My B5 A4 1.8T recently started overheating. The level was full so I bled it in case some air got in the system. Still overheats. I pulled the t-stat and tested it. It opened but I replaced it anyways just to cross that off the list.

While I had the bottom radiator hose off I ran some water through the radiator. It flowed through however using my garden hose it filled faster than it would drain. I'm not sure if that's normal. I filled it up and bled the system (via the heater hose with the hole in it with the heater on high) and it's still overheating.

When I got to work, the temp gauge was reading 3/4 to H. I grabbed my radiator hoses and the top one was really hot while the bottom one was still cool.

Also my waterpump is fairly new, I changed it less than 50k miles ago and it's the upgraded metal impeller one.

Any ideas?

 

Two things come to mind here. First it sounds like your thermostat is bad even though you replaced it. Second you said you ran "tap" water thru it. I am almost positive you have to have G12 coolant with either de-ionized or distilled water. Any other type of coolant and you risk serious damage to your water pump and other components. I would replace the thermostat and flush the cooling system and re-fill it with G12/ de-ionized water. I would also double check the temp rating of your t-stat. It is possible that your pump is starting to go out.

 

Did you find your problem??