COOLING SYSTEM DIAGNOSIS

Transcription

1 54 July 2008

2 COOLING SYSTEM DIAGNOSIS BY PETER CROSBY Even when working properly, many cooling systems run perilously close to overheating. If (or when) a system blows its top, the cause can often be found by looking at the basics that all systems require for efficient operation. For a cooling system to function properly it needs several things: proper coolant mixture and level, sufficient pressure, adequate coolant flow and sufficient airflow. When and where a vehicle overheats provides a tech with valuable diagnostic clues as to which of these may be missing. Driving at highway speed, for example, can offset a cooling fan or fan clutch failure that prevents sufficient airflow at slower speeds. A failure to overheat at highway speeds may also confirm adequate coolant flow, including proper thermostat operation. When a vehicle arrives in the service Photoillustration: Harold A. Perry; photos: Jupiter Images lane with a complaint of overheating, the first thing to do is to verify the problem. More than one vehicle has arrived with a temperature gauge problem rather than an actual overheating condition. If it s not puking coolant upon arrival, the service advisor should determine why the customer believes his vehicle is overheating, and under what conditions. Does it get better or worse the faster he drives? When did the problem start? Other information that will speed the diagnosis includes details of previous repair efforts. Did the customer add water or coolant, or attempt to flush the system? How far did he drive after the vehicle began to overheat? After reviewing this information, the July

3 tech should then check the coolant level and condition, both in the overflow reservoir and in the radiator itself. If the level is low in either location, this adds some credibility to the customer s claim of overheating. If the overflow tank is full but the radiator is low, the cause may be more than just a radiator cap with a failed check valve. If there s Dex- Cool in the cooling system, it may be an indication that serious problems have already begun. Coolant condition and concentration are critical. Has the customer completely plugged up the radiator by continually topping it up with hard water, or created mud by mixing DexCool with a traditional green coolant? Many drivers mistakenly believe that if a 60/40 mix of coolant and distilled water is good, 100% coolant must be even better. It s not; ethylene glycol doesn t transport heat nearly as efficiently as water. Test strips or a refractometer should be used to gauge the true condition and concentration of the coolant. The use of distilled water is always recommended when mixing coolant and topping up, yet try to find a gallon of distilled water in most shops! The next step is to duplicate the overheating condition. Is the vehicle actually overheating? Some vehicles, notably the Ford Focus or others with small front fascia openings, simply run hotter than A cooling system pressure tester, whether hand- or mechanically operated, is the gold standard for diagnosing engine overheating problems. The tool can be used to quickly determine whether a system will hold the necessary pressure and, if it doesn t, to help locate the source of coolant leaks. others. Some vehicles may not overheat, but a loss of system pressure may cause them to run 15 to 40 above where they should. Does driving faster or reducing engine rpm help bring the temperature back down? How long does it take after a cold start for the engine to begin overheating? If the coolant level was indeed low, where did the stuff go? There aren t that many answers. Either it went out the tailpipe, dripped onto the driveway or is collecting someplace it shouldn t be. Is it mixed with the engine oil or transmission fluid? Are the carpets wet, indicating a heater core failure? Something as Photos & illustrations: Peter Crosby Normally Closed or Spring-Pressed Vacuum Radiator Cap Open or Weighted Vacuum Radiator Cap Vacuum Spring Lower Sealing Gasket Vacuum Spring Lower Sealing Gasket Weight Spring Vacuum Radiator caps come in two styles constant pressure and drop-center. The cap performs three different functions: First, it keeps the cooling system sealed from outside contaminants. Second, by keeping pressure on the cooling system, it raises the boiling point of the coolant. Third, the cap allows coolant to go to the expansion recovery tank when the coolant gets hot and expands, and pressure increases beyond a set limit. As the system cools down and pressure drops, the cap allows coolant to return to the cooling system from the coolant recovery tank. 56 July 2008

