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Heater-core hose blown off

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I am having yet another issue with my 98' Polo Classic (6kv2). A couple of days ago I drove it through town for about an hour (heavy traffic), and stopped in the parking lot.

As soon as I go out of the car I see a puddle building under the car. The hose connecting the thermostat housing to the heater core inlet was blown off the heater. (I suspect it happend just as I was parking) 

Clamp was there, hose did not seem to have any damage. Placed it back on, refilled engine with coolant/water - seems ok now.


The question is what caused this in the first place?

I've checked the thermostat - seems OK (does not overheat, return radiator hose is hot). In fact both thermostat, housing and temp sensor are brand new (2 weeks old).

There does not seem to be milky residue on the engine cap (water in oil). No oil in the expansion tank. No white smoke coming out of the tailpipe.

If seems to build a bit of white foam in the expansion tank when I am accelerating and water is pouring on it.

Could it be a head-gasket issue?

Does this polo series have any pressure release valve in the cooling system?


Thank you

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  • JCD37


  • caretakerplus


  • dvderlm


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You appear to have (more or less) eliminated a head gasket problem BUT - sometimes the checks that you have made could produce a different result, given time.


The pressure release valve that you are looking for is in the coolant filler cap, but these things tend to fail in the open position, preventing the system from pressurising. Quite often the only problem is that dirt has become trapped under the valve and is nothing that a good wash in water won't cure.


Is there any chance that the clip holding on the hose was simply not tight enough? If it has the sprung type hose clip on it, I would suggest that you change it for a 'Jubilee' clip.


I'm always a little curious when people have problems after fitting new parts - especially items such as sensors and thermostats. If these are genuine VW items, or of a reputable make, there shouldn't be any problems, but there are a lot of spurious aftermarket parts out there, generally sold at 'bargain' prices.


From cold, the basic check that the thermostat is working, is to start the engine, place one hand on the return hose and one on the flow hose.

What you should find is that both hoses remain cool. When the thermostat opens, the flow hose will become hot, but as cool water is flowing from the radiator to the engine at that point, the return hose will still be cool - for a while.


Suggest that you keep a close eye on the coolant level for a while - it may well drop a little if there is air in the system, but should eventually remain constant. DO NOT overfill the system, or every time that your engine gets hot, it will dump coolant from the pressure relief valve, then when the engine cools down, the level will be lower - probably at the 'normal' level.


It is recommended that you run 50/50 coolant (50% coolant 50% water)




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Thanks for the reply!

I did a test this morning - sealed a plastic bag over the expansion tank, and left the car idle for 2-3 minutes. The bag did not inflate - in fact, when I revved to 6000 it was actually being sucked in (which I would think is consistent with the pump sucking more water). I think this rules out the head-gasket.

I've also did the test you recommended for the Thermostat. With engine cold, both top and lower hose remain cold. When engine is warm - both of them really hot - the lower one (radiator->thermostat) is a bit colder (I imagine it's normal as it does return from the radiator).

The thermostat was changed together with the whole housing + temp sensor. It came as preassembled pack from Calorstat by Verner - I have no idea about their quality (I understand Verner is OEM supplier for Volvo or something, dunno about their Calorstat brand).

The clip could have been re-seated by the mechanic recently, but I doubt it - I think it was there for years. I'm going to put an extra Jubilee clip just to be sure.

Regarding the cap (image). The pressure valve is the black thingy? If so, where does it vent out the pressure to? I couldn't find any hole/hose leading outside.


With coolant recently replaced (50-50), there could still have been air in the heater.

Could it be that this trapped air expanded and caused the extra pressure which forced out the hose?

Thank you!

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Air compresses. Water doesn't. That cap does not have a 9 digit VAG part number. I thought it would look like this, vents past threads


See the source image

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It is probably a case of 'the camera never lies' - except when you want it to !

If you tilt your 'Verner' cap to the same angle as the genuine one (in dvderlm's picture) it will probably look the same.

If there is air in the heater matrix, it will not cause extra pressure, but would stop or reduce the the heater output and when eventually it works its way out of the matrix, it will be replaced in there by coolant, causing the level in the header tank to fall.


Pressure in the cooling system is derived solely from the coolant expanding when heated, this compresses the air in the space between the coolant level and the top of the header tank (reservoir). It is this air that is dumped when the pressure release valve opens - this valve also incorporates an anti-vacuum valve which allows air back into the header tank as the engine cools down, preventing a vacuum forming in there which could well allow atmospheric pressure to crush hoses or even the radiator core/heater matrix.



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So in this case, can a bad concentration of coolant to water cause a higher than normal pressure? (I.e if coolant expands easier than water, and the mechanic could have accidentally done a 75/25 coolant-water mixture)?

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