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2008 Hyundai Sonata 2.4 - poor heat #5217882 09/20/19 11:40 AM
Joined: Jul 2014
Posts: 1,366
Number_35 Offline OP
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My friend complained of poor heat in his '08 Hyundai Sonata. The car is in great shape with only 17x,000 km (so not much over 100K miles).

Checked it out yesterday evening. Lots of hot air pouring out of the vents at present, but it's been hot here. Winter, when you need the heat, is the real test.

In diagnosing poor heat I look at three things:

1. Is the vehicle coming up to temperature? Yes, per the temperature gauge (just a hair under halfway between C and H) and per my ScanGauge (c. 90 C).

2. If so equipped (and the Hyundai is), is the blend air door operating correctly? Yes, it seems to be. The air coming out of the vents goes pretty quickly from hot to ambient when the temperature control is turned from hot to cold, and vice versa.

3. Is there good flow through the heater core? Both hoses to the heater core were hot, indicating that coolant is passing through the core fine. To be sure, we unhooked the hoses at the heater core connections at the firewall, attached a long rubber hose section to each pipe, and flushed both ways with a garden hose. Blew compressed air through to clear the water out. Didn't get much if any sediment out, and the flow was good. (Captured all of the coolant in a big pail, so I don't think we missed any sediment.) Bless Hyundai for good access to the connection points. Don't know if the V6 is as accessible, but the inline-4 was great.

But here's the thing - of course the coolant will come up to temperature in this balmy weather. What we don't know, but suspect, is that the (original) thermostat is weak and is opening prematurely, not allowing the coolant to get hot enough in cold weather conditions. But how does one diagnose that when it's hot out? Yes, one can remove the T-stat and boil it in a glass pot with a thermometer and watch for it open, but at that point why not just replace it? Is there an easy way to test for a weak thermostat in-car when the weather is hot enough that the thermostat will definitely open anyway? Would way rather work on the car now in late summer/early fall than once it's cold out!

Thanks in advance.

Re: 2008 Hyundai Sonata 2.4 - poor heat [Re: Number_35] #5217890 09/20/19 11:51 AM
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JC1 Offline
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I would replace the thermostat. When was the last time the coolant was flushed?

I would also disconnect the hoses to the heater core and flush those out with a garden hose.


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Re: 2008 Hyundai Sonata 2.4 - poor heat [Re: Number_35] #5217902 09/20/19 12:12 PM
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Pelican Offline
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As JC1 said, replace the thermostat, that seems to be the only thing not working right.

Re: 2008 Hyundai Sonata 2.4 - poor heat [Re: Number_35] #5217906 09/20/19 12:13 PM
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WobblyElvis Offline
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Keep your hand on the radiator inlet hose from a cold start. It should suddenly become much warmer when the engine gets up to temperature. This should happen just as the temp gauge approaches operating temp. If this checks out and there is still not sufficient heat in the winter, I would suspect the blend door.

Re: 2008 Hyundai Sonata 2.4 - poor heat [Re: Number_35] #5217942 09/20/19 12:44 PM
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Lolvoguy Offline
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I agree, no point in second-guessing a wear item that's easily accessible.
When in doubt, change it out!

Also, some cars just have weak heaters. We used to have a 97 Escort with one.
Sure, the A/C in the summer was cold enough to freeze a polar bear, but come winter you'd really have to bundle up to go for a drive!
And of course, the heater was great in warmer temps (when you didn't need it).
Tried changing thermostats, coolant flushes etc, but nothing helped.


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Re: 2008 Hyundai Sonata 2.4 - poor heat [Re: WobblyElvis] #5217969 09/20/19 01:04 PM
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Number_35 Offline OP
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Thanks, excellent suggestion - I should have thought of this! We'll do a cold start with the ScanGauge hooked up, and note when the upper rad hose starts to feel warm. The thermostat is rated at 180 F (83 C). If the temperature changes in the hose much before that, we'll change the T-stat.

Re: 2008 Hyundai Sonata 2.4 - poor heat [Re: Number_35] #5218016 09/20/19 01:54 PM
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Snagglefoot Offline
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The scan gauge is only as accurate as the temp sensor the computer is wired to. Maybe use a temperature gun to check the temp at the thermostat housing.


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