Heat Only Works When Driving (Key Causes and Fixes)
Chris is Head of Content for FindTheBestCarPrice and is based out of Philadelphia, PA. As a seasoned automotive industry analyst and car enthusiast, he ensures the highest level of quality across all our content and curates our picks for the best deals each month.
Chris studied information systems and marketing at Drexel University and writes about a wide range of topics ranging from car buying tips to troubleshooting common mechanical issues.
When he’s not thinking about cars, he likes to stay in with his dog and make an “attempt” to finish a crossword puzzle (he’s not quite at the Saturday/Sunday level…yet). As a former cheesemonger, Chris still has a “sharp” passion for all things cheese, and his fridge is always loaded with it!
For most individuals, each season elicits a mechanical quandary of sorts. Among the most vexing of these issues is the malfunction of one's automobile's heating system during winter.
This complication is a common yet baffling variation when the heater functions solely when the vehicle is in motion.
Depending on the nature of the problem, it is probable that either the thermostat or the heater core is the root cause for the heat solely functioning while driving and thus calls for replacement.
However, if the issue is more critical, such as a faulty water pump, you must anticipate significant fixes.
We have comprehensively studied the inner workings of automotive heating systems, particularly this predicament, to ensure you remain cozy in your vehicle regardless of its mobility.
This article will cover all the factors responsible for why heat only works when driving. Sit back, read, and enjoy!
Table of Contents
What Is a Heater Core?
Akin to a radiator, the heater core is a contrivance employed to warm the interior of an automobile.
Situated within the dashboard, it receives scorching coolant from the engine, which heats the air circulating within the cabin.
The heated fluid subsequently circulates back to the engine to be cooled. In certain vehicles, the heater core also furnishes warmth to the floor vents.
The failure of a heater core is the catalyst for inadequate cabin heating and the possibility of engine overheating.
A leaking heater core results in coolant seepage within the cabin, which poses a risk of harm to the interior upholstery.
What Is a Thermostat?
A thermostat regulates the temperature of a system, such as an engine, by regulating the flow of coolant to the system.
Positioned between the engine and the radiator, it incorporates a wax pellet that swells and shrinks with temperature changes.
As the engine heats up, the wax pellet expands, and the valve opens to permit coolant circulation through the radiator, thereby cooling the engine.
Upon attaining its peak operational temperature, the thermostat enacts the occlusion of the valve to avert coolant circulation through the radiator.
Should the thermostat become jammed in the open position, the engine would be prone to overheating since coolant would continually circulate through the radiator.
Conversely, if the thermostat malfunctions and remains fixed in the closed configuration, the engine would be rendered incompetent in reaching its optimum temperature, culminating in overheating.
Heat Only Works When Driving (Causes)
The heating mechanism of your automobile operates in conjunction with the engine's cooling system. Instances of irregular cabin temperature can often be linked to predicaments with the coolant.
If your vehicle's heating system is functional solely when the car is in motion, the underlying causes could be attributable to the following malfunctions:
1. Airlocked Coolant System
When your automobile's heating system functions exclusively during motion, it may be an aftermath of air pockets that impede sufficient coolant circulation. These air pockets could arise from insufficient coolant or a faulty radiator cap.
Nevertheless, the root cause of such a circumstance is frequently the insufficiency of coolant levels that culminate in lackluster engine performance, overheating, and the emission of irregular heat or cold air from the cabin vents.
2. Clogged Heater Core
Although a blocked heater core is not frequently encountered with car heaters, it could ultimately serve as the underlying cause.
The heater core utilizes hot engine coolant from the cooling system to generate warmth within the cabin. Once this component becomes obstructed, it could lead to various car heater complications.
The process for heating your vehicle involves the water pump circulating hot coolant through heater hoses and delivering it to the heater core.
The blower motor is then utilized to move air over the heater core fins, which produces warm air emanating from the A/C ducts.
In some vehicles, a valve opens to permit hot coolant to flow through the heater core when you activate the heat function. The coolant, generally heated to around 200 degrees, distributes the hot air necessary for your car's interior.
The obstruction of your heater core may result from debris accumulation or inadequate cooling system maintenance.
Consequently, seeking a professional's assessment is preferable when grappling with issues involving the heater core. Accessing this component can be arduous (located deep behind the dash in most cars), and the repair process could be messy.
3. Thermostat Is Stuck Open
The thermostat in your vehicle serves the critical function of regulating the engine temperature by modulating the flow of coolant out of the engine.
However, a buildup of debris and corrosion can impede its proper operation, causing it to remain unclosed.
Consequently, the engine may not attain the requisite temperature, which, in turn, can render the heater incapable of generating adequate warmth. In effect, your car's heat will only work when driving or cease to emit warmth even when the vehicle is stationary.
4. Low Coolant
The function of coolant extends beyond engine temperature regulation to include the transmission of heat from the engine to the radiator, which helps maintain suitable temperatures for your car's interior.
Coolant is essential for absorbing and transporting heat to the radiator for dissipation as the engine generates heat.
When your car's heater only provides heat while in motion, it may indicate insufficient coolant levels. This could indicate underlying issues within the vehicle, as a well-functioning coolant system is crucial for multiple components in the car.
Consequently, the absence of heat during idle often indicates a malfunction within the coolant system. It is imperative first to inspect the coolant level, but one should not open the cooling system if it's under pressure or hot, as this may lead to personal injury.
Inadequate maintenance or leaks in the coolant system are common reasons for low coolant levels.
5. Radiator Leak
A leak in any part of the radiator system can trigger various issues mentioned earlier. The leakage leads to a loss of coolant, which can result in problems with the heater, such as air blockages and other unfavorable side effects caused by insufficient coolant.
Detecting the leak's location is a relatively straightforward process that involves visually inspecting the radiator and checking for coolant seepage in areas where hoses are connected.
The solution depends on the leak's origin. If the leak originates from a crack or other damage on the radiator, it may require the expertise of a mechanic or the need for a replacement.
However, if the source of the leak is the end of the hose, a simple tightening of the clamp holding the hose should resolve the issue.
6. Defective Heater Valve
The next probable culprit is a malfunctioning heater valve, which functions similarly to the thermostat but diverts coolant to the heating core instead of the radiator.
To diagnose this problem, run your vehicle with the heat on for approximately 10 minutes, then locate the heater valve.
It is usually positioned on the firewall behind the engine. Carefully touch the hoses on either side of the valve. The valve is defective if the hose on the engine side is hot and the heater side hose is superb.
To rectify the issue with your heater valve, you must first determine if it is cable, vacuum, or electronically operated.
Identifying the operating mechanism will enable you to examine the corresponding heating valve. You can manually adjust it, use a vacuum, or fix it electronically to resolve the problem.
How To Test and Unclog a Heater Core
To determine if a heater core blockage is responsible for the issue, run your car idle for approximately ten minutes while the heat is on.
Then carefully examine the hoses on the firewall behind the engine leading to the heater core. If the hoses exhibit varying degrees of warmth, then it is highly probable that a blockage is present.
Fortunately, a straightforward method of resolving this issue involves using a flushing agent.
These agents have formulated cleaners that can safely penetrate your cooling system and alleviate blockages, including those affecting the heater core.
