Rick's Free Auto Repair Advice

My car overheats

How to fix a car that overheats

It’s blistering hot out there and about every 3rd question on automotive forums ask “My car overheats”  There’s no simple answer as to why an engine overheats. We have to divide it into two sections; a mechanic cause and an electrical cause.

On the mechanical side.

The very first item to check is coolant level. Low coolant makes the car overheat at idle but cool down at highway speeds. That’s because the water pump runs at a high enough speed to force the remaining coolant through the engine faster. At idle, it’s pumping slower and there’s less coolant to pump, so it overheats.

The thermostat controls when coolant flows out the top of the engine and into the radiator. Once it hits the radiator, the coolant flows across and down to the lower radiator hose where it’s sucked back into the engine by the water pump. Here’s what can go wrong.

Clogged fins on the A/C condenser and radiator fins:
I often find the front of these covered in dead bugs and road crud. If air can’t past through the fins, the radiator can’t cool. And since the condenser coil is mounted in front of the radiator, I clean that first. It’s easy to do with a garden hose and pistol type sprayer. Just spray a high pressure jet of water down the cooling fins to wash off the crud.

This is a fairly primitive device. Most newer cars have a thermostat rated at 195 degrees. It should start to open at about 170 degrees and be fully open by 195, allowing full coolant flow. Thermostats fail by opening too early, which can sometimes prevent the engine from warming up. That also increases gas usage since the engine stays cold much longer. Or, it can fail by not opening up enough (or at all) which prevents coolant flow. When a thermostat fails in the closed position, the engine heats up very fast and makes boiling sounds. That happens because you’re literally heating up the water but preventing it from circulating. You can check a thermostat, but it’s pretty much a waste of time since you have to remove it from the vehicle to test it. And, new thermostats are about $12, making the test pretty worthless. Using the theory of replacing the most likely and least costly items first, I usually replace the thermostat when I encounter an overheating engine.

People always discount how important the radiator is in an overheating situation. The cooling tube passages are very small. If you haven’t been good about changing coolant at recommended intervals, they can clog up with corrosion. If you own one of those newer non-contact infra-red

car overheats, engine overheats, what's causing overheating

Non-contact infra-red thermometer

thermometers, use it to check the condition of your radiator. With the engine running and thermostat open, just point it at the fins near the upper radiator hose. Then scan it across and down the radiator. You’ll looking for cold spots. That indicates blockage. If you find any, don’t waste your time trying to clean the radiator. The flushing chemicals can remove some of the buildup, but experience shows that by that point the corrosion has eaten up so much metal, that you’ll be left with pinholes in the metal tubes. A radiator that’s working should show hotter near the location where the upper radiator hose enters, and much cooler readings towards the bottom where the coolant leaves.

Water pump:
If you have access to the radiator cap, you just remove it and watch the coolant flow. Increase idle speed and the coolant flow should increase with it. If you can’t get access to the radiator (no cap on newer systems that use the overfill bottle as the cap), you’ll have to clamp the upper radiator hose to feel coolant movement. Careful, it’ll be hot, so wear gloves.

Radiator Cooling Fans:
These come in two varieties: mechanically driven by the engine, or electric.

Mechanical Fan
Mechanical fans always have a clutch mechanism. The clutch determines how much engine rotation is transferred to the fan blades.

radiator fan clutch, viscous clutch

Viscous fan clutch controlled by bi-metallic coil

At idle, the clutch provides full rotation to the fan blades to suck in as much air as possible and provide maximum airflow across the radiator cooling fins. But at highway speeds, the force of incoming “ram” air is actually greater than the sucking provided by the fan. In fact, at highway speeds, the fan slows down airflow, so the clutch disengages and the clutch free-wheels. This is done to increase fuel efficiency and maximize airflow.
Car makers use two types of fan clutches; a viscous controlled by a bi-metallic strip, and a viscous clutch controlled by electrical current.

engine overheats, why engine overheats, fix overheating engine

Electrically controlled viscous fan clutch

Both types can fail. And when they do they can slip at idle resulting in far less airflow than you need. If you have a mechanical fan and experience overheating at idle but not at highway speeds and your coolant levels are where they should be, suspect a bad fan clutch.

Electric Radiator Fans
Next are the electric fans. The early models have fairly simple operating circuits. The PCM read the engine coolant temperature and provided ground to the fan relay when it determined the system needed more cooling. The relay provided power to the fan and it ran up to speed.

Older systems:
To diagnose these older systems, start by checking the fuse for the radiator fan. If it’s good, check the operation of the fan itself. Start the vehicle and turn the A/C to MAX. The fan should start up. If it doesn’t you know there’s a problem. Start checking the relay. You can do that by finding another relay with the same part number inside the fuse box and swapping them. If the fan still won’t run, you’ll need a non-contact infra-red thermometer and a digital multimeter. Locate the battery voltage pin of the fan relay socket going to the relay switch (see relay layout here). Then located the pin with battery voltage going to the relay control coil. Next, start the engine and measure the temperature of the water outlet with the thermometer. As it reaches 195, use your meter to test for power on both of those pins. And check for ground on the PCM side of the relay. If the PCM isn’t providing ground, the relay can’t operate and no power will flow to the electric fan. The weak link in these system is usually the engine coolant temperature sensor. There’s a way to test it, but the sensor is only about $20 and it takes less time to replace it than test it.

Late model systems:
But those were the early systems. The newer

Recharge A/C system, A/C blows warm, A/C not working, How to fix A/C

Click on this to open PDF

systems are far more complicated. The PCMs in these system take into account not only engine temperature, but A/C pressures, the position of the accelerator pedal (cuts off A/C during heavy acceleration to reduce engine load), and transmission fluid temps. Click on the PDF below to see how complicated they’ve become. But that doesn’t mean you can’t diagnose them. Just follow my step by step instructions
Also notice that many of these systems use two speed fans. So the fact that an electric radiator fan is running at idle doesn’t mean it’s going to run at full speed later.

Final note:
Car makers install plastic shrouds around their fans to provide maximum wind tunnel sucking properties. And they install baffles around the radiator to direct air through the radiator instead of around it. If anyone has every removed the radiator and not replaced those components in exactly the right place, that can disturb the airflow and reduce cooling.
Also, most late model cars are equipped with an air dam under the front bumper. This deflector is there for a reason—to deflect air upwards towards the radiator. If yours is missing and your vehicle overheats at highway speeds, the missing air dam may be the culprit.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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