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It is 105 degrees on the 79 and the needle is climbing. This is the most common call we get in August, and what you do in the next five minutes decides whether this is a $200 day or a $4,000 one.

The First Five Minutes

Heat has to go somewhere. If the radiator is not moving it, the heater core is the only other radiator on the car, so you use it. It is miserable in August and it works.

• Air conditioning off. The compressor adds load and the condenser sits right in front of the radiator, dumping more heat into it.
• Heater and blower on full. This alone can drop coolant temperature enough to get you off the freeway.
• Get out of stop and go. Moving air helps. Sitting at a light with no fan is the worst place to be.
• Pull over and shut it off if the needle keeps climbing or you see steam. Then call someone.
• Leave the cap alone. A hot system is pressurized and the coolant is well over boiling. People end up in the emergency room over this every summer.

If the temperature light came on and stayed on, the car needs to be looked at before it is driven again. We would rather come to you than meet the same car on a flatbed two days later.

The Four Things That Usually Did It

Overheating feels dramatic but the cause is almost always one of four ordinary parts.

Cause	What You Notice	Typical Repair
Coolant leak	Low reservoir, sweet smell, a puddle, steam	$180 to $700 depending on the part
Stuck thermostat	Overheats fast, heater blows cold, gauge swings	$220 to $500
Cooling fan not running	Fine on the freeway, hot in traffic and at lights	$350 to $900
Failing water pump	Whine, weep hole dripping, coolant loss with no obvious leak	$500 to $1,200

Those ranges are for the cars we see most in Riverside County. A V6 with the pump buried behind a timing cover runs higher, and a truck with an easy pump runs lower. We quote the actual car before anyone turns a wrench.

Why Our Heat Makes It Worse

A cooling system that is 20 percent weak gets away with it in March. In August there is no margin left. Ambient air near 110 degrees means the radiator has far less temperature difference to work with, air conditioning runs constantly, and a lot of driving here is uphill out of the valley with the car loaded.

That is why the same car that was fine all spring boils over on the first triple digit weekend. Nothing broke that day. The reserve just ran out.

What We Check, in This Order

• Pressure test. Cold system, pumped to spec, watched for a drop. This finds leaks that never show a puddle.
• Thermostat behavior. Watching the upper hose and live temperature data as it opens, or fails to.
• Fan operation. Commanded on with a scan tool, then verified with the air conditioning on.
• Coolant condition. Color, contamination and the wrong coolant mixed in, which is more common than people think.
• Combustion gas test. If the first four look fine, we test for exhaust gases in the coolant, which points at a head gasket.

That last test is the one that matters most. A head gasket diagnosed on a Tuesday is a repair. A head gasket found after somebody drove it another week is often a whole engine, and our engine replacement cost guide covers what that actually runs.

What It Costs to Ignore It

A hose is a cheap part. A head gasket on a four cylinder is usually $1,800 to $3,000 with machine work, because the head comes off, goes out to be checked and resurfaced, and goes back with new bolts. If the head warped badly or the engine ran out of oil in the process, it becomes an engine.

Every one of those jobs started as a $30 part and two more miles of driving.