I had added onto an earlier Thermostat thread, however it was recommended to start a separate thread specific to my 2004 C5 vert.

I have a 2004 A4 vert and noticed the last two times I have been out driving and stuck in heavy traffic, that my engine temperature reached the red zone. Earlier this week, it happened the second time after a 15 minute ride to the post office in mid-70s, Florida weather, enough where the Check Engine light came on so I pulled over, shut the car off for 5 minutes and then restarted and drove home with the A/C off. The temps still creeped up near 240 but never went back into the red. Now I've noticed with every drive, the temps get up to 240 after about 10 minutes.

My question, is what is the proper order to diagnose the issue? I've also read a bunch of threads on this, but want to make sure I'm diagnosing the problem correctly. Obviously, I need to (1) check the radiator for any type of debris, etc. preventing the air flow. I have a very nice "Elite Engineering" screen installed underneath the front fascia that has done a great job in keeping plastic bags, leaves, etc. out, however since I drive west coast Florida roads, I'm sure there may be a bunch of sand and other smaller debris collected in the radiator over the past 18 years. After checking the radiator, I read I should also check the hoses, overflow tank pressure cap, thermostat, and water pump. But in what order for best diagnosis? To make things more fun, I installed a set of braided hoses and a Callaway Honker CAI way back in 2008, so the hoses are almost 14 years old. They look great, but I can't tell if any of the rubber hoses inside have failed or collapsed. In addition, I swapped out the OEM stat for a low temp stat after I installed the Callaway Honker CAI and had a CoW A4 tune.

I performed a secondary test yesterday. The car was idling in the driveway with the AC running with an outside, late afternoon temperature of 75 degrees. Below are the temps:

4:55 PM - Start
5:00 PM - 218 Degrees
5:05 PM - 219 Degrees
5:10 PM - 220 Degrees (Vertical Top of Gauge) - 1st Fan Turns On
5:15 PM - 230 Degrees
5:20 PM - 235 Degrees
5:25 PM - 240 Degrees
5:30 PM - 240 Degrees
5:35 PM - 245 Degrees - 2nd Fan Turns On
5:40 PM - 250 Degrees
5:50 PM - Near 260 Degrees Redline (After Engine Revs)

Appreciate any guidance as I'm considering going all-in this winter and performing a full proactive engine maintenance run with new (silicone) hoses, new DeWitts radiator, new belt, new plugs, new balancer, and new water pump. Outside of this recent engine temp issue, the car runs great.

Thanks!

 

After checking the radiator AND A/C condenser for debris, I'd change the thermostat. After that many years, if it's still original, replace it. The coolant temp sensor would be next. Honestly, I'd do both at the same time. Also, the high speed fans should be on at 235°, NOT 245°. But I doubt that's the primary problem. Going from cold start to 218° in just 5 minutes is mighty quick, if idling, and on the first start of the day!! Hope this helps.....

 

If the check engine light illuminated what DTC is stored ??…that may help in the diagnosis….what part of west Florida are you in ??…if you’re near Cape Coral/Fort Myers area bring the car by and I’ll look at it for you !!..also if you changed the thermostat you may need to “burp” the cooling system…procedure below.



1. Fill the system through the surge tank opening.
2. Fill half the capacity of the system with 100 percent DEX-COOL® coolant.
3. Slowly add clean drinkable water to the system until the level reaches to the base of the neck.
4. Start the engine.
5. Idle engine for 1 minute.
6. Install surge tank cap.
7. Cycle the RPM, idle to 3000 in 30 second intervals until engine coolant reaches 99°C (210°F).
9.Shut off the engine.
10. Refer to step 3 above to remove the surge tank cap.
11. Start the engine.
12. Idle engine for 1 minute and fill surge tank to 1/2 inch above COLD FULL mark on the radiator surge tank.
13. Install the surge tank cap.
14. Cycle the RPM, idle to 3000 in 30 second intervals until engine coolant reaches 99°C (210°F).
15. Shut off the engine.
16. Top off coolant as necessary, 1/2 inch above FULL COLD mark on the radiator surge tank.
17. Rinse away any excess coolant from the engine and the compartment.
18. Inspect the concentration of the coolant.

