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Discussion Starter #1
I've been doing research on this over the past few weeks, but it seems to be getting more persistant.

Occassionally, but not all the time, after a cold start I'll drive about 10 mins/5 miles and the engine starts overheating. At the same time, if I turn on the heater, ice cold air blows thru. I've come to the conclusion that the heater core is well on it's way out, and/or there may be some sort of blockage in the coolant system, but I'm not sure where. I've recently chased down and fixed coolant leaks in the lower radiator hose, rear engine coolant flange, and from the expansion tank itself, and I haven't found any more leaks, but I still get the overheating issue. My solution for longer trips has been to pull over and shut off the engine for a minute or two while it cools, maybe open the coolant cap slowly to release some pressure. Sometimes this fixes it, sometimes not.

While driving home after a near-overheat today, I heard a new sound, pulled over, and found the expansion coolant tank nearly completely filled with what appeared to be surges of coolant flowing into the tank. Shut the car off a minute, turned it back on, kept driving. It began tipping over the 190 degree mark, but suddenly came back down to a steady 190, and all of a sudden I had air hotter than I've felt in a while coming thru the vents. :hmmm:

My first step would to do a heater core flush, but I'm not sure I have the proper equipment for that, so I figured I'd just take it to my local trusty garage and have them check it out.

Is there anything else I should check for or have them check for? Are there perhaps any other reasons this might be happening?
 

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When's the last time your water pump/timing belt was done? Where did you get the pump? The heater core has nothing directly to do with overheating, but a bad pump could certainly affect the heat.
 

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Check antifreeze level, and thermostat. I used laser thermometer once to diagnose clogged radiator. They are about $20 from HarborFreight.
Check bubbles in coolant tank.
 

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Discussion Starter #4
Timing belt was done just under 3 years ago at 90k miles, I now have under 125k. Got the ECS timing belt kit with metal impellar.

Coolant was topped off recently after getting the lower radiator hose replaced 2 or 3 weeks ago after having a leaky old one, levels have remained full since.
 

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No special tools required other than a shop vac. Plenty DIY's on heater core flushes, but essentially "gently" force water in through the outflow. I used a garden hose and an adapter I fashioned out of other hose.

I had a pretty clogged heater core so I needed the shop vac to evacuate all the coolant (and subsequent water) to make room for some CLR. Had to do the CLR treatment a few times to completely clear the heater core.

Goes without saying, but bleed the system when your done.




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Because you had hot air coming from the vents, your heater core is not clogged.
You have other issues, either from water pump not circulating or something (read head gasket) is causing air pockets and impede on the flow.
Did you ever overheat this car, since you said you kept having coolant leaks?
 

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My solution for longer trips has been to pull over and shut off the engine for a minute or two while it cools, maybe open the coolant cap slowly to release some pressure. Sometimes this fixes it, sometimes not.
Your symptoms sound like pump or thermostat related, even though the parts are fairly new.

I wouldn't expect much to be gained by letting off coolant pressure, and this often just promotes dangerous boiling. Plus, the latent heat that is released causes a temperature surge- try holding a cooling system hose and then loosen the reservoir cap, you'll feel the hose get suddenly hotter, if the coolant is at running temp.
 

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Discussion Starter #8
Thanks for all the help so far. Seems a little early for the water pump to fail, but I guess those things happen.

Just gonna add a couple more details. I've been hearing a sound like there's water sloshing around in the dashboard for a while now, never really gave it much thought, but seems like the bit of research I did pointed to the heater core.

Also, I personally have never completely overheated the car to the point where there's steam billowing out of the hood. My near-overheating issues have only seemed to happen in the extreme cold, usually less than 25 F, and only after cold starts. The leaks I had were never severe enough to completely drain the coolant, and for a second I was contemplating a bad garage job, but even before I had the leak in the lower radiator hose fixed, I once or twice experienced this no heat/near overheat issue.

I was also wondering what correlation the lack of heat and overheating had. As I said, the heater was blowing out ice cold air while the temp gauge was bouncing around over 190, then suddenly a blast of hot air came out and the temp gauge sank back to 190 where it should be. Ran a few more errands today, and haven't encountered any above-normal temps since.
 

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Sure sounds like poor circulation. Typically, I'd say the impeller had separated from the shaft, but that's not supposed to happen with an aftermarket, metal-impeller pump. Still...
 

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Poor circulation, I agree. Either the pump has separated impeller, or you have an air pocket that keeps the thermostat from seeing the right temperature, and therefore it stays closed. Until things get really hot, and then it finally opens.
There is really not a way to find out for sure where the lack of circulation is coming from, you need to run some tests. Since you say you hear the sloshing through the core, I am thinking bad head gasket. Do your hoses get really hard after the engine warms up? Do you experience hard starting or starting with a couple of seconds of misfiring?
 

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Coolant nearing the point of boiling/overheating suggests that the coolant doesn't get recirculated through the radiator (where it cools down).
At the same time you either had an air pocket in the system, or the heater core had a blockage.
The heat returning out of the blue would suggest that the blockage worked itself out (maybe from constant pressure) - which it means you have now good circulation through the heater core (hence the water pump still does its job).
How recently did you fix that lower radiator hose?
Did you still have instances of near-overheating issues? If yes, then more than likely your T-stat is the main culprit in this whole charade - and it was all along since the coolant didn't/doesn't circulate through the radiator.
You should still do a heater core flush - bring it back to be fully decongested again.
 

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A bad thermostat will stop the flow through the radiator, but will keep the flow through the heater core. So he would have heat inside the car at all times.
 

