Need help diagnosing a heater issue....

[ItsADiesel]

I recently noticed a severe lack of heat coming out of the vents on my 2002 Jetta TDI. I stuck a temp probe in the vents on full blast and the air was only 86 degrees. I thought the heater core might have been clogged up, so today I flushed the heater core and replaced the temp gauge while I was at it. I put it back together, topped off the coolant, and got the car to operating temperature, and I still had 80 degree air in the vents. I popped the hood to see if I could squeeze the radiator hoses to burp the air out of the system and lower radiator hose was cold; while the top one was hot.

So what would cause a car to be at operating temp, with a cold lower radiator hose, and no heat out of the vents to speak of?

Thanks, Fred.

 

[Vince Waldon]

"Blend door" is the perfect search term here... lots of great threads on this fairly common problem with aging MKIVs.

 

[ItsADiesel]

"Blend door" is the perfect search term here... lots of great threads on this fairly common problem with aging MKIVs.

I've actually had that issue in the past and it was the first thing I checked when I lost heat. The arm for the blend door is attached as it should be.

Plus, the blend door issue wouldn't account for the cold lower radiator hose

 

[Vince Waldon]

The common issue with the blend door is that the sealing foam on the blend door deteriorates over time.

The heater core still works, hence the cold OUT hose from the core, but the heat is not being properly diverted to the interior of the car because of the missing foam.

Here's one thread with some pictures, to get you started:

http://forums.tdiclub.com/showthread.php?t=225704

EDIT: I see now that you're talking about the lower rad hose being cold...my mistake. It's also pretty common for the lower rad hose to stay cool... except on a warm summer day the t-stat stays mostly closed... these engines don't put off much waste heat.

The heater core is in a different loop than the rad anyways... so to confirm you're getting warm water to the heater core it's the heater core hoses that are important... and one should feel cooler than the other if the core is working properly. May need an IR thermometer to detect the difference however.

 

[ItsADiesel]

Bump...... anyone else have some knowledge?

 

[JB05]

Cabin air filter clogged?

 

[CoolAirVw]

Are you getting foam out your vents? Is the thermostat sticking open?

 

[Smalls]

80-85 Degree air is going to be pretty close to optimum in the Mk4. Like a previous poster had stated, these cars don't produce much waste heat. They are really thermal efficient.

If it gets in the 20's and below, and you get stuck in traffic with the blower on high, expect to lose temperature and the thermostat to shut.

Make sure you have a good thermostat as well. Don't go by the gauge for 100% accuracy,

 

[Alchemist]

So what would cause a car to be at operating temp, with a cold lower radiator hose, and no heat out of the vents to speak of?

Thanks, Fred.

The engine is not hot enough for the thermostat to open, but the gauge is telling you it is up to normal temperature.

 

[ItsADiesel]

80-85 Degree air is going to be pretty close to optimum in the Mk4. Like a previous poster had stated, these cars don't produce much waste heat. They are really thermal efficient.

I'm not sure if you have ever actually used a temperature probe in the vents before, but 80-85 degree air is down right cold by the time it hits your face. We did a temp reading in my buddy's vents on his mk4 GTI and it was reading 165 degrees. The air in my vents on the other hand, 86 degrees.

 

[ItsADiesel]

Are you getting foam out your vents? Is the thermostat sticking open?

I actually did get a small piece of foam a few weeks back..... what does that tell me? No idea what the thermostat is doing right now, but my gauge on the dash says I'm getting to operating temperature and the lower hose is still cold.

 

[ItsADiesel]

The engine is not hot enough for the thermostat to open, but the gauge is telling you it is up to normal temperature.

Sooooo possibly an air bubble?

 

[UhOh]

I was going to post that there's an issue with the thermostat sticking closed but wasn't sure that that would account for a lack of heat inside the car (would account for lower radiator hose being cold, but I couldn't map this to the cabin heat issue).

Suspect there may be TWO things contributing to your problem. Good idea, however, to first make sure you don't have air trapped. If you got foam blowing out then, yes, you've also likely got a blend door issue: both my vehicles' blend doors seem fine- they're northern climate cars in which case I'm wondering whether cars exposed to warmer climates are more likely to encounter this issue.

 

[turbovan+tdi]

What do the heater hoses feel like? If inlet is hot and the outlet is cold, heater core is plugged. I like to use CLR, let it soak for an hour and backflush.

EGR system working?

I'd change the thermostat and the green sending unit with OEM type units.

 

[ItsADiesel]

What do the heater hoses feel like? If inlet is hot and the outlet is cold, heater core is plugged. I like to use CLR, let it soak for an hour and backflush.

EGR system working?

I'd change the thermostat and the green sending unit with OEM type units.

I literally just flushed the heater core (as stated in the OP). The EGR system has been deleted. I replaced the thermostat spring 2014, so I kind of doubt it went bad already, but it's possible. I just replaced the temp sensor when I did the heater core flush.

 

[ItsADiesel]

I just read up on the blend door foam issue and I think I might give that a go later this week. The car does have 201k miles, so it wouldn't surprise me if the foam was non existent at this point.

Here's a couple links for anyone that is also interested in tackling the blend door foam issue.....
https://forums.tdiclub.com/showthread.php?t=413341

http://forums.vwvortex.com/showthread.php?5984439-Mk4-blend-door-foam-repair-diy

 

[turbovan+tdi]

I literally just flushed the heater core (as stated in the OP). The EGR system has been deleted. I replaced the thermostat spring 2014, so I kind of doubt it went bad already, but it's possible. I just replaced the temp sensor when I did the heater core flush.

Flushing can work but sometimes they are bad enough they need cleaning. I use CLR, blow out the water, hook up some hoses and fill with CLR, let it sit an hour and back flush. Hasn't failed me yet if the core was plugged up.

Depending on what thermostat you used, it could have gone bad. Aftermarket and some OE replacement parts are just not as good as they should be,