I have a 2015 Mazda 3 2.2L Diesel, and during winter, it’s like the heater doesn’t want to work at all. It’s blowing nothing but cold air, and it’s making my drives miserable.
The only time I get warm air is when I’m on the motorway doing about 50 mph, and even then, it takes around 20 minutes to get there. Once I’m back to city speeds, it cools down again after about 10 minutes. Most of my trips are at lower speeds (20-30 mph) and can last up to 40 minutes, so I’m left freezing the entire time.
Coolant levels seem fine, and it works okay in warmer weather. Is this normal for a diesel engine, or could something like the thermostat be faulty? Any ideas?
Koa said:
If the engine gets warm but the cabin doesn’t, it could be a clogged heater core.
I hope it’s not that. I’ve heard fixing a heater core can be pretty expensive.
Check out some videos online about flushing the heater core. Replacing it should be the last option. Sometimes the wrong coolant can cause gunk buildup.
Diesels in winter need some help to keep warm, especially at low speeds. Try covering part of the grille to help maintain the engine temperature. It’s a common trick for diesel engines. The thermostat probably isn’t the issue here.
@Shay
It’s usually between 0°C and -10°C (32°F to 14°F) for several months here. So, not crazy cold but cold enough to be uncomfortable during my commute.
If the thermostat isn’t the problem, would it still be worth replacing? They seem relatively cheap and not too hard to change. I might also flush the coolant system to check for any blockages.
@Darby
Replacing it likely won’t help. I’ve owned and driven plenty of diesels, and what you’re describing is typical for winter. Use some cardboard or a grille cover—it’ll make a huge difference. If the thermostat were bad, you’d have issues in more than just cold weather.
@Shay
Would covering the grille affect airflow to the engine? Also, back in the summer, I had a coolant overheating issue on a long drive. The coolant was boiling and over max. I released some pressure, let it cool, and it didn’t happen again. Could that be related?
@Darby
Grille covers don’t block air intake; they’re designed to help in cold weather. If you’re unsure, have a mechanic check it out. As for the summer overheating, that’s something worth mentioning during your next visit to the shop. Could be connected.
This really sounds like your thermostat is stuck open. Normally, the thermostat stays closed until the engine warms up, then it opens to circulate coolant. If it’s stuck open, the coolant flows all the time, so it never heats up properly at idle or low speeds. This matches the symptoms you described.