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[Solved] VW Golf 4 1.6 16V 2000: Thermostat Issue, Engine Not Warming Up, Radiator Hoses & Fan Gears

Kubool96 7158 12
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  • #1 16992576
    Kubool96
    Level 7  
    Hello, I have a problem that the engine did not warm up in my Golf. I bought a new thermostat, cost PLN 34, it turned out that it does not open at all and the water boiled, I bought a new one (cost PLN 45), I also bought a temperature sensor in the head and a thermo-switch for fans in the radiator, now the car runs, but the temperature hint can be lifted up to 90C. and the upper hose from the radiator is very hot, and the hose that goes from the thermostat is cold, the same after driving about 20 km. I would like to add that I checked the fans briefly and only 2nd gear works, that is to the max, because as I waited for the system to vent, the fans did not turn on even once. I am asking for advice.
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  • #2 16992689
    jacekwwy
    Level 19  
    I do not know about you, but in Poland it is very hard to vent, so maybe try to vent better
    Ps. You didn't boil it by accident?
  • #3 16992718
    Kubool96
    Level 7  
    rather, it is well vented because it walked for a good 25 minutes, the first time and the first thermostat, the water boiled but maybe for 2 seconds and I immediately turned off the engine. Later I bought the second thermostat and so far only the bottom cold hose.
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  • #4 16992724
    jacekwwy
    Level 19  
    And you checked the water pump
  • #5 16992733
    Kubool96
    Level 7  
    I think the water pump is good because the car drives all the time, after replacing the thermostat, such actions ...
  • #6 16993756
    RADU23
    VIP Meritorious for electroda.pl  
    Kubool96 wrote:
    rather it is vented well because it walked a good 25 minutes

    Did he set the air circulation to the maximum warm during venting?
  • #7 16993930
    Strumien swiadomosci swia
    Level 43  
    Kubool96 wrote:
    I bought a new thermostat for PLN 34

    First of all, did you look if you have a damaged thermostat bumper in the middle of the housing?
  • #8 16994132
    Kubool96
    Level 7  
    that's how I set it to warm air to the max
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  • #9 16994138
    Strumien swiadomosci swia
    Level 43  
    RADU23 wrote:
    Did he set the air circulation to the maximum warm during venting?

    No matter G4 has no valve on the circuit, only mixing flaps, the heater is always hot.
  • #10 16994164
    Kubool96
    Level 7  
    yes i have a good thermostat housing.

    Added after 1 [minutes]:

    I think to try tomorrow if the car's heating is turned off, the hose will get warm inside.
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  • #11 16994166
    Strumien swiadomosci swia
    Level 43  
    Kubool96 wrote:
    yes i have a good thermostat housing.

    Have you looked inside or not ripped out?
  • #12 16994587
    Kubool96
    Level 7  
    Yes. is like new :)
  • #13 17000310
    Kubool96
    Level 7  
    the problem was solved, it turned out that the water was not boiling, only the pressure released it from the tank because I checked it on the unscrewed tank, after turning on the tank the fans turned on, greetings! :)
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Topic summary

✨ The discussion revolves around a thermostat issue in a 2000 VW Golf 4 1.6 16V, where the engine fails to warm up properly. The user replaced the thermostat twice, but the upper radiator hose remains hot while the lower hose stays cold after driving. They also replaced the temperature sensor and thermo-switch for the fans, but only the second fan speed operates. Suggestions include checking the water pump, ensuring proper air circulation during venting, and inspecting the thermostat housing for damage. Ultimately, the user discovered that the water was not boiling; instead, pressure was released from the tank, and the fans activated when the tank was closed.
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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

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?

  1. Fill coolant, start engine, and install the expansion tank cap.
  2. Let it idle until the gauge nears 90°C and fans engage.
  3. 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]

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]
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