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Right.
that's why I said "at the same time" he could have had a partially blocked heater core. The return of the heat all of a sudden would suggest an unblocking.
Also, that's why I asked if he still had the near-overheating condition after repairing the radiator lower hose. If the coolant still doesn't circulate through the radiator, the t-stat is possibly still the culprit. I didn't contest anyone else's opinion.

Just gonna add a couple more details. I've been hearing a sound like there's water sloshing around in the dashboard for a while now, never really gave it much thought, but seems like the bit of research I did pointed to the heater core.

As I said, the heater was blowing out ice cold air while the temp gauge was bouncing around over 190, then suddenly a blast of hot air came out and the temp gauge sank back to 190 where it should be. Ran a few more errands today, and haven't encountered any above-normal temps since.
 

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Discussion Starter #14
How recently did you fix that lower radiator hose?
Did you still have instances of near-overheating issues? If yes, then more than likely your T-stat is the main culprit in this whole charade - and it was all along since the coolant didn't/doesn't circulate through the radiator.
You should still do a heater core flush - bring it back to be fully decongested again.
The hose was fixed on the 14th, so 15 days ago. As an update, made a quick run over to a buddy's place late this evening, returned home maybe 2 hours later. Engine dead cold on both starts, both times it nearly overheated by the time I got to my destination.

Edit: lulz... just browsing ECS's thermostat section and found this in the description for the thermostat:
If your vehicle is overheating but not blowing warm air from the heater, the problem could be a thermostat that is stuck closed.
 

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The gurgling noise in the dash is because there is air in the system, this won't cause the other problems, but needs to be corrected.

It is unlikely for the thermostat to have an intermittent fault, your symptoms indicate that some of the coolant is freezing.
Do you have the correct ratio of G12/G13 to water ?

Can you give a better explanation of how you determine that it is overheating, and what does it go to on the gauge ?

I suggest you flush the cooling system, and replace the coolant with G12/G13 at the correct mix with distilled water, then bleed it.
I don't think there is a problem with the heater core, but it wouldn't hurt to flush it.

http://www.passatworld.com/forums/61-b5-information-base/383113-diy-bleeding-cooling-system-heater-core-flush.html
 

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Try bleeding like Tom says.
ECS is wrong, a blocked thermostat SHOULD NOT affect flow through the heater core. If anything, I would say it should improve it since all the flow goes only one path: heater core, instead of radiator and heater core. They probably know, just want to sell more parts ;)
 

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Discussion Starter #19
It is unlikely for the thermostat to have an intermittent fault, your symptoms indicate that some of the coolant is freezing.
Do you have the correct ratio of G12/G13 to water ?

Can you give a better explanation of how you determine that it is overheating, and what does it go to on the gauge ?
Last person to top off the coolant was the garage after fixing the lower radiator hose, not sure what mixture if any is in there. There is definately a pretty solid pink color to the liquid though, so I assume its probably at least half coolant or more.

After starting the car from a freezing cold start, the engine warms up normally as I drive, sits at 190 for a couple minutes, then after some more driving, the needle starts tipping past 190, and will reach the redline of the temp gauge before I decide to pull over and shut it off for a few minutes to prevent the catastrophic overheat. After the needle starts tipping past 190, the air coming thru the vents turns ice cold, even with hot selected on the dash.

Yesterday when I was driving mid-day, the temp needle was bouncing around just under the redline, then suddenly decided to return to normal 190 and I simultaneously experienced a blast of air thru the vents hotter than I've felt in a while. My hot air in the vents is never really that hot anyway, which is why I originally suspected the heater core.


When you start the car after sitting for a while, do you get any misfires?
No misfires at all. Haven't had misfires since I replaced my throttle body back in summer.


Flushing is all good and dandy, but I'm pretty sure my garden hose is frozen as well since it's been below freezing temperatures here for almost a week.

Also, if the coolant was freezing, wouldn't it then just unfreeze after sitting around in a boiling hot engine for ~10 minutes?
 

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After starting the car from a freezing cold start, the engine warms up normally as I drive, sits at 190 for a couple minutes, then after some more driving, the needle starts tipping past 190, and will reach the redline of the temp gauge before I decide to pull over and shut it off for a few minutes to prevent the catastrophic overheat. After the needle starts tipping past 190, the air coming thru the vents turns ice cold, even with hot selected on the dash.

Yesterday when I was driving mid-day, the temp needle was bouncing around just under the redline, then suddenly decided to return to normal 190 and I simultaneously experienced a blast of air thru the vents hotter than I've felt in a while. My hot air in the vents is never really that hot anyway, which is why I originally suspected the heater core.
Allowing your engine to overheat repeatedly like that isn't doing it any good.

If the needle is "bouncing around" and suddenly jumping up OR down, you most likely need to replace your coolant temperature sensor. But, that's not the worrisome part. The fact that the temperature appears to rise coincident with sudden loss of cabin heat really can only point to a lack of circulation. The thermostat fails open 99% of the time, and I've never heard of one being intermittent. Your idea of a blockage, which has at this point has repeatedly blocked and cleared itself, I don't buy either. That pretty much narrows it down to a large amount of air in the system, or a bad water pump. IMHO, you need to stop driving the car and get this resolved before you do some serious damage.

One additional blue-sky thought: Where did you get the radiator hose? I've never seen it happen on a Passat (or any other modern car) but I once had a 70's Fiat with a badly deteriorated radiator hose. At highway speed, the water pump would pull enough suction to collapse the hose and block the flow, leading to symptoms like yours. Any chance? Comments, anyone else?
 
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