On this page, you’ll find a short video outlining the top reasons your vehicle has no heat at idle, some mechanics’ advice for diagnosing, and you’ll also find links to tools and parts you’ll need to fix it like a pro.
Common Causes
Heating issues are always a popular topic as the summer fades into the distance. Some heating issues are a simple fix, some are a little more work, and some are an unbelievable amount of work.
Three of the most common causes of no heat at idle are:
- Low coolant level
- Airlocked coolant system
- Faulty thermostat
Low Coolant Level
Low coolant is, on the face of it, an easy problem to solve – top it up, right? Yes, for most, that’s all that’s required to fix the issue. But some systems will require a little help – a system bleed. More on this below.
There’s a second concern with low coolant, and it surrounds the question of why it’s low. Now some engines just use a little coolant, which is OK, but an engine that requires a coolant top-up regularly needs investigation.
Airlocked Coolant System
Low coolant and air locking are closely linked. In some cases, they are cause and effect; when coolant runs low, the system tends to swallow gulps of air which airlocks the system and prevents coolant from moving around the circuit.
The result is no heat from the heater when the engine is at idle; at higher RPM the coolant pump has the power to overcome the airlocked system, and heat usually improves a little.
Some vehicles will self-purge the air after a coolant top, but some won’t. And for those that won’t, the system will need a bleed.
Low coolant and air locking for many vehicles aren’t the root cause of the problem; they are the result. Here’s a list of common root causes of both low coolant and air locking.
- Faulty coolant cap
- Leaking hose clamp
- Hose leak
- Faulty thermostat
- Faulty fan
- Radiator leak
- Heater core leak
- Head gasket leak
Faulty Thermostat
A thermostat is a valve that lives inside the coolant circuit and automatically opens and closes at a predetermined temperature. Its job is to help the coolant system get up to working temperature quickly. It does this by remaining closed until the engine heats the smaller volume of coolant.
A faulty stat that fails in the open position forces the engine to heat a larger volume of coolant, which takes much longer to feel the effects inside the cabin. In addition, when the vehicle idles, the coolant cools down quickly, affecting cabin temperature.
Anyhow here’s a table of common causes, diagnoses, and repair of no heat at idle.
| Causes of no heat at idle | Diagnosis | Repair |
| Low coolant level | Check level | Top up coolant |
| Air locked system | Check coolant system | Bleed coolant system |
| Faulty thermostat | Test stat | Replace stat |
| Faulty heater core | Check system temperature | Replace heater core |
| Faulty water pump | Replace pump | Replace pump |
| Faulty head gasket | Test head gasket | Replace head gasket |
Issues like coolant top-ups and bleeding air from the system are easy fixes. Replacing a water pump or thermostat is a more challenging task, and replacing a heater core requires dash removal, which is a day-plus job and a right pain in the ass.
A head gasket is also a possible cause and should be checked if other possible causes have been eliminated. Typical symptoms of a failed head gasket include regular coolant top-ups, air locking, and white smoke from the tailpipe.
Tools
A No-spill funnel makes filling and purging air from the coolant system easy. If you’re going to flush or change your coolant, this tool will make you look like a pro.
Combustion leak test kit. Head gasket failure isn’t always so obvious. In those cases, use this chemical leak tester. Check for combustion chamber leaks into the coolant system. Comes with a test tool and chemical; enough product here for about a dozen tests.
Perfect coolant system and rad cap test kit. Coolant leaks out, and air sneaking into your coolant system are common causes of overheating, poor heat, and intermittent heat. Very often, the issue is simple, a bad coolant cap or loose hose clamp.
This little tool, while not premium quality, is perfect for the occasional user. It pressurizes the system and the gauge, and your ears will tell you if there’s a leak. Check that it suits your vehicle before pulling the trigger.
Remote hose clamp tool made by Astro. Some coolant hose clamps are impossible to access without the use of this ingenious tool.
US-made Valvoline, a leading supplier of top-quality anti-freeze coolants. Valvoline antifreeze is premixed, so ready to fill or top up your coolant system as needed. Safe to use in all vehicles and mix with all colors of coolant. It’s conveniently delivered to your door by Amazon.com. Picture links out to Amazon.com.
About the Author
John Cunningham is a Red Seal Qualified automotive technician with over twenty-five years of experience in the field. When he’s not writing about car repair, you’ll find him in his happy place – restoring classic cars
You may find these links helpful:
- Beginner car maintenance page
- Car repair and troubleshooting index
- OBD fault code list
- Tools and parts page
- About the Author
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.