 

I just checked the DTC codes and had 3 codes for the HVAC, and 10 codes for the LDCM and RDCM (5 codes for each side). Not sure if the HVAC codes have anything to do with the temperature issue, but the codes are B0361, B0363, and B0367 which are linked to the left and right actuator feedback.

 

Those DTC’s have nothing to do with your cooling issue !!

 

Beyond just checking the radiator, it would help to actually clean it. Best method I have found is to go to the HVAC section of your local hardware store and grab a can or two of AC coil cleaner. It comes in an aerosol spray can. It's safe on the aluminum of the radiator, and washes clean without leaving any residue. Even if you don't have any large debris like leaves or bags 18 years of built up crud can restrict a lot of airflow as well as provide a lot of insulating to the metal of the radiator.

Once everything is clean, and you have verified the fluid in the system is clean and purged using the procedure above it will give you a good starting point to see what the problem is. C5s are notoriously hot-blooded to begin with, and even a properly functioning cooling system will struggle to keep up in certain conditions. Being a bottom-feeder design they pick up the absolute HOTTEST air available right from the surface of the road. If you're driving on fresh black asphalt on a hot sunny day you're probably ingesting air that's close to 140 degrees before it even hits the radiator.

Another thing to look at is your oil temperature. I'll usually run my car with the oil temp pulled up on the DIC. If the oil temp is high it can be causing the high water temps. The stock C5 cooling system uses the water system to cool the oil. Excessive internal friction in the engine (from a bad bearing or something) can cause a rise in oil temperature that can overpower the water system.

 

Agreed, but those are the only codes that appeared even though the check engine light came on so I'm not sure where to start the diagnosis. I will check again, however which module would generate cooling codes? SDM?

 

None of those DTC’s will illuminate the MIL but something like an ECT sensor will…the coolant temp must exceed 282 degrees for 20 seconds for the MIL to illuminate…the MIL will extinguish if the car is started 3 times and the PCM did not see a fault…is the MIL on right now ??…there is not really a “diagnostic process” you would use like for other driveability issues but other than checking for blockage of the radiator/condenser, burp the system, check the radiator cap and maybe look at replacing the water pump…you really don’t know if the ECT sensor is accurate but you could do a resistance check after the car sits over night…the coolant temp should be close to the outside air temp…see what the DIC shows for a coolant temp and check the temp in relation to the resistance chart…did you try running the heater and blower all the way up and see if the temperature decreases somewhat ??…there are no modules that will generate cooling codes…the SDM is the Sensing and Diagnostic Module which is the airbag module.

 

10-PCM codes, but I don't know if any of the coolant related codes would set a check engine light

 

Have you done ANYTHING to open, drain or insert anything into the cooling system?

Has the surge tank recently been empty for any reason?

The first thing I would do would be to make sure there is not an air lock in the heads. There is a very specific FSM procedure to eliminate air from the heads if the cooling system has been opened or drained.

Your 2004, if stock, does not have rear head coolant air bleed connections like 2000 and earlier models have, which makes it more difficult to remove air and more important to follow FSM procedure.

 

If you turn on your heater full blast (temp and fan), does your temp reduce? If it does, then your water pump and thermostat are likely ok and would lean towards a plugged radiator. This is of course you didn't do anything that would have introduced air into the system.

 

The only work was my annual oil change and check up in July at the dealership. Not sure if they introduced any air into the system if they topped off the coolant, etc. I will perform the heater test and see what happens. My concern is that I have 10+ year old braided hoses. They look great, but you can't tell if the rubber on the inside has failed or has collapsed.

 

Do the high temps only occur in slow city traffic, I would suggest taking your car out on the highway to see if the temps still climb up. If the temps stay normal during highway driving it would point to one or both of your electric fans not coming on in slow traffic. The electric fans are turned off at 35mph so they will not impede air flow, if temps still climb at highway speeds, I would check all those things already mentioned in previous posts.

 

Nice offer, Rob!!