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I recently had my car engine, radiator and thermostat replaced as previous engine overheated due to a radiator leak.

It seems to be working fine now however the heat isn't coming on when the engine is on. However when I start driving for a minute and it comes on and works fine.

Is this normal or should it be giving heat when stationary regardless of whether you are driving or not?

just to add, I timed it today, its was about 2c, I started the car and after 10 minutes despite being on the highest heat setting i could only detect a small increase in heat and I think the slight heat came after i revved it a little. i got fed up and just moved off. I may be wrong but it seems the heat increases only when driving.

  • Stationary as in when you first start the car, or literally at each stop light the heat stops? – JPhi1618 Dec 1 '17 at 18:53
  • If you stop, does the heat stop flowing? – Pᴀᴜʟsᴛᴇʀ2 Dec 1 '17 at 19:42
  • @JPhi1618 stationary when i first start. i timed it today and after 10 minutes it only became a little warm despite being on the hottest setting. again only after a driving for some minutes did it heat better. – James Wilson Dec 2 '17 at 2:22
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I am guessing that you have an air pocket in the cooling system. When the cooling system/ engine was replaced they did not remove all the air. While the car is at idle the coolant flow is insufficient to provide heat. You can prove this theory by placing the car in park and increasing the engine RPM and feel if the heat increases. You need to bring it back to whoever did the work and see if they can "burp" the air out of the system

  • This. Some vehicles are worse than others. In fact, I owned a 94 Pontiac with a 3100, and it had a coolant bleeding port. It took over 45 minutes of idling and adding fluid to get it bled. Another friend blew his head gaskets because he thought he had enough coolant when in fact he was a gallon low on a similar GM product. – Doug Dec 10 '17 at 1:43
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Once the engine has warmed up, you should get heat from the heater regardless of whether you are moving or not - for the first few minutes after starting from cold you won't, as engine doesn't have enough heat in it to heat up the incoming air.

  • I timed it today, its was about 2c and after 10 minutes despite being on the highest heat setting it was only a little warm and I think the slight heat came after i revved it a little. . i got fed up and just moved off. Might this be suggestive of something other than simply warming up from cold? – James Wilson Dec 2 '17 at 2:28
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The problem is not what heat setting you use or if you are driving or not. You won't get warm air if your engine is still cold. Cars warm up slowly when idling and it can take 10 minutes or more depending on outside temperature etc. That's perfectly normal. Especially diesel models get warm even slower.

When you drive, engine uses much more fuel, so it gets warm fast. For example my car does 0.6liters/hour when idling but it uses 6+liters/hour if I am driving 100km/h speed. That is more than 10 times more fuel burned. Therefore engine gets warmer faster.

If you get warm air when engine temperature needle is showing normal temperature. Then everything is fine and you don't have to worry about this.

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Usually there's a vacuum actuated valve which controls the diversion of coolant into the heater core. It's possible that the vacuum tube is either loose or leaking, and isn't able to provide enough vacuum to actuate the valve at low engine speed.

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Yes. If your temperature gauge is steady somewhere in the middle (where it "usually" is), your water pump is likely not pushing enough coolant through your heater core.

Likely causes for this not working are underdriven pulleys (you would have installed such a thing), air in the system, or a failing water pump.

  • soolus, once there's more heat the temperature gauge rises to just under half. however you say that's where it is usually(normally) meant to be but at the same time you say thats suggestive of a problem. seems like a contradiction, if thats where the gauge is meant to be, please clarify. i believe a new water pump was installed with new engine and i dont know of any underdriven pulleys installation. does this suggest their is air in the system? – James Wilson Dec 2 '17 at 2:37
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At idle RPM in cold winter weather, the engine is not producing enough heat to overcome cooling from the cold outside ambient air temperature.

Assuming a typical 4-cylinder sedan gas engine idling at about 900 RPM, it is firing 15 times per second at a likely lean fuel mix because all it needs to do to maintain that RPM, is overcome the friction of the bearings and the engine accessories.

When you get out on the road, the engine goes up to about 3000 RPM, firing 50 times a second on a rich fuel mixture, so much more heat is being produced.

The heater is likely producing some warm air, but it's not warm enough to matter to you. If you have the blower on high, the small heater core is trying to heat a huge moving air mass with limited thermal energy. Set the blower on low and it will feel warmer.

Also setting airflow to recirculate will help warm the cabin more quickly, though the recirculated cabin air may also fog and freeze the windows with moisture.

Turning on additional electrical loads like a rear window defroster and the vehicle lights will add load to the engine via the alternator, increasing the idle speed and fuel richness so it produces more heat. This effect will be limited, though, since the defrost heater and all the lights are about 2000 watts, or 2.5 HP of power for an automobile engine likely rated for 90-300 HP.

Slightly pressing the accelerator so it idles at 3000-4000 RPM will also increase the heat output somewhat, but it doesn't take much fuel to push an engine with no load up to this high idle RPM, so the warming effect is not as great as actually driving down the road at highway speed at this RPM.

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