. Valve stem-to-guide clearances also are closer to reduce oil consumption and emissions, too. Plus, many engines today have aluminum heads with overhead cams. Such engines don’t handle higher than normal temperatures well, and are very vulnerable to heat damage if the engine gets too hot.

    Anytime temperatures climb beyond the normal range, the engine is running in the danger zone.

    Consequences of Overheating
    If the engine overheats, the first thing that will happen is a gasoline engine will start to detonate. The engine will ping and start to lose power under load as the combination of heat and pressure exceed the octane rating of the fuel. If the detonation problem persists, the hammer-like blows may damage the rings, pistons or rod bearings.

    Overheating can also cause preignition. Hot spots develop inside the combustion chamber that become a source of ignition for the fuel. The erratic combustion can cause detonation as well as engine run-on in older vehicles with carburetors. Hot spots can also be very damaging and burn holes right through the top of pistons.

    Another consequence of overheating may be a blown head gasket. Heat makes aluminum swell almost three times faster than cast iron. The resulting stress can distort the head and make it swell in areas that are hottest, like those between exhaust valves in adjoining cylinders, and areas that have restricted coolant flow like the narrow area that separates the cylinders. The typical aluminum head swells most in the middle, which can crush the head gasket if the head gets hot enough. This will cause a loss of torque in the gasket allowing coolant and combustion leaks to occur when the head cools.

    Overheating is also a common cause of OHC seizure and breakage.

    Wait, there’s more. If the coolant gets hot enough to boil, it may cause old hoses or an age-weakened radiator to burst under the increased pressure. Pistons may swell up and scuff or seize in their bores, causing serious engine damage. Exhaust valve stems may stick or scuff in their guides. This, in turn, may cause valves to hang open which can damage pistons, valves and other valvetrain components. And if coolant gets into the crankcase, you can kiss the bearings and bottom end of the engine goodbye.

    A HOT warning lamp should never be ignored. Though a few high tech cars like Cadillacs with the Northstar engine can disable cylinders to "air-cool" the engine and keep it running at reduced power in the event of coolant loss, most engines will suffer serious damage if they overheat. Turn the engine off, let it cool down and try to find and fix the cause before risking further travel.

    Causes of Overheating

    Overheating can be caused by anything that decreases the cooling system’s ability to absorb, transport and dissipate heat, such as a low coolant level, loss of coolant (through internal or external leaks), poor heat conductivity inside the engine because of accumulated deposits in the water jackets, a defective thermostat that doesn’t open, poor airflow through the radiator, a slipping fan clutch, an inoperative electric cooling fan, a collapsed lower radiator hose, an eroded or loose water pump impeller or even a defective radiator cap.

    One of nature’s basic laws says that heat always flows from an area of higher temperature to an area of lesser temperature, never the other way around. The only way to cool hot metal, therefore, is to keep it in constant contact with a cooler liquid. And the only way to do that is to keep the coolant in constant circulation. As soon as the circulation stops, either because of a problem with the water pump, thermostat or loss of coolant, temperatures begin to rise and the engine starts to overheat.

    The coolant also has to get rid of the heat it soaks up while passing through the block and head(s). So the radiator must be capable of doing its job, which requires the help of an efficient cooling fan at slow speeds.

    Finally, the thermostat must be doing its job to keep the engine’s average temperature within the normal range. If the thermostat fails to open, it will effectively block the flow of coolant and the engine will overheat.

    What To Check
    • Thermostat - Severe overheating can often damage a good thermostat. If the engine has overheated because of another problem, the thermostat should be tested or replaced before the engine is returned to service.

    One way to check the thermostat is to start the engine and feel the upper radiator hose (or use an infrared noncontact thermometer to read its temperature). The hose should not feel uncomfortably hot until the engine has warmed-up and the thermostat opens. If the hose does not get hot, it means the thermostat is not opening.

