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Mike, Technician
Category: Audi
Satisfied Customers: 39198
Experience:  Technican - Business owner and Diagnostics for 34 years +..
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2002 AUDI A4 1.8T coolant not circulating. top radiator

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2002 AUDI A4 1.8T coolant not circulating.

top radiator hose hot bottom hose cold

has brand new water pump, brand new thermostat

have bled and bled, have flushed the heater core and the radiator

Roman :

Hello , Here to answer automotive questions and make you feel comfortable and become familiar with your product

Please explain how you bled the system? There is air trapped, did you open the bleeder in the top hose long enough to allow a solid stream of coolant to come through.


in doing so have the heat on hot and fan running on low to ensure the coolant circulates through the heater core?

Customer: replied 3 years ago.

what info do you need


Customer: replied 3 years ago.

yes many times


when do so make sure to let the car sit over night then top off the coolant with a cold engine. Try that. Sometimes these Audi and VW are tough to bleed let me know

Customer: replied 3 years ago.

I have done that I have been working on this for over a week. I have let it sit over night with the coolant reservoir elevated several times. I have squeezed all of the hoses.


When the top hose gets hot the bottom hose is stone cold. It seems like the top of the engine will boil the coolant tha tis in it but the bottom hose is still cold.


when I had the tstat out I felt the water pump and looked at it with a mirror and it is has a metal impeller and does not spin

you notice the impeller not spin then there you.
Customer: replied 3 years ago.
Relist: Answer quality.
Hello and Welcome -

My name is XXXXX XXXXX As you have relisted your question, I can help you with this.

It is a common issue with these models whereby the water pump impeller breaks up -or, it breaks on the spindle and while the outer of the pump operates, the actual impeller is not doing so - The centre is spinning but the impeller isn't and hence you get the problems you describe.

This has been known many a time to happen with even new pumps - it looks like you have this situation Im afraid.

You are going to have to remove the pump and physically check its condition now then and go from there with what you find... let me know how you go..

Let me know if you need anything more on this at all -

Meantime -- -

You need now to click on the OK rating to Excellent ratings service for me now, Thank you for that... and you can get back to me anytime if you need / wish..

Best Rgds, MIKE.

Please note, you are rating the content of my "answer" - my help to you - not the "system" which can experience delays etc which are not under my personal control -or delays caused by "time -off" which we all have on occasion - Thank you

Customer: replied 3 years ago.
While I was replacing the Tstat I stuck my finger in the opening to try and spin the impeller. It was solid I could not spin it with my fingers. Which I figured was good because of being timing belt driven.

Would you explain the electrical Tstat and what makes it open/close and possibly a good test for the radiator. I want to be able to exclude them because the pump is less than 30 days old. And the top of Tstat gets but bottom of Tatar stays cold
Hello again -

Thank you for your reply and further info..

I understand what you are saying - that makes sense yes - I thought , maybe wrongly , that you said earlier that the pump impeller you could see wasn't turning- with the engine running - in which case, what I said would have applied .. As you say, it wont turn when you do as you have - you would have to crank the engine to check - look at it that way...and its worth doing that - to see if the impeller is actually turning...before doing much else.

The electrical t'stat is operated ultimately by the Ecu - via input signals from the Ect sensor - which gets its info from the temp of the water... So there are areas for problems there - in that circuit.. You would get a fault code though if you have a fault in that circuit anywhere.

A good test for the rad.... not easy at all Im afraid. Radiators can be very misleading - because you can give them a good flush - with hosed water - and water is gushing out the bottom hole - so you think its ok.... but the rad is as you will know, made up up lots of fins -- so the water can be just coming through them - and out... whereas several /lots of fins can be blocked - and causing your issues.

The best way I know of to test for a blocked radiator is to bung the holes- and fill it with boiling water - If it gets hot everywhere - feel every part of it - then you know its ok. If there are large areas of cold , you know its clogged -

You can test it on car - with the engine running - and use a laser or other temp probe- but that doesn't rule out other blockages in the system on car - such as in the pipes - a jammed shut t/stat or clogged radiator matrix.... So its best to do it "manaully" - as above.... I use a hot pressure washer - simply to get the hottest water I can - but you may not have one of those - so just use the hottest water you can. It will work.

Given your last sentence- ie, ..."and the top of Tstat gets but bottom of Tatar stays cold..."... I strongly suspect either the stat isn't working - either because it itself is faulty - or the circuit -You may need to start on checking that circuit... with the use of diagnostic tools I suggest really - though you can check the connections and so on first -
I hope that helps you further, let me know how you go -

Best Rgds - MIKE.

.................. Please do click on the OK / Excellent ratings button for me now - as thats how Im credited for helping you - Thank you in advance for that.

You can get back to me anytime you might need too after that - you are always welcome.

Rgds - MIKE...
Mike, Technician
Category: Audi
Satisfied Customers: 39198
Experience: Technican - Business owner and Diagnostics for 34 years +..
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