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Discussion Starter · #1 ·
Dear folks

My 1999 passat is overheating. Replaced thermostat with new vw. Removed water pump and checked, no impeller issues (metal impellers). No leaks. top hose is super hot, bottom is cold. Starts good, temp rising normally reaches 90 and stays there for a good 20-30 minutes when idling. After which or if you drive for more than 10 km, it starts creeping up and continues to overheat. Checked everything, head gasket is good.

Any ideas please
 

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Pull back one of the coolant hoses going to the heater core (on drivers side nozzle, IIRC) and you'll find a bleed hole. Add coolant to the reservoir until it starts coming out this bleed hole, then reseat the hose. Take a test drive and see if this solves your problem.

If you're still having overheating issues after that, drain your coolant into a suitable container, take your thermostat back out (PITA, I know, but not as bad as other B5s), then drill a 2mm hole in the lip of the thermostat and reinstall it. The hole will help all the air within the cooling system to move upward towards the heater core bleed hole so it can be bled out.
 

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What engine? V6 also has 2 vents on the block.
PITA to get at, plus Aluminium. 5.5mm easy to strip the socket.

Will cause overheating in a V6 if not correctly vented.
 

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If you stop the car and let it idle does the engine temp come down fairly quick? As crazy as it sounds there have been people who've had the impeller be loose on the shaft so when driving they pumped little coolant but at idle they pumped enough to keep the engine at the correct temp.
 

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Discussion Starter · #6 ·
Thank you all for your insights. The vehicle has a 1.8T engine. We have done literally done everything imaginable except for the bleeding which I will certainly do first thing tomorrow morning. The engine heat does not go down that fast too, You can shut it off for 20-30 minutes, come back and it is right in the centre (90 degrees C).

I may add, that the coolant flow test with the thermostat in is negative (No flow through the top reservoir hose even with revving the engine, NONE). We tested it w thermostat removed , there is plenty of flow but as expected, the vehicle runs cold and will never warm up except if you are stuck in traffic. The faster you go on the highway, the cooler it gets. Does this mean something. We removed the water pump, it is in very good shape, metal impellers and tight to the axle as a brand new water pump is. Why are we having a negative flow test with a good water pump...
 

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The flow tests you did are meaningless, normally there is little if any flow through the reservoir while the engine is running.

CHECK OR REPLACE THE CTS, and its wiring.

There is a slight possibility that the thermostat is faulty, and probably nothing else could cause what you have described.
 

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Discussion Starter · #8 ·
we have directly measured the temp and compared to the temp sensor reading, it matched, engine top measures 115 and temp sensor measuring about 120 and goes higher from here. Top pipe connected to top rad hose, also gets very hot, bottom hose, is cold around 50 or so. Thermostat is a new vw, tested outside in hot water over the stove, and it functions as it should be.

Has any body have experience of drilling a hole in the thermostat.......
 

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We tested it w thermostat removed , there is plenty of flow but as expected, the vehicle runs cold and will never warm up except if you are stuck in traffic. The faster you go on the highway, the cooler it gets.
If the actual temperature is too high with the thermostat in and is too low with it removed, the thermostat is faulty or installed incorrectly.
All else being equal, there is no other possibility.
 

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Discussion Starter · #10 ·
I see your point and have wondered about this. The thermostat is installed as per the vw provided diagram. Thermostat pin is facing the water pump/engine (this vehicle has an external water pump),

Is this correct or not
 

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Discussion Starter · #11 ·
good evening friends. Thank you very much for your great advise. Problem solved. I followed the advise, bleed the coolant via the heater core hose hole and also drilled a hole in the thermostat. Now I have good flow test, vehicle warms up normally and holds the temperature at 90 degrees without over heating, tried it in heavy traffic and high speed for a long time, doubtful Thomas, it came through very nicely. I am ever grateful for all of you who provided the suggestions, thank you all.
 

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Discussion Starter · #13 ·
I bled the system via the heater core hose hole and also drilled a hole in the thermostat. This has totally fixed the problem. I have been driving the car now for two days with 70 KM both high traffic and high speed and no problem at all. Thank you again
 

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For the benefit of others who might read this thread.
Drilling a hole in the thermostat and bleeding the system DID NOT and could not fix this problem.
Maybe the thermostat was in backwards, or maybe its operation was impeded in some way.
 

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I bled the system via the heater core hose hole and also drilled a hole in the thermostat. This has totally fixed the problem. I have been driving the car now for two days with 70 KM both high traffic and high speed and no problem at all. Thank you again
Since a portion of your cooling system had a void occupied by air (as evidenced by your lower radiator hose never getting hot) when your cooling system had issues, did you happen to notice how much additional coolant was necessary to fill the system after drilling a hole in the thermostat lip?

Sometimes it's good to have an AEB. No other B5 1.8T has a thermostat that doesn't require removing the timing belt for access. :cool:
 

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All B5 1.8T have thermostats that don't require removing the timing belt for access, now the water pump is another story.
 
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Discussion Starter · #18 ·
good morning Friends. Great discussion and to wrap up. The bleeding and drilled thermostat hole has solved the problem. In checking with a mechanic, the drilling is not a bad idea as it helps mainly with deairing the system more than establishing a flow. Please keep in mind, most if faced with this problem and the lack of flow, will directly think water pump WHICH is not the case. Yes, agree, it is great to have a car with external water pump as such.
 

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good morning Friends. Great discussion and to wrap up. The bleeding and drilled thermostat hole has solved the problem. In checking with a mechanic, the drilling is not a bad idea as it helps mainly with deairing the system more than establishing a flow. Please keep in mind, most if faced with this problem and the lack of flow, will directly think water pump WHICH is not the case. Yes, agree, it is great to have a car with external water pump as such.
I don't know what game VWDOCTOR is playing, but this is false information, see post #14.
 
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