    Another way to test the thermostat is to remove it and dip it into a pan of boiling water (it should open). The exact opening temperature can be checked by using a thermometer.

    If the thermostat needs to be replaced, install one with the same temperature rating as the original. Most cars and light trucks since 1971 require thermostats with 192- or 195-degree ratings. Using a cooler thermostat (160 or 180) in an attempt to "cure" a tendency to overheat can increase fuel and oil consumption, ring wear and emissions. On newer vehicles with computerized engine controls, the wrong thermostat can prevent the computer system from going into closed loop resulting in major performance and emission problems if the engine fails to reach its normal operating temperature.

    • Cooling system leaks - Loss of coolant because of a leak is probably the most common cause of overheating. Possible leak points include hoses, the radiator, heater core, water pump, thermostat housing, head gasket, freeze plugs, automatic transmission oil cooler, cylinder head(s) and block.

    Make a careful visual inspection of the entire cooling system, and then pressure test the cooling system and radiator cap. A pressure test will reveal internal leaks such as seepage past the head gasket as well as cracks in the head or block. A leak-free system should hold pressure for at least a minute or more.

    It’s important to pressure test the radiator cap, too, because a weak cap (or one with too low a pressure rating for the application) will lower the coolant’s boiling point and can allow coolant to escape from the radiator.

    • Fan - With mechanical fans, most overheating problems are caused by a faulty fan clutch - though a missing fan shroud can reduce the fan’s cooling effectiveness by as much as 50 percent (depending on the fan’s distance from the radiator), which may be enough to cause the engine to overheat in hot weather or when working hard.

    Defective fan clutches are a common and often overlooked cause of overheating. The shear characteristics of the clutch fluid gradually deteriorates over time, with an average loss in drive efficiency of about 200 rpm per year. Eventually slippage reaches the point where effective cooling is no longer possible and overheating results. (On average, the life of a fan clutch is about the same as a water pump. If one needs to be replaced, the other usually does too.)

    If the fan clutch shows signs of fluid leakage (oily streaks radiating outward from the hub of the clutch), spins freely with little or no resistance when the engine is off or wobbles when the fan is pushed in or out, it needs to be replaced.

    With an electric cooling fan, check to see that the fan cycles on when the engine gets hot and when the air conditioner is on. If the fan fails to come on, check the fan motor wiring connections, relay and temperature sensor. Try jumping the fan directly to the battery. If it runs, the problem is in the wiring, relay or sensor. If it fails to run, the fan motor is bad and needs replaced.

    • Water pump - Any wobble in the pump shaft or seepage would call for replacement. In some instances, a pump can cause an engine to overheat if the impeller vanes are badly eroded due to corrosion or if the impeller has come loose from the shaft. The wrong pump may also cause an engine to overheat. Some engines with serpentine drive belts require a special water pump that turns in the opposite direction of those used on the same engine with ordinary V-belts.

    • Belts & hoses - Check belt tension and condition. A loose belt that slips may prevent the water pump from circulating coolant fast enough and/or the fan from turning fast for proper cooling.

    The condition of the hoses should also be checked and replaced if they are over 5 years old.

    Sometimes a lower radiator hose will collapse under vacuum at high speed and restrict the flow of coolant from the radiator into the engine. This can happen if the reinforcing spring inside the hose is missing or damaged.

    • Radiator - The most common problems radiators fall prey to are clogging (both internal and external) and leaks. Dirt, bugs and debris can block air flow through the core and reduce the radiator’s ability to dissipate heat. Internal corrosion and an accumulation of deposits can likewise inhibit coolant circulation and reduce cooling. A good way to find clogs is to use an infrared thermometer to "scan" the surface of the radiator for cold spots. If clogged, the radiator should be removed for cleaning or be replaced.

    Backflushing the cooling system and/or using chemical cleaners can remove rust and hard water scale, but may do little to open up a clogged radiator.

