# 2.5 overheating after 20 minutes of driving, blowing heat intermittently



## southafricanmatt (Feb 3, 2006)

2009 new jetta 2.5 liter! 

Ok guys, I'm scratching my head on this one. Please give me some insight on this! Especially if you've had the same problem and solved it!!

I've replaced:

1. new thermostat - replaced with a dealer OEM
2. new water pump
3. new radiator
4. flushed the system out - from the inlet and outlet hoses going into the engine until the water ran clear. TheRe was some residue in the system (which looked like aluminium hydroxide, like someone had put an additive in the system before). 
5. put a cylinder head tester on it and the cylinder head gasket is good! 

The lower radiator outlet pipe goes warm but not hot intermittently, so it goes cool most of the time like the thermostat is not opening! 

So I went back to the dealer and got a new thermostat thinking it was a faulty thermostat they gave me. Took out the new thermostat that I had just replaced, put in a new one... thought the problem was solved, because after I started the car the heat was blowing nice and hot and the lower radiator hose was also hot!! I was exstatic!!! 

BUT the PROBLEM persists!!! I drove it the next morning and overheating after 10-15 minutes of driving :banghead::banghead::banghead::banghead:

PLEASE GIVE ME SOME INSIGHT! Thanks chaps!


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## southafricanmatt (Feb 3, 2006)

Also I use g12 from the dealer! Don't think the previous owner did though as the previous fluid was green! 

The fans work perfectly and come on when it hits 190 degrees. 

It doesn't overheat at idle only when driving after 10-15 minutes,

the inside of the vehicle only blows heat when i rev the car, at idle absolutely no heat!

all the fuses are working perfectly!!


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## le0n (Jun 12, 2013)

sounds tricky.

have you tried running it temporarily without the thermostat?

am i correct in assuming that you did all of these repairs because of the overheating symptom?

edit: do you think your belt tensioner is doing its job? any lack of tension there can keep the pump from turning properly.


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## southafricanmatt (Feb 3, 2006)

Thanks for the reply Leon.

Yes I performed all the work previously listed due to the overheating condition. 

Great thought regarding the tensioner (hopefully that's the issue), the belt was squeeking a bit, I will check this later on today.

No I haven't tried to run it without the thermostat in it


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## southafricanmatt (Feb 3, 2006)

the new belt tensioner did not do the job! 

Ready to drive this car off a cliff! :banghead:

The bottom radiator hose is ice cold, it was blowing heat inside, top hose hot, then I took it for a small test drive, water was bubbling like mad in the reservoir, and the fans were blowing off cold air off the radiator.


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## gugu1981 (Jul 25, 2011)

if the upper hose is hot, that means the thermostat is working, right?

if air through the radiator is cold, and lower hose is cold, does that mean the radiator is clogged for some reason?




Peter



edit: maybe take the radiator off, turn it upside down, run some water pressure from the outlet back in, see if anything gets washed out through the inlet?


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## Ronny Bensys (Apr 17, 2014)

I'm not very sure but some thermostats can be installed backwards. I have no idea whether our thermostats are designed in a way that prevents this. What I am pointing out are you sure you've installed it in proper direction and everything is in good order inside housing?


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## southafricanmatt (Feb 3, 2006)

Thanks for the input Ronny and Peter. 
To answer your question, I have installed the thermostat pointed towards the engine correctly with the bead in the upwards position. I do see the thermostat opening and closing by observing the water reservoir fluid level going up and down as the thermostat opens and closes. 
-----------------------------------------------------------------------------------------------------
Now for the update on this mission! 

I decided to flush the system a second time, this time including the heater core (i got so much crap out of there it wasn't even funny - it looks like the previous owner put a sealant in the cooling system to seal off a previous leak). 

After I had flushed it, I started it and ran the car up to 190F and the heater inside the car was blowing hot hot heat! And the lower radiator hose was nice and hot! The fans were kicking in and blowing nice hot heat off the radiator - problem solved I assumed! I was ecstatic! 
Took the car for a drive and then after about 20 minutes of driving again it started to overheat!! 

I parked the car, checked the lower radiator hose - ice cold. 
radiator fans are working but now blowing cold air off the radiator. 
the heater didn't seem as hot as it was initially before the test drive. 
------------------------------------------------------------------------------------------------------

What do you guys think?? Still clogged somewhere else? air pocket and needs to be burped more? I'm almost about to start stripping this thing down to the cylinder head and I don't want to!