4 Many cooling systems feature pressurized surge tanks (left photo above), rather than a simple radiator cap and adjacent coolant recovery tank. Dedicated surge tank fittings allow the use of a pressure tester (right) on these systems. The tank is connected to the rest of the cooling system and must hold pressure when pressurized. The surge tank cap has no relief function, unlike a conventional radiator cap. simple as a cracked hose to an overflow tank can cause a vehicle to gradually lose coolant and eventually overheat. Vehicles that were notorious for overheating include Chevrolet Vegas and certain Jaguars, simply because they didn t have large enough overflow tanks. They tended to spit out a little coolant after every hot shutdown. The system eventually became low on coolant, then the vehicles would start to overheat. EGR coolers on some newer Ford diesels are prime suspects when coolant begins to disappear, or is found mixed with the oil. Simply watching the coolant below the neck of the radiator can provide useful information. Is there a constant stream of bubbles? Does the level drop when rpm is increased, providing some indication the pump is working? After topping off the system, and burping it, if necessary, a pressure test is the next and by far the most important thing a tech should do when diagnosing an overheating complaint. A coolant/ water mix boils at a higher temperature than water alone, but it s pressure anywhere from 5 to 15 psi that does the real work of keeping coolant from boiling. Each psi of pressure raises the boiling point by approximately 3. -testing the system before and after repairs is essential. Check your technical information resources for the location of cooling system bleeder valves. On some vehicles, it s impossible to remove all of the air from the cooling system without following the bleed procedure and using the bleeder valves. Many water pumps are not as easily accessible as this one. It may be virtually impossible to diagnose a damaged water pump with worn or broken impeller blades or a slipping impeller without removing and visually inspecting the pump. Use methodical troubleshooting techniques to rule out as many other causes of overheating as possible before tearing a vehicle apart. 58 July 2008

5 If the system can t hold pressure, coolant will boil in the cylinder head and next to the cylinder walls, forming vapor bubbles, and the engine will quickly overheat. Realistically, no further diagnosis can be done until the system is repaired to the point where it can hold pressure. If the pressure tester hand pump must be pumped for an excessive amount of time before the system is pressurized, this often indicates the presence of trapped air in the system. A block test should immediately follow a pressure test, since the estimate for the repair may skyrocket if a failed head gasket or cracked head is indicated. Having a complete set of adapters, including those needed for newer vehicles equipped with surge tanks, is a requirement for proper pressure testing. Radiator caps perform three vital functions, yet commercial pressure testers check only one the pressure at which the cap releases. Caps must seal around the top lip on the radiator neck and hold vacuum as well, otherwise coolant will never be recovered from the overflow tank. Pulling a vacuum on the recovery port with a hand-held vacuum pump is one way to test this seal, as well as to identify any other vacuum leaks in the cooling system. The radiator cap check valve must also operate properly for coolant to be recovered. One way to test the valve is to drain some fluid from the radiator (with the system sealed) and see if coolant is pulled in from the overflow bottle. Some shops automatically replace the radiator cap on every car that comes in for cooling service because so many fail one or more of these tests. A vehicle that doesn t recover coolant properly quickly enters a downward spiral of coolant loss and overheating, In the case of systems filled with DexCool, severe corrosion also can result from the low-coolant condition. On vehicles that overheat only when idling or while moving slowly in traffic, both the low-speed and high-speed cooling fan stages must be checked for proper operation. High-speed fan operation should be commanded above a certain temperature, or when the a/c is turned on. The wild card in many overheating situations is the thermostat. A sticking stat will prevent adequate coolant flow through the radiator. Diagnosis is rela- While the majority of late-model cooling systems rely on electric cooling fans, many trucks and other heavy-duty vehicles still feature engine-driven cooling fans. A complaint of overheating during low-speed driving may be due to a failed cooling fan clutch or a slipping belt. This fan looks like it was only a few miles away from becoming a projectile. Circle #34 60 July 2008

