Cooling System and Drive Belt Service

Belts, hoses and coolant are cheap compared with what they protect, and they fail in predictable ways. A broken drive belt can stop the alternator and water pump at once. A failed hose can overheat an engine and ruin it in minutes. This guide covers how to inspect these parts, replace them, test the cooling system and refill it without trapping air.

What You Will Be Able to Do

• Inspect a drive belt, tensioner and pulleys, and replace them correctly
• Judge the condition of hoses and find coolant leaks
• Choose the right coolant, and drain, refill and bleed a cooling system
• Pressure test a cooling system and diagnose a thermostat or water pump
• Know which refrigerant work needs EPA certification

Warning: Never open a radiator cap or pressurized coolant cap on a hot engine. The system runs under pressure, and hot coolant flashes to steam the moment that pressure is released. It can spray out and cause severe burns. Wait until the engine is cool to touch, cover the cap with a rag, turn it slowly to the first stop, let any pressure escape, then remove it.

Drive Belts

A serpentine belt uses one long belt to drive the alternator, water pump, A/C compressor and, on some vehicles, a power steering pump. When it fails, all of them stop.

Inspect with the engine off and the key removed. Look for:

Modern EPDM belts lose rib material without obvious cracking, so a belt can look fine and still be worn out. Use a belt wear gauge in the rib grooves to check.

Tensioner and Pulleys

An automatic tensioner keeps the belt tight. Check these:

1. Arm movement. Use the correct tool to move the arm through its travel. It should move smoothly.
2. Damping. When you release it, the arm should return with control, not snap back. A weak tensioner lets the belt flap and chirp.
3. Pulley bearings. With the belt off, spin each pulley by hand. Feel for roughness, and rock it to check for play.

Replace a weak tensioner when you fit the new belt, since a new belt on a bad tensioner fails early. While the belt is off, check the idlers and the alternator pulley. An overrunning alternator pulley should spin freely one way and lock the other.

Replacing a Belt

1. Photograph the routing, or find the diagram on the underhood sticker.
2. Find the tensioner release point.
3. Rotate the tensioner to slacken the belt, and slip the belt off.

Warning: Keep your fingers out from between the belt and pulleys when you release the tensioner. The spring is strong. If the tool slips, it can trap your hand.

1. Fit the new belt, leaving the easiest pulley until last.
2. Release the tensioner slowly and check that every rib sits in its groove.
3. Start the engine and watch the belt track straight. Listen for noise.

A chirp on a new belt usually means a bad idler, a misaligned pulley or wrong routing.

Hoses

Squeeze each hose when the engine is cold.

• Soft or spongy: The hose is weak from the inside.
• Hard or brittle: It has aged and can crack.
• Bulges or swelling: It is about to fail.
• Wet spots or crust at clamps: Green, orange or white residue means dried coolant, which confirms a slow leak.

Check clamps and the heater hoses at the firewall. Replace any hose you doubt, and replace old hoses on age, since they often fail from the inside.

Coolant

Coolant, also called antifreeze, carries heat, prevents freezing, raises the boiling point and protects metal from corrosion.

Do not go by color. Dyes vary between makers, so use the coolant the service information specifies.

Never mix coolant types. Different chemistries can react, forming gel or sludge that clogs the radiator and heater core, and they can lose their corrosion protection. If you change type, flush the whole system first.

Use a 50/50 mix of concentrate and distilled water, or a factory premix. Tap water leaves mineral deposits. A 50/50 mix protects to about -34°F and, with a 15 psi cap, raises the boiling point to about 265°F.

Replace coolant on the manufacturer's schedule. Traditional coolants last about 2 years or 30,000 miles, while long-life types last 5 years or 100,000 miles or more. Test the protection level with a refractometer, and use test strips for additive condition.

Warning: Coolant is poisonous to people and animals, and it tastes sweet. Clean up spills, and never leave it in open containers. Collect it and follow your shop's disposal rules. Do not put it down a drain.

