2002 C5 coupe with almost 100,000 on the odometer. Cruising for about 30 minutes between 50 and 70 mph when I noticed the temp gauge all the way over to hot. Pulled over to find no leaks or excessive heat coming from the engine bay but upper radiator hose was only warm and almost no pressure and overflow tank was full and only warm and no pressure. I figured a thermostat was needed but my shop mechanic drove it with no problems or overheating. I still think it’s the thermostat sticking and would appreciate any ideas.

 

Sounds like it's the Thermostat. No circulation of Coolant.

 

The IPC will drive the coolant gauge to max indication if the coolant temp data becomes unreliable. If there was a problem with the coolant temp circuit, you should have codes to support this? Did the mechanic check for DTCs?

What was the temperature reading when the mechanic drove the car? Did he confirm circulation and check temp with an IR thermometer?

 

He did check temps and radiator was pressurised. He could not get it to overheat and said the was flow. I’ve also noticed the temp rises at least 20 degrees at red lights and returns to normal once moving again. This started just before the overheating problem so now I’m thinking one or both fans are going out or possibly a collapsed hose. Any thoughts would be appreciated

 

Btw thanks Merv for the response

 

What does this mean? Did he check for circulation? Pressure and temperature at each hose? Thermostat housing?


...except you have not established that you have a over-heating problem. Did the system puke coolant? Did the coolant overtemp indicator illuminate? Check gauges indicator? Chime? Any DIC warning message? What are your temperatures at idle/parked. Are your fans operating (both run a the same time in high or low)?

Cooling Fan Control

The engine cooling fan system consists of two electrical cooling fans and three fan relays. The relays are arranged in a series/parallel configuration that allows the powertrain control module (PCM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage and ignition 1 voltage from the underhood electrical center. The ground path is provided at G102.

During low speed operation, the PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the cooling fan 1 relay coil, closes the relay contacts, and supplies battery positive voltage through the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left cooling fan is through the cooling fan 3 relay and the right cooling fan. The result is a series circuit with both fans running at low speed.

During high speed operation the PCM supplies the ground path for the cooling fan 1 relay through the low speed cooling fan relay control circuit. After a 3-second delay, the PCM supplies a ground path for the cooling fan 2 relay and the cooling fan 3 relay through the high speed cooling fan relay control circuit. This energizes the cooling fan 3 relay coil, closes the relay contacts, and provides a ground path for the left cooling fan. At the same time the cooling fan 2 relay coil is energized closing the relay contacts and provides battery positive voltage on the cooling fan motor supply voltage circuit to the right cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.

The low speed cooling fan is commanded on when the coolant temperature reaches 108°C (226°F). It is turned off if the coolant temperature lowers to 104°C (219°F). The high speed cooling fan is commanded on when the coolant temperature reaches 113°C (235°F). It is turned off if the coolant temperature lowers to 108°C (226°F). When the A/C is on and the coolant temperature reaches 85°C (185°F), the low speed cooling fan will be turned on at vehicle speeds less than 56 kPh (35 mph).

Engine Coolant Indicator(s)

COOLANT OVER TEMPThe IPC illuminates the COOLANT OVER TEMP indicator in the message center when the following occurs:

• The PCM detects that the engine coolant temperature exceeds 124°C (256°F). The IPC receives a class 2 message from the PCM indicating the high coolant temperature.
• The IPC will also illuminate the CHECK GAGES indicator and a chime sounds when this condition exists.

 

Thank you for the information. I seriously doubt it if the shop tech did much of anything since it drove normally for him. I had no lights or codes come up when I noticed the temp gauge pegged. I’m convinced it wasn’t overheating at all but there is an issue with the cooling. Picked up the car and drove around to get it up to operating temp of around 187 which it stayed at until stopping for a minute or two and it would climb about 5-10 degrees then back to 187 while moving. I’ve also noticed the fans running high when shutting down.

 

Ok, so are you the original owner of the car? The stock thermostat is 187 degrees, so if that you are maintaining 187 degrees with a stock thermostat ...it's because someone has changed the fan settings, it's a battle between the thermostat and running the crap out of the fans.

 

After getting my service report I see the tech installed a new radiator cap so that would explain the low pressure. How do you check and adjust the fan settings?

 

You will need HPTuner or someone that has tuning capabilities

 

By radiator cap, I assume you mean the surge tank cap was replaced?

You did not answer the question if you are the original owner.

Is the "shop tech" and the "mechanic" the same person in this story?

Sounds like you did not inquire what steps the tech/mechanic took to confirm the cooling system was operating correctly, but you are here seeking advice on what could be wrong?

You check the fan operation by verifiying the fans operate per the factory configuration. Did you read the information I posted about the fan operation?

 

Maybe you ought to make sure the AC condenser and radiator fins aren’t packed full of dirt, grass leaves etc. overheating at speed is usually an airflow problem.
The front spoiler supposedly needs to be in place as well.