How to determine component causing overheating?

Question

Car: 2008 Audi S5 4.2L

Background: Fan has been working very hard for the last 3-4 months (albeit it was summer, but even on cooler nights it was on max), temperature on dash dial steady in the middle. Recently (last 2 weeks), radiator fan stays on quite a lot longer after turning the car off (~5 minutes, originally didn't stay on when car is turned off).

Issue: Now, the car temperature fluctuates between the center and 3/4 level (I haven't seen it near the red line, nor would I let it). The needle isn't erratic, it slowly creeps up then down.

Tests:

1. Coolant is the correct color (pink).
2. Coolant level in overflow tank has not dropped, and is at max
3. Turning the a/c heater on max for a while causes the temp to drop, and the fan to be silent, and me to be sweaty (so there is heat from the heater core)
4. Turning the a/c heater off causes the fans to ramp up again, and the temperature to rise

The components I've identified are (in order of price and time to replace):

1. Faulty engine coolant temp sensor
2. Faulty thermostat
3. Radiator blocked
4. Faulty water pump
5. ??

From researching on this car, it seems like the thermostat is a common issue. However, it was replaced by the previous owner ~2 years ago (some people have replaced it twice). If a thermostat only partially opens, will this cause excess heat? I don't get any fault codes.

Since turning on the a/c heater brings down the temperature, this leads me to believe the water pump is working (I can also see the overflow pipe from the radiator pumping coolant into the tank). However, I'm not sure the likeliness of an corroded impeller causing a lower than required performance.

I'm not sure how to test a radiator being blocked, and am not sure how common a radiator problem is.

What's a good next step? I'm thinking try replace the thermostat (not an easy job, it's part of the water pump assembly). As these components are all quite expensive, I'm not sure what further diagnosis I could do...

Answer 1

A test you can do is to measure the heat from several places:

• Both upper/lower radiator hoses
• Both inlet/outlet heater hoses.

I'm thinking you may have either a clogged radiator or a bad thermostat. If the thermostat is going bad (sticking closed), you'd most likely see the top radiator hose being a lot cooler than the inlet side heater hose. If the radiator is clogged, the lower radiator hose will probably be cool (or most likely look warm) to the touch (though don't just go grabbing it without knowing the temperature of it ... may be a little uncomfortable).

More than likely it's the thermostat, at least this is where I'd lean.

The thermostat itself is not part of the water pump.

Also, remember, your A5 has two coolant pumps ... one mechanical (main) and one electric (auxiliary). The auxiliary one is used for after the car is turned off (I'm assuming it runs when the fans run to circulate the coolant when the engine has stopped running). If the thermostat isn't allowing coolant to circulate properly, I'd suspect it would give you your symptoms.