FAQ
TL;DR: VW Golf 4 heaters use 0 coolant shut-off valves; “the heater is always hot.” If your upper radiator hose is hot and the lower is cold, bleed correctly with the expansion tank cap on so fans can engage. [Elektroda, Strumien swiadomosci swia, post #16994138]
Quick Facts
- Symptom snapshot: hot upper hose, cold lower hose even after ~20 km indicates thermostat/circulation or bleeding issues. [Elektroda, Kubool96, post #16992576]
- Golf 4 heater core is always in the circuit; HVAC uses mixing flaps, not a water valve. [Elektroda, Strumien swiadomosci swia, post #16994138]
- Bleeding with the expansion tank cap off can prevent fan activation and mimic “boiling.” Close the cap to finish bleeding. [Elektroda, Kubool96, post #17000310]
- Radiator fans have two speeds; a stuck first speed can mask correct operation during warm-up. [Elektroda, Kubool96, post #16992576]
- Inspect thermostat housing “bumper/stop” for damage or misfit before blaming the pump. [Elektroda, Strumien swiadomosci swia, post #16993930]
How do I fix a Golf 4 that won’t warm up and shows a cold lower radiator hose?
Bleed the system with the expansion tank cap fitted so pressure builds and the fans can engage. Verify the thermostat orientation and housing stop. Then road-test until the gauge stabilizes near 90°C and both hoses warm progressively. If the lower hose stays cold, recheck the thermostat and circulation. [Elektroda, Kubool96, post #17000310]
Do I need to set cabin heat to MAX when bleeding a Golf 4?
No. The Golf 4 heater core is always in the coolant loop. The HVAC uses air-mixing flaps, so heater temperature selection does not open or close a coolant valve. “The heater is always hot.” [Elektroda, Strumien swiadomosci swia, post #16994138]
Why did my coolant seem to boil during bleeding, but fans never turned on?
With the expansion tank cap off, the system can’t build pressure, so the boil point drops and the fan switch may not see trigger temperature. Fit the cap; the fans should then engage as designed. [Elektroda, Kubool96, post #17000310]
What does a hot upper hose and cold lower hose mean on this engine?
It often means the thermostat is not opening or the system still contains air. This pattern persisted for ~20 km in the reported case until the bleed was completed correctly. [Elektroda, Kubool96, post #16992576]
Could the water pump be the culprit even if the car drives fine?
Yes. A worn or loose impeller can reduce flow without immediate overheating while driving. If bleeding and thermostat checks fail, inspect pump condition during timing service. [Elektroda, jacekwwy, post #16992724]
How can a bad thermostat housing cause false diagnosis?
A damaged or missing internal stop ("bumper") can prevent the thermostat from seating or moving correctly, mimicking a stuck thermostat. Always inspect the housing interior when replacing the stat. [Elektroda, Strumien swiadomosci swia, post #16993930]
My fans only run on high speed—what does that indicate?
It suggests first speed is inoperative, so fans only jump to max. During warm-up or bleeding, this can hide normal stage-one behavior and confuse diagnosis. Repair stage one to restore proper staging. [Elektroda, Kubool96, post #16992576]
What is the radiator thermo-switch on a Golf 4?
It’s the temperature switch in the radiator that commands the fan stages based on coolant temperature. If replaced with the cap off, verify bleeding and pressure so it can sense correctly. [Elektroda, Kubool96, post #16992576]
What’s the quick 3-step way to bleed this cooling system correctly?
- Fill coolant, start engine, and install the expansion tank cap.
- Let it idle until the gauge nears 90°C and fans engage.
- Cool down, top off to the mark, and recheck both radiator hoses for warmth.
[Elektroda, Kubool96, post #17000310]
Is idling alone enough to purge air from the system?
Often yes, but only with the cap on so pressure and temperature rise sufficiently for the fan switch to trigger. In the case discussed, idling ~25 minutes with the cap off failed to start fans. [Elektroda, Kubool96, post #17000310]
I replaced the thermostat twice and still have issues—what next?
Confirm the thermostat orientation and housing condition, then bleed with the cap installed. If symptoms persist, test fan stages and consider pump inspection. Avoid extended idling if the gauge spikes. [Elektroda, Strumien swiadomosci swia, post #16993930]
Does the cabin heat setting affect coolant flow on the Golf 4?
No. The climate system mixes air at the flaps. Coolant always flows through the heater core, regardless of the knob setting. [Elektroda, Strumien swiadomosci swia, post #16994138]
What’s an edge case I should watch for during bleeding?
A non-working low fan speed can delay fan activation, making you think the engine is overheating or the thermostat is stuck. Check both stages before deeper teardown. [Elektroda, Kubool96, post #16992576]
How was the original problem in the thread finally solved?
By realizing the coolant wasn’t actually boiling; the open cap lowered pressure and faked the symptom. Closing the cap allowed pressure, the fan switch triggered, and the issue cleared. [Elektroda, Kubool96, post #17000310]
