I often get this issue where a car runs fine when idling or at low speeds. However, the car overheats when you accelerate or drive at highway speeds. What you may not know is that this issue can cause serious and expensive damage, like engine seizures.
A car overheats only when accelerating topped with proper oil and coolant for the following reasons.
- Sticky thermostat
- Air pocket in the system
- Partially clogged radiator
- Radiator cap failure
- Blown head gasket
- Water pump failure.
I’ll get more into the culprits and fixes for car overheating, then go back to normal at idle or low speed. Plus, I’ll show you the basic things to rule out before attempting any repairs.
Car Overheats Only When Accelerating: Troubleshooting Tips
Higher engine loads from acceleration dumps more heat into your vehicle’s cooling system much faster. Typically, your cooling system keeps up through increased coolant flow, pressure, and auxiliary heat management.
But in most cases, the cooling system doesn’t combat the excess heat as it should. Refer to the table for a quick breakdown of potential causes and fixes.
| NO | Potential Cause | Solutions |
| 1 | Sticky thermostat | Replace the thermostat |
| Apply silicone lube if corroded | ||
| 2 | Air pockets in the cooling system | Burp out trapped air |
| 3 | Partially clogged radiator | Flush your radiator |
| 4 | Radiator cap failure | Swap with an OEM replacement |
| 5 | Blown head gasket | Replace the head gasket |
| 6 | Water pump failure | Install a new water pump |
You don’t want to tear into repairs if it’s something basic. So, start your diagnosis by checking:
- Fluid levels: Low oil and coolant level reduces your system’s ability to absorb and transfer heat. So, top up and ensure no leaks are draining the system.
- Coolant hoses: Inspect your coolant hoses. Hoses may collapse and reduce flow at high RPMs. Replace the hoses if damaged.
If that doesn’t fix the problem, explore my list of 7 potential causes and fixes for a car that overheats when revving. Here we go:
1. Sticky Thermostat
Your vehicle’s thermostat controls coolant flow through the radiator. If it’s sticky, it won’t open when hot, restricting coolant flow through the radiator. Less coolant causes a spike due to thermal load from acceleration.
Your thermostat is defective if it remains closed or partially closed in boiling water. Watch the following video on how to test a thermostat in boiling water:
Solution
Replacement is the most reliable fix in the long run. Still, you can try applying silicone lube to the valves and seals. It may free up corrosion or residue, causing stickiness.
2. Air Pockets in The Cooling System
Air pockets rapidly expand under heat from acceleration. This displaces your coolant, reducing the amount available to circulate and absorb heat. Besides overheating, the primary signs of trapped air in the system are bubbling noises from the coolant reservoir.
Solution
- Bleed out the trapped air in the cooling system.
Watch this video on two ways to burp air out of your vehicle’s cooling system:
3. Partially Clogged Radiator
Blocked radiator fins decrease the cooling surface area. This limits the ability of your cooling system to reject spikes in heat under load.
Solution
You have to clean the partially clogged radiator to get rid of the situation. Follow these steps:
- Step 1: Use a plastic scraper to remove large debris on fins and core openings.
- Step 2: Disconnect the radiator hoses to avoid engine damage.
- Step 3: Flush your radiator with a pressure washer from the inside out. You can use a radiator cleaner solution for the best result.
4. Radiator Cap Failure
A faulty cap can’t maintain proper system pressure, lowering your coolant’s boiling point. The high temperatures from acceleration vaporize the coolant, forming bubble pockets.
As I mentioned before, air pockets reduce your coolant’s ability to absorb and transfer heat. This causes your engine to overheat.
Solution
Check for proper sealing by removing debris that could prevent a tight seal. Then, pressure test the cap. If it fails, swap the radiator cap with an OEM replacement.
Watch the following video on how to pressure test your radiator cap:
5. Blown Head Gasket
Combustion pressure rises dramatically under acceleration loads. If a head gasket seal is weak, it can’t contain this spike load, causing the coolant and oil to mix.
The coolant/oil mixture can’t transfer heat effectively. Typically, the tell-tale sign is oil that’s milky or emulsified. Also, you can do a combustion test as shown in the video below:
Solution
Replace the head gasket if blown. Use quality OE-specified multilayer steel gaskets for best results.
6. Water Pump Failure
Acceleration increases your engine’s thermal load. So, your water pump has to work harder to maintain a sufficient coolant flow.
However, if the pump fails, it can’t meet the increased flow demands. This reduces its ability to draw heat away from hot spots.
Symptoms of a water pump failure are:
- Coolant in If your car overheating while idling, you might find our articles on car overheating while idling helpful.Our article on car overheating while idling explores some of the common reasons why this issue can occur, such as low coolant level,thermostat malfunctioning, faulty head gaskets, etc. To learn more about these potential causes and what you can do to fix them, check out our articles on car overheating while idling. Also car overheats when driving but not idling might help sometimes. oil due to pump shaft seal and housing failure.
- Loose pulley from a wobbly impeller.
- Chattering/grinding noise indicating a seized or damaged bearing.
Also, inspect the mounting surface for cracks.
Solution
Install a new water pump and use new gasket sealants.
If your car is overheating while idling, you might find our articles on car overheating while idling helpful. Our article on car overheating while idling explores some of the common reasons why this issue can occur, such as a low coolant level, thermostat malfunction, faulty head gaskets, etc. To learn more about these potential causes and what you can do to fix them, check out our articles on car overheating while idling. Also, a car overheating when driving but not idling might be helpful sometimes.
FAQs
Let’s look at FAQs related to car overheats only when accelerating.
Q. Is there a temporary fix for an overheating car during acceleration?
Yes. Drive slowly without high RPM until proper repairs can be made. Also, you can improvise airflow by driving with the hood partially open.
Q. How do I know my engine is overheating without the thermostat?
You’ll see steam coming from the engine compartment. Modern cars will indicate an overheating warning light on the dashboard.
Q. Why doesn’t a faulty fan play a role in overheating during acceleration?
At high speed, your car relies more on airflow from moving than the cooling fan. Your fan is a backup system that provides cooling at lower speeds or when idling.
Conclusion
Identifying and fixing cooling issues when accelerating will avoid catastrophic engine failure. With these repair tips, your car’s cooling system should normalize regardless of the driving condition.
I know the root cause may not be obvious at first. So, be systematic and patient in testing components. Also, after repairs, monitor the temperature closely to ensure the problem is fully resolved.