Why Is My Car Overheating In Traffic?
- Tyler Ellis
- 2 days ago
- 5 min read
Overheating that only shows up in slow traffic is one of the most common “it drives fine on the highway” headaches. You’re cruising at speed, everything feels normal… then you hit a red light or get stuck behind a line of cars & the temp gauge starts creeping up like it’s testing your patience on purpose.
If you’re asking, Why Is My Car Overheating In Traffic?, it usually means your cooling system can’t shed heat efficiently when airflow is low. At highway speeds, natural airflow through the radiator helps a lot. In traffic, you’re relying heavily on cooling fans, proper coolant circulation, & a clean/healthy radiator to keep temperatures stable.
The good news: this problem is typically diagnosable with the right checks, & catching it early can prevent a much more expensive engine repair.
Why Is My Car Overheating In Traffic? What’s Different About Traffic vs. Highway Driving
When you’re driving fast, air is forced through the radiator & A/C condenser, pulling heat out of the coolant. In stop-and-go traffic, airflow drops dramatically, so the system has to “work harder” using:
Electric cooling fans (or fan clutch on older setups)
Proper radiator & condenser airflow (no blockage)
Correct coolant level & mixture
A thermostat that regulates flow correctly
A water pump that circulates coolant efficiently
So if the car only overheats at idle or low speed, the failure is often tied to airflow & fan operation, not necessarily the engine itself.
What Causes This Problem?
Cooling fans not running correctly (most common)
If your electric fans aren’t turning on, aren’t running at the right speed, or are cutting out intermittently, temps will climb quickly in traffic.
Common fan-related causes:
Failed fan motor
Bad fan relay
Blown fuse
Faulty coolant temperature sensor input
Fan control module issues (on some vehicles)
Wiring/connectors with high resistance or corrosion
A fan system can also partially fail, like low-speed not working but high-speed does, which can create “sometimes overheats, sometimes doesn’t” behavior.
Low coolant level or a slow leak
Low coolant can still “seem fine” at speed because the radiator gets extra airflow, but in traffic the system has less margin.
Low coolant is usually caused by a leak, such as:
Radiator seam or end tank seep
Water pump seepage
Thermostat housing leak
Hose connection seep
Heater hose leaks
Reservoir cracks or cap sealing issues
Even a small leak can gradually create air pockets, which makes overheating more likely during low airflow conditions.
Thermostat problems (stuck closed, stuck partially, or sluggish)
A thermostat regulates coolant flow. If it’s sticking or not opening fully, coolant flow may be restricted, especially when heat load increases.
Traffic overheating can happen if:
The thermostat opens late
It doesn’t open fully
It sticks intermittently
Clues often include temperature spikes that come & go, rather than a steady climb.
Radiator airflow restriction (external blockage)
Sometimes the radiator is fine internally, but airflow can’t pass through like it should.
Common causes:
Bugs/leaves packed into the condenser/radiator fins
Bent fins restricting airflow
Debris between the A/C condenser & radiator
Aftermarket grille guards or accessories blocking airflow (depends on design)
In traffic, you don’t have the “forceful” airflow that helps overcome restrictions, so the issue becomes more obvious.
Radiator restriction (internal clogging)
Over time, coolant passages can restrict due to:
Corrosion
Old coolant that wasn’t serviced
Mixed coolant types creating sludge
Debris from component wear
An internally restricted radiator can cool “okay” in ideal conditions, but struggle in traffic or when the A/C is on.
Water pump circulation issues
A weak or failing water pump may not circulate coolant efficiently at idle.
Possible causes:
Impeller erosion or damage (some designs are more prone)
Belt slip affecting pump speed (belt-driven systems)
Bearing wear causing wobble or leakage
This can look like: fine at speed, hotter at idle, sometimes worse with A/C on.
Air pockets in the cooling system
Air doesn’t transfer heat like coolant does. If there’s trapped air, you can get hot spots & unstable temperature behavior.
This can happen after:
Coolant service done without proper bleeding
A leak that allowed air in
Repeated low coolant events
A/C condenser heat load (overheats when A/C is on)
When the A/C is running, the condenser dumps heat in front of the radiator. If fans are weak or airflow is restricted, the radiator can’t keep up, so temps rise mostly in traffic.
This is a huge clue: if it overheats faster with A/C on, think airflow/fan performance first.
How to Fix It?
Fixing “traffic-only overheating” starts with verifying the basics, then narrowing down whether the problem is airflow, coolant level, circulation, or restriction.
Confirm the symptom safely
We verify:
How fast temps rise at idle
Whether the A/C makes it worse
Whether the temp drops when you start moving again
Whether fans engage at the correct temperature
Check coolant level, condition, & obvious leaks
We inspect:
Coolant level in reservoir (when cold) & overall condition
Signs of dried coolant residue around hoses & fittings
Radiator seams, thermostat housing, water pump area
Pressure cap sealing condition (a bad cap can reduce boiling margin)
If you want a general reference for maintenance & cooling-system warning signs, this is a good starting point: https://www.marblefallsautocenter.com/blog
Verify fan operation (including high & low speeds)
This is the big one. We confirm:
Fans turn on when commanded
Correct fan speed behavior (many vehicles have multiple speeds)
Relays, fuses, & wiring integrity
Scan tool commands (when applicable) to confirm control logic
A fan that runs “sometimes” is often a relay/module/wiring issue, not just the motor.
Pressure test the cooling system
A pressure test helps locate slow leaks & confirms the system holds pressure (pressure raises the boiling point & prevents steam pockets).
If it fails pressure, we find the leak first, because overheating will keep returning until the system is sealed.
Inspect radiator & condenser airflow condition
We check:
External fin blockage
Bent fins
Debris packed between condenser & radiator
Anything physically obstructing airflow
Confirm thermostat & circulation
If fans & airflow are good, we look deeper:
Thermostat operation & temperature behavior
Water pump signs (leaks, noise, circulation concerns)
Radiator restriction suspicion (temperature differential testing, flow clues)
Repair the root cause & verify stability
Common repairs for traffic overheating include:
Replace fan motor/relay/control module (as needed)
Repair coolant leaks & refill/bleed properly
Replace thermostat
Replace radiator if restricted or leaking
Replace water pump if circulation/leak issues are confirmed
Coolant service if contaminated or overdue
After repairs, we verify the car holds normal operating temperature at idle with A/C on & during stop-and-go conditions.
For more info on services & diagnostics, you can also check: https://www.marblefallsautocenter.com
Why Act Now
Overheating isn’t a “drive it until it gets worse” problem. Heat is how engines get expensive.
Waiting can lead to:
Warped cylinder head
Head gasket failure
Coolant pushing out & creating chronic low-coolant events
Catalytic converter stress from overheating & misfires
Being stranded in traffic when it finally spikes too high
Also, every overheating event makes the next one more likely. Seals, hoses, & plastic components don’t love repeated heat stress.
If you’re asking Why Is My Car Overheating In Traffic?, the smartest move is diagnosing it before it turns into a full overheating breakdown.
Schedule Cooling System Diagnosis With Marble Falls Auto Center
If your temperature climbs in traffic, Marble Falls Auto Center can test fan operation, pressure test the cooling system, check for leaks & restrictions, & get you a clear plan to fix it correctly before engine damage happens.
Schedule your visit here: https://www.marblefallsautocenter.com/contact-us