Diskman

Half way home last night, the A/C started blowing hot air only. I glanced down at the instrument panel and noticed that the temp gauge is around the 3/4 mark. I've never seen it above 1/2 in the past. I then shut down all nonessential electrical load and the engine temp gradually dropped back to just over the 1/2 mark. Within a few minutes it climbed back up again. Luckily I was close enough to home that it never went above 3/4 mark. I'm wondering if the thermostat is sticking. But why did the A/C blow hot air only? Is there a mechanism that disables the A/C compressor with elevated engine temp? Will hook-up the VAG-COM tomorrow to troubleshoot it further.

s4master1

Once the temp reaches 105 degrees celsius, the compressor gets deactivated to lower heat generating and lower load in the engine.

Mister Bally

Confirming what S4master said and adding the following. It appears that you thermostat had stuck. My condolences since you may as well do the timing belt replacement as it has to come off.

However, I have also had the $25 temp sensor fail causing no a/c, radiator fans on full blast but the temp gauge will be pegged to the left (ice cold).

Your profile say 2005 A8 so that is what I'm going by. FSI engines on newer models have an easier to access thermostat. And, those usually fail in an open mode.

ryan_qttro

if the car gets too hot it will turn the heater on to help cool the coolant. Most likely your thermostat is sticking closed. I've seen alot of A8 thermostats stick close.

Diskman

Ran car for over 30 minutes today in 90-degree heat while monitoring with VCDS. The AC was set at 64F. Coolant temp started off at 33C then climbed to 113C. Compressor was still engaged and engine temp remained at 1/2 mark. Both fans were running at ~54% duty-cycle. No blockage (external) on condenser. Timing belt and water pump assembly were dry. Will monitor it a couple more days but my gut feeling is that the thermostat is sticking. Was hoping to delay the TB/WP change until next year. Vehicle only has 46K miles on it.

s4master1

before you start thinking on removing parts get a temp reading from the instrument cluster (17) and the a/c unit (08), and compare them to the ecm (01),
if all 3 within 5 degrees difference you can proceed with the diagnosis, if larger difference is present I replace the coolant sensor first.

don,t feel so bad about timing belts.

Brozee

Wow, great pic S4; is that really how much the chain piece can wear? How many years/miles were on that car?

Diskman

s4master1,

Both ECM and Instrument cluster read 109C but Auto HVAC (8) did not have an advanced measurement option. Still think it's the thermostat?

Thanks for suggestions!

s4master1

120k

s4master1

i believe is in channel 8 for temp reading in the a/c. eirthe way.
set the heater at max with the econ button depressed and see how hot is the air coming out of the vents, if really hot it means the water pump is moving the coolant, at the same time measure or touch the radiator on both sides and see if is cold on one side or both. if the radiator is cold and the heater is hot your thermostat is stuck close.