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## donjuan1jr (Oct 8, 2008)

Try a dealer type tool to expel any possible air out of the system...lots of companies sell em

one i have is made by airlift.


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## Ronny Bensys (Apr 17, 2014)

southafricanmatt said:


> air pocket


This. Something is happening between thermostat housing and expansion tank. Your refill procedure was wrong. I've read that with lower radiator hose cold if you virtually unscrew thermostat housing you will find no coolant inside. Did you check how many liters of coolant you filled?


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## 325 (Jul 17, 2005)

*having the same concern, does anyone knows what he found?*

Iam having the same concern with the lower radiator hose gets cold and the cooling fans stays on longer. Does anyone knows what he found for his problem....thanks


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## Ronny Bensys (Apr 17, 2014)

Lets ask southafricanmatt what his attempts ended with. The best way as far for this issue is to use vacuum refill device. These are about less than $100 online.


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## Mwood1164 (Mar 19, 2020)

*Solution!*

For those that happen Across this post trying to solve this problem on one they are working on as I did. Only to get to the end of the post and find that no solution was posted. (just rude). Well if you find yourself with these symptoms before replacing everything as this guy did and I did pull off your oil cooler and you'll find it is clogged allowing your tstat to end up with an air pocket and not open. 

AND TO ALL THOSE THAT POST IN ANY FORUM WHEN YOU FIX YOUR PROBLEM TAKE A MINUTE AND POST THE SOLUTION PLEASE.


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## garryt (Dec 22, 2016)

Mwood1164 said:


> For those that happen Across this post trying to solve this problem on one they are working on as I did. Only to get to the end of the post and find that no solution was posted. (just rude). Well if you find yourself with these symptoms before replacing everything as this guy did and I did pull off your oil cooler and you'll find it is clogged allowing your tstat to end up with an air pocket and not open.
> 
> AND TO ALL THOSE THAT POST IN ANY FORUM WHEN YOU FIX YOUR PROBLEM TAKE A MINUTE AND POST THE SOLUTION PLEASE.


fully agree with that:thumbup:


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## JaxPlanet (Sep 17, 2018)

+1 to finishing out a troubleshooting post. OP probably made a stupid mistake and was too embarrassed to post it. :laugh:


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## in2turbos (Jan 1, 2004)

To all that have run across this I am going to tell you the solution as I've seen this three times in the last nine months,,. im a technician at a vw dealer in New Orleans 35 years experience, I also changed all these parts and the car still runs hot or overheats. The problem is the engine is full of slag in the cooling system at the cylinders. basically the engine is stopped up and has no cooling capacity. To fix this drain the coolant add 2 bottles of CLR and fill with water run this in the engine for at least 30 minutes or so then drain and then blow out the system with compressed air, you will have to flush with water and blow it out many times to get results. you will get a pile of muck and rust scale out of the engine, also blow out the heater core now, fill with water and do it again, now you replace your radiator and heater core and if your lucky you wont have to change your radiator and heater core twice. i changed all my parts first and did the CLR twice. As a rule I don't post my job here but I felt the need to save the vw peeps with the 2.5L engine. btw I went through hell with this first car.:banghead:


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## JaxPlanet (Sep 17, 2018)

in2turbos said:


> To all that have run across this I am going to tell you the solution as I've seen this three times in the last nine months,,. im a technician at a vw dealer in New Orleans 35 years experience, I also changed all these parts and the car still runs hot or overheats. The problem is the engine is full of slag in the cooling system at the cylinders. basically the engine is stopped up and has no cooling capacity. To fix this drain the coolant add 2 bottles of CLR and fill with water run this in the engine for at least 30 minutes or so then drain and then blow out the system with compressed air, you will have to flush with water and blow it out many times to get results. you will get a pile of muck and rust scale out of the engine, also blow out the heater core now, fill with water and do it again, now you replace your radiator and heater core and if your lucky you wont have to change your radiator and heater core twice. i changed all my parts first and did the CLR twice. As a rule I don't post my job here but I felt the need to save the vw peeps with the 2.5L engine. btw I went through hell with this first car.:banghead:


Thank you for breaking your rule for us!


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## in2turbos (Jan 1, 2004)

Ok


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## abadassdog (7 d ago)

Good evening I just replaced, water pump, was bad and pervious owner didn't put antifreeze. thermostat, radiator. Flushed Heater core 
Now it still overheating but when I disconnect heater and join both hoses the overheating stopped. Runs great did a 50 mile run freeway and streets. 
When I reconnected Heater overheating return. 
Is there a way to check heater core. Note puts out heat. Any solutions??


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