Draining, Refilling and Bleeding

1. Let the engine cool completely.
2. Open the radiator drain and catch the coolant. Disconnect the lower hose if needed.
3. Close the drain and refit the hose.
4. Open any bleed screws.
5. Fill slowly with the correct coolant.
6. Close each bleed screw as a solid stream of coolant comes out.
7. Run the engine with the heater on maximum until the thermostat opens, and watch the level. Add coolant as air works out.
8. After it cools, top up the reservoir to the cold mark and recheck for leaks.

Many modern engines trap air easily, so a vacuum fill tool is often required. It pulls the system into a vacuum, then lets coolant flow in. Air pockets can cause overheating and no heat, even when the system is full. Follow the service procedure.

Testing the System

Pressure Test

With the engine cool, fit a pressure tester to the radiator or reservoir. Pump to the rating on the cap, commonly 13 to 18 psi, and do not exceed it. The system should hold for about 10 minutes.

If the pressure drops, look at the hoses, radiator, water pump weep hole, heater core and core plugs. If it drops with no visible leak, suspect a head gasket or an internal leak. Test the cap as well. A weak cap lets coolant boil early.

The Thermostat

The thermostat blocks flow to the radiator until the engine warms up, then opens.

Fit a new thermostat the right way round, with any jiggle valve or bleed pin at the top so air can escape. Use a new gasket or seal.

The Water Pump

On a belt-driven external pump, the weep hole lets coolant escape when the internal seal fails. Coolant dripping from the weep hole means the pump needs replacing. A rough or wobbly pulley means the bearing is failing.

When you replace a water pump, inspect or replace the related parts: the thermostat, coolant hoses, the drive belt, or the timing belt and tensioner if the pump is timing-driven. Refill with fresh coolant. Some pumps are driven by the timing belt, and others are electric and controlled by the engine computer.

Overheating Checks

Work from simple to complex:

1. Check the coolant level when the engine is cold.
2. Look for leaks and test pressure.
3. Check that the cooling fans turn on.
4. Check the radiator for blocked fins or a clogged core.
5. Test the thermostat and water pump flow.
6. Run a combustion leak test if nothing else explains it.

Warning: Electric cooling fans can start at any time, even with the engine off. Keep clear of them, and unplug the fan if the service procedure allows when you work nearby.

Air Conditioning Is a Separate Certification

The A/C compressor runs off the drive belt, so you will see it during this work. You can inspect the belt and listen to the clutch. The refrigerant is another matter.

Anyone who services a motor vehicle air conditioner for payment or barter must be trained and certified under EPA Clean Air Act Section 609, and must use approved recovery equipment. This is different from Section 608, which covers stationary and other non-vehicle equipment. Releasing refrigerant on purpose is illegal. Many newer vehicles use R-1234yf, which is mildly flammable and has its own equipment rules. If a job requires opening the A/C system and you are not certified, hand it off.

Common Mistakes

Check Your Understanding

1. A customer's car has no heat and reads cold on the gauge after 20 minutes of driving, with P0128 stored. What is your first suspect?
2. You find coolant at the water pump weep hole. What do you do?
3. An engine overheats right after a coolant change, but the system is full. What might you have missed?
4. A customer asks you to add orange coolant to a green system to top it up. What do you do?

Answers

1. A thermostat stuck open.
2. Replace the pump, because it means the internal seal has failed. Inspect the belt, hoses and thermostat, and refill with fresh coolant.
3. Air trapped in the system. Bleed it using the bleed screws or a vacuum fill tool.
4. Do not mix them. Check what the vehicle requires, and top up with that coolant, or flush and refill if the wrong type is already in the system.

Key Takeaways

• Never open a cap on a hot engine. Wait, then release pressure slowly.
• Check belts with a wear gauge and test the tensioner, since a belt can look fine and still be worn out.
• Use the specified coolant, not the color. Never mix types, and use distilled water.
• Pressure test the system, and bleed out air after any refill.
• A stuck-open thermostat causes low temperature and P0128. Weep hole leakage means a failed pump seal.
• Motor vehicle A/C work requires EPA Section 609 certification.