All Things Mk5 > How to Guides / Troubleshooting

Edition 30 Overheating

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exodus_ste:
Okay well by my reckoning, it could well imply that the stat isn't opening and allowing coolant to flow through the rad.

Not 100% sure how the cooling system works on these but i'm sure the pump will be working. It will pump coolant round the heater matrix system before the stat is supposed to open.

Think this also suggests it's the b**tard stat on the front of the engine thats the problem, not any of the inlines. Just my guess though, wouldn't wish that job on many people.

pudding:
I'm pretty sure manual ED30s don't have the inline thermostat, well, mine doesn't at least.  Not sure if the DSG might has one, but worth checking/replacing if yours is a DSG.

The OP's problem sounds like the main pump to me.  The classic the faster you go, the hotter it gets.  Coolant system pressure is about 1 bar, so that is some pressure for the impeller blade to overcome.  What can happen with the plastic ones (as GVK said) is the impeller spins on it's shaft rather than moving the water.  The mechanical bond breaks down over the years.

The mechanic was wrong to hold off on the belt/pump change as the pump is the heart of the coolant system, and the belt is long over due as well.

You might also want to check the secondary pump as well, which lives in the primary thermostat area.  That pump supplements the mechanical one and also circulates the coolant after shutdown to prevent hot spots.  @exodus_ste , the fans can come on as part of that process, so I wouldn't be concerned about that.  So long as they don't stay on permanently.  The after run circulation process usually lasts for approx 5 mins.

As for the fans not coming on, if both of them work via an output test, it proves electrical continuity at least, so you need to investigate the signals from the sensors.

exodus_ste:
I thought a manual had one inline stat, and DSG 2?

Pretty sure the coolant system pressure doesn't put extra load on the impeller of the pump. The whole system is running at 1 bar, and thats due to expansion from the temperature. The pump would see the load if it itself was creating the pressure. These are circulation pumps not positive displacement.

That said, if the shaft/impeller connection is a weak point that could still be the culprit.

I will stand corrected though if someone can tell me that it isn't the main belt driven pump that circulates coolant round the heating system. If running the climate at the highest temp allows coolant to flow around the matrix then doesn't that suggest that the pump is working?

Anyway, running with the cap of the reservoir and looking for a return flow will confirm.

Thanks pudding, however i have no idea how to check the output of the sensors. Can easily check for a resistance but not sure how it that and the actual temp correlate.

pudding:
My manual only has the main thermostat.  Above the gearbox is just a plastic housing with a temperature sender in it.  When I bought an inline stat from the dealer, it did actually come up on his screen that it wasn't applicable for my chassis number, but I bought it anyway  :grin:

Yeah a visible return flow in the expansion tank is another sign the pump is working, but unfortunately the symptoms of an impeller spinning on it's shaft aren't there at idle usually, only at higher engine rpm where pulling/pushing the water presents more of a resistance.

That is a good point about the heater matrix. Another symptom would be luke warm heating at motorway speeds compared to idle. Is the heater matrix in permanent circulation, or only after the stat opens up?  Some older VWs didn't get a matrix flow until the stat had opened up.....hence the frosty experience over winter when they jammed shut.

I would use VCDS to check the sensor output.  Engine coolant temp for the main stat and I *think* the second sensor could be in the HVAC controller, I can't remember.  The sensors normally sit at 85 ish degrees on a run.

cpk:
Just in case anyone was waiting with baited breath for the resolution of my problem  :wink: - it turned out to be the thermostat.  It was replaced (only after all the other expensive work), and the temperature gauge reaches 90C and stays there.  The fan starting up after switching the engine off has been cured too.

On the down side, the car appears to have lost its cruise control at some point during the process.  That's probably another thread though...

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