    When refilling the cooling system, be sure you get it completely full. Air pockets in the head(s), heater core and below the thermostat can interfere with proper coolant circulation and cooling. If the cooling system has no bleeder valves to vent air, you may have to temporarily loosen a heater hose to get all the air out of the system.

    • Excessive exhaust backpressure - A clogged catalytic converter is usually the culprit here, but don’t overlook the possibility of a crushed pipe or a collapsed double wall pipe. Check intake vacuum at idle. If it reads low and continues to drop, inspect the exhaust system.

    • Retarded or overadvanced ignition timing (may also contribute to detonation and preignition).

    • Overheated incoming air - On older vehicles with a carburetor or throttle body injection, check the operation of the heated air intake system on the air cleaner. If the temperature control valve is stuck so only heated air from around the exhaust manifold is drawn into the air cleaner, it may contribute to detonation and/or overheating. Also check the heat riser valve for manifold heat on older V6 and V8 engines. If stuck shut, it may be overheating the intake manifold.

    • Dragging brakes - A caliper that’s sticking or a parking brake that isn’t releasing may be making the engine work too hard.

    • Overworking the engine. The cooling systems in many passenger cars today are marginal and have little excess capacity to handle extra heat generated by towing or high speed mountain driving in hot weather.

    more here http://www.arrowheadradiator.com/ove..._and_cures.htm
    Shogun tricks and tips for the E32 series are HERE!

  2. #2
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    good read


    i remember reading a few months ago that an engine will only use 20% of the energy in gasoline for propulsion

    25% for diesels

  3. #3
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    What about underheating? My temp gauge never gets past 1/4 regardless of how long I drive and I have no heat.

    Stuck open thermostat? I have the M30B35, by the way.

  4. #4
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    Quote Originally Posted by the cheat View Post
    What about underheating? My temp gauge never gets past 1/4 regardless of how long I drive and I have no heat.

    Stuck open thermostat? I have the M30B35, by the way.
    your engine needs to get to normal operating temp from time to time. will wear out piston rings etc and you'll start burning oil


    i aint no expert btw

  5. #5
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    It's no E30.
    True, and your spark plugs will be drowned in fuel, as the car dumps more fuel when the engine temp is cold.

    I finally put the tstat back in, and after only 1000 miles without it, the new spark plugs look almost toasted. And the car is slower. At least I won't be tearing up the cat any more.



  6. #6
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    Engine temperatures

    Shogun is the man. Having said that, I would add the following. Do not think you can solve a problem by running a higher a higher concentration of anti-freeze. Pure water has a higher specific heat than anti freeze, so it will cool the egine much more efficiently. But, it freezes and will allow corrosion to form.(It also takes more pump hp to circulate it around your engine) Add enough anti freeze to meet your climates needs. If you live in a climate that never sees freezes, pure water and a corrosion inhibitor is your best bet

  7. #7
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    Quote Originally Posted by E30NEIN View Post
    True, and your spark plugs will be drowned in fuel, as the car dumps more fuel when the engine temp is cold. I finally put the tstat back in, and after only 1000 miles without it, the new spark plugs look almost toasted. And the car is slower. At least I won't be tearing up the cat any more.
    I had this experience 2 months ago on my E36 M3, thermostat was broken and in open position on a 500 miles trip half way, so I drove back all the way home with an open t-stat = 1/4 on the temp gauge only, much more fuel consumption as a result.
    Replaced the t-stat and also replaced the plastic t-stat housing with a alu housing.
    Shogun tricks and tips for the E32 series are HERE!

  8. #8
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    Mr. Shogun, thank you for the many repairs made easy on my 735iL over the past several years. I have a slight overheating issue. I recently had my head reworked after a blown gasket and a major warp. Things were fine until I started to smell coolant and found several loose hose clamps. After tightening and adding coolant, my temp gauge runs between 1/2 and 3/4 if idling. During several attempts to bleed cooling system, I get a geyser of coolant from the cap open expansion tank as soon as the thermostat opens. Is this normal? Or is it caused by a big air bubble? I read somewhere about a secondary electric water pump, I don't believe my car has one but I haven't been through it thoroughly yet.

