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Hello all,
I have a coolant temperature issue with my 03 C5. The temperature will slowly climb to approximately 227 degrees and then immediately drop to around 217 degrees. Would this be a thermostat issue?
Thanks,
Larry
Last edited by runnjump; Feb 25, 2026 at 07:22 PM.
Your temps are normal. New cars run hotter than your used too.
C5 Cooling Fans Operation 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
Hello all,
I have a coolant temperature issue with my 03 C5. The temperature will slowly climb to approximately 227 degrees and then immediately drop to around 217 degrees. Would this be a thermostat issue?
Thanks,
Larry
I agree this appears to be normal operation of the electric radiator cooling fans. I presume you are describing sitting at idle or at low speeds with the AC off?
Question: What do you mean by "immediate" when describing how quickly the temperature drops from 227 F?
I’m just curious about running at 227°. I drive in hot summer weather with the AC on all the time and never go that high. This time of year it hovers 196-200°…
Is your coolant full? Have you cleaned your radiator and AC condenser coils?
I’m just curious about running at 227°. I drive in hot summer weather with the AC on all the time and never go that high. This time of year it hovers 196-200°…
Is your coolant full? Have you cleaned your radiator and AC condenser coils?
..and you are absolutely certain that your tuner did not adjust your fan settings?
My coolant is full, but I have not cleaned my radiator and AC coils. But I will. Thanks for the suggesstion.
Cleaning the fins on both are reeeely important on these hot rods. You can jack the front up and look up into the radiator shroud and bet you find a bunch of crud near the top of each corner. That’s a good starting point. Then remove the radiator hold down and use some air and water.
Cleaning the fins on both are reeeely important on these hot rods. You can jack the front up and look up into the radiator shroud and bet you find a bunch of crud near the top of each corner. That’s a good starting point. Then remove the radiator hold down and use some air and water.
These wands are priceless!
Thank you for the cleaning suggestion! I will order the Blow Out/Wash