6 tively easy: Remove the thermostat, drive the vehicle and see if the problem goes away. Thermostat operation and opening temperature can also be tested in hot water, but the most expedient thing may be to install a new stat. Thermostats frequently stick after customers flush a cooling system with caustic chemicals and rinse it out with mineralladen tap water. Using the correct thermostat is also a requirement. Some of today s vehicles actually have two separate thermostats, and others are equipped with a dualaction thermostat that seals off the bypass flow once the desired temperature is reached. A wrong thermostat can leave this passageway open, leading to inadequate flow through the radiator. Incorrect thermostat installation, such as installing it upside down or backward, is another recipe for overheating. A clogged heater core preventing bypass operation will cause thermostats on certain GM vehicles to not open properly, because the bypass flow is used to heat the thermostat. Thermostats with lower opening temperatures can help in perennially hot climates but, again, are no fix for an underlying problem. Air pockets can be another cause of overheating. Typically, air pockets will develop after the cooling system has been drained and refilled while some other problem is being fixed, and not as an original complaint. Check your technical resources to see if the vehicle has any bleeder ports. Nissan cooling systems can be particularly difficult to fill without creating air pockets. Techs have adopted a variety of tools and techniques to properly burp certain Nissan models, including raising the front end, using super-long funnels and vacuum lift systems, etc. If an air pocket is present, the coolant temperature sensor output will swing wildly as it s alternately exposed to air, then coolant, then air. Many thermostats come predrilled with a small hole and so-called jiggle valve designed to prevent air pockets by allowing air bubbles past the thermostat, so they can reach the highest point in the cooling system. Some techs drill a small hole in thermostats that lack this feature so an air pocket can t form below the thermostat and prevent it from operating properly. Damaged water pumps with worn or broken impeller blades or a slipping impeller are virtually impossible to diagnose without removal and visual inspection. Using methodical troubleshooting techniques, you ll be able to rule out as many other causes of overheating as possible before tearing a vehicle apart, beginning with causes that are easiest to test. For instance, a wintertime complaint of intermittent heat often precedes a completely failed head gasket. Bubbles of combustion gases pushed into the coolant will often prevent proper heater operation. Truly intermittent overheating is of-

7 ten caused by a head gasket just starting to fail as well. Further discussion of overheating problems caused by head gasket failures is beyond the scope of this article; however, chemical testing of the coolant and/or use of a four- or fivegas analyzer seem to be the fastest and most reliable methods of detecting head gasket failure. Some domestic manufacturers make nearly identical pumps in both righthand and left-hand versions for use in systems with reversed flow. Getting one with the wrong impeller is rare, but it does happen. Loose belts, or improperly routed serpentine belts, are other rare but real causes of overheating. Gauging coolant flow to determine water pump condition is one of the toughest tasks when diagnosing overheating. Soft radiator hoses can simply be squeezed and felt for flow pulsations. Cooling the radiator with spray from a garden hose will help only if adequate coolant is flowing through the radiator. When the engine is cold, a radiator hose can be removed and the flow of coolant can be directed into a bucket for a rough gauge of flow. Comparing the temperature of radiator inlet and outlet hoses provides some indication of coolant flow, but a clogged radiator will exhibit the same signs as a lack of pumping action. Measuring the actual pressure on one of the heater hoses (at 2500 rpm) while deadheading the other is another approach. Look for 10 to 15 psi. Some techs will splice a piece of clear plastic hose into one of the heater hoses for a similar look-see. As always, beware of scalding hot coolant that can ruin your day! A review of Techhelp requests on iatn, the website for automotive technicians ( suggests that inoperative cooling fans and failed fan clutches are the leading causes of overheating problems attributable to inadequate airflow. Debris collected in the radiator or between the radiator and a/c condenser is a close second. Both conditions are relatively quick and easy to diagnose. The bottom line is this: Overheating problems can be solved by sticking to the basics: checking airflow, coolant flow, pressure, proper fluid and level. Much of what the cooling system does is invisible to the eye and can only be deduced. How and when a vehicle overheats offers vital clues to making the proper deductions. Don t assume there s only one cause of the overheating condition when a vehicle comes in. Pay particular attention to radiator caps; they cause or contribute to many cooling system problems. Duplicating an overheating problem and checking for more serious damage like a failing head gasket is the best way to prevent comebacks. This article can be found online at Circle #36