    - - - Updated - - -

    Additional information: new water pump and fan clutch installed recently

  9. #9
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    All E32 have an auxiliary water pump which is mounted below the heater valves engine bay left side. This aux water pump is basically to ensure constant flow of hot coolant into the heater core, no matter what the RPM of the engine is, as the main water pump is belt driven and dependant on RPM. The aux water pump makes 950 l/h when heater is on.
    Maybe you try another round of coolant bleeding, also the coolant must be warm in order to bleed the heating section.
    Gale has a write up here http://www.nmia.com/~dgnrg/page_19.htm


    M30 6 Cylinder Engine Coolant System Bleed
    There is a lot of discussion about how difficult it is to correctly bleed these straight 6 engines.Some say to raise the front of the car, face it uphill etc… Never on this car has any of that been done. If the system is re-filled and then bled correctly then the following simple procedure will suffice. If you are constantly getting air locks you need to look for other reasons as to why, coolant leaks, pressurisation etc…This car never uses any water at all, period. The coolant is changed every year rather
    than two years as stated and only genuine BMW coolant is used.The temperature needle rests a needle’s width below the centre line no matter what,idling for two hours, stuck in a traffic jam, air con on all day it makes no difference.If the system has no leaks, the pump is good, belts tightened correctly, correct radiator cap, bleed line free, auxiliary fans and switch working, viscous fan operating correctly, you should never have a problem with the cooling system.This car has the added benefit of 100% inert softened water straight from Sizewell B’ plant and no it doesn’t glow in the dark. It advisable once the system is known to be fine to use softened water or “water wetter” additive although if the correct coolant is used it’s not a 100% necessity.
    The following is based on you knowing you have no water leaks, blocked radiator, failing water pump etc... i.e. you have either overhauled the system or know it to be in operating correctly and are either changing the coolant or topping up after pump
    change etc…if you know you are loosing coolant from somewhere and just a small top up is required to keep you going it should not be necessary to perform a system bleed. The main, repeat main issue with bleeding this system is the small bore return line from the LH top corner of the radiator back to the expansion tank becoming blocked, or kinked / damaged. The only way natural bleeding of this system occurs is via air going through this pipe back to the expansion tank as in most other car systems like this.To check your pipe is clear simply disconnect it at the radiator end, remove the radiator cap and blow through it, it shouldn’t be hard to blow, just the resistance felt through it being a small bore pipe. It can become blocked quite commonly at the entrance point inside the expansion tank as well. Original fitment is of crimp clamp, simply replace with standard jubilee type. If you are totally re-filling the system please ensure enough coolant is available for a 50/50 mix with water and that all the hoses and plugs are reconnected, it’s easy to forget one! M30 Engine coolant capacity is 12.5 litres, you will always have some resident in the heater matrix and generally for this car 4 litres of coolant is purchased therefore 8 litres total approx (50/50) is used to change the system, more if you have changed the radiator, drained the block etc…
    When ready to re-fill the system start with ignition on and temp wheels on hot. This should run the auxiliary heater pump ensure coolant in pushed through the heater matrix. Remove the radiator cap, undo the bleed screw on top of the thermostat housing, undo the small bore return line at the joint from the rubber hose to the plastic hose giving you about six inches loose from the radiator corner, try to keep this hose upright, a screwdriver wedged into the radiator top and hook this pipe around it generally works if you don’t have an assistant. This makes it the highest point to bleed on the system. Slowly fill the system with correct 50/50 mix of antifreeze and water, keep filling
    until spillage comes from the thermostat bleed screw and tighten this screw. Keep filling until coolant comes out of the top of the raised small bore line, this ensures the radiator is basically full. Re-connect the line. Make sure the coolant level is correct on the expansion tank; leave it a couple of minutes to settle. This car is always started up with the expansion tank cap off allowing you to check that coolant is circulating by seeing a small stream of coolant coming out of the small bore return line into the expansion tank, if so everything is fine. Re-fit the cap, job done!
    Here with pics http://www.bmw7resource.co.uk/TechDo...d%20System.pdf

    In case the aux water pump does not work at all, it might make trouble to bleed the heating section = heater core. So also check the aux water pump.
    The IKHA documentation has the following statement:

    An electrically powered auxiliary coolant pump (mounted in the engine compartment, near the brake booster) is used to ensure an adequate supply of hot water to the heater coresat all times. The control module keeps the auxiliary coolant pump off:

    • Following a cold start, until one of the heater cores reaches 86oF (30oC), AND
    • Until the left or right Y-factor reaches 30%
    Once the pump is started, the control module keeps it operating until the left or right Y-factors drops below 25%.

    This is from E31, but you see nicely explained the coolant flow by Timm http://www.meeknet.co.uk/e31/E31_Hea...eplacement.htm

    Aux pump repair http://www.meeknet.co.uk/E31/AuxPump...mp_Repairs.htm
    Shogun tricks and tips for the E32 series are HERE!

  10. #10
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    This is good info. I may be tackling an overheating 5 series with new water pump, t-stat and radiator.
    ______________________________
    1992 750iL

  11. #11
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    BMW Expansion Tanks, Caps, and Bleeding Explained https://www.bimmerforums.com/forum/s...ding-Explained
    Shogun tricks and tips for the E32 series are HERE!

  12. #12
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    Just out of curiosity, does anybody else’s e32 temp gauge go right of the middle mark upon hot restarts, then go back down to the middle mark?

  13. #13
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    That sometimes happens on my 750 also on very hot days, when I switch off hot engine and restart a few minutes later, the temp is hotter, as the heat stays there and no visko and no aux fan is running, after engine has been started, the gauge goes down again.
    That is why some people modify the aux fan, that it runs a bit after the engine has been stopped to cool down the engine bay.
    I started such an idea in 2007 on the German forum http://www.7-forum.com/forum/showthr...light=Nachlauf
    In post 9 someone made a diagram, post 65 shows the final diagram and that is working since 2012, in 2014 he even added that the aux water pump is running to move the coolant, final revised drawings in post 84 dated 29.12.2018

    Comments:
    Meine Erfahrung!
    Ich habe das schon mit einem Infrarotthermometer geprüft, die Temperaturen nach dem Abstellen sind mit dem Nachlauf am gesamten Kopfbereich weniger. Normal ohne Nachlauf hatte ich immer nach dem Abstellen so um die 95° bis 100° und jetzt mit dem Nachlauf 80° bis 85° weiter fallend. Der Nachlauf ist bei mir 5min eingestellt, wenn ich messe wenn die Zeit um ist, liege ich schon bei nur 70° bis 75° am Kopfbereich der Wasserkühler ist meistens schon auf 40° gefallen. Man beachte auch, dass der Luftstrom sehr deutlich außen am Fahrzeug zu spüren ist, gerade wenn ich im Sommer aus dem Auto steige und an der Tür stehe, kommt mir da sehr warme Luft entgegen. Also die Luftzirkulatin unter der Haube ist auch nicht zu verachten.

    My experience! I have already checked with an infrared thermometer, the temperatures after parking are less with the fan system on the entire cylinder head area. Normal without that additonal fan cooling I had always after stopping around 95 ° to 100 ° Celsius and now with the additional cooling 80 ° to 85 ° C and further dropping. The time is set for 5 minutes, when I measure when the time is up, I'm already at only 70 ° to 75 ° at the head of the water cooler it is usually already fallen to 40 ° C. Also note that the airflow is very noticeable on the outside of the vehicle, just when I get out of the car in the summer and stand at the door, I get very warm air. So the air circulation under the hood is quite good.
    Shogun tricks and tips for the E32 series are HERE!

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