Odd cooling issue

[dougkehl]

Recently my QSW developed a weird cooling issue that I cannot seem to pinpoint.

Over last weekend I went to Mk1Madness out in PA as I do every year, but I brought the QSW which I just finished painting to camp in this year instead of my Mk1. The location of Mk1Madness is 2.5 hours from my house and I drove a solid hour and fifteen minutes before I hit the toll to enter into Easton, PA. That whole hour and fifteen minutes the car ran cool as a cucumber, but within a minute or two of leaving to toll gate the temp gauge started to rise to an uncomfortable level. This only occurred while I drove at highway speeds, if I slowed the coolant temp seemed to drop.

This car has a brand new aluminum radiator and fan, low temp fan switch, low temp thermostat, external oil cooler, heater core and the water pump was done maybe a thousand miles ago when I did the timing belt. And the coolant system has been thoroughly bled and their should be no air.

The car shows no normal signs of a blown head gasket which was my first suspicion, but there is no smoke. Not at idle, not under a load. There is no oil in the coolant and there is no coolant in the oil. It starts smooth while cold, starts smooth while up to temp and runs smooth as butter now that I've replaced all the lifters as of last week. I've blown head gaskets on few cars, but it exhibits none of the same symptoms. I also suspected the new-ish water pump to be faulty, but I'm not so sure about that.

I ran the car yesterday in my driveway just idling to see what it would do.. The gauge showed the temp getting kind of hot and then the fan kicked on for like 15-30 seconds like normal and then shut off and it repeated this consistently until I shut the car off.. The whole time the temp gauge was higher than normal.

I know these use the same temp gauge as Mk2 Golfs/Jettas and I've read plenty of those go bad randomly, but I'm not sure that's it because of the fluctuations the gauge is indicating.

Any other ideas as to why the coolant would rise in temp while driving, but not idling would be very helpful.

[Berserker]

Sounds like a loose impeller on the water pump. Pumps fine at low RPM, but slips on the shaft at higher RPM.

[dougkehl]

Sounds like a loose impeller on the water pump. Pumps fine at low RPM, but slips on the shaft at higher RPM.

This is kinda what I was thinking in terms of a faulty water pump. I noticed yesterday while watching the coolant flow from the radiator to the expansion tank through the small hose, the flow seemed to surge and then slow down even though the engine was running at a steady 900 rpm.

[Noisy Cricket]

Check the bleed hose from the top of the radiator to the surge tank. I had weird issues with my QSW until I discovered that a chunk of red silicone came free from somewhere and plugged the hose.

If that hose doesn't flow freely, you will have mostly air in the engine and radiator unless you remove the upper hose at the radiator and fill through there. And then any air still in the system will migrate to the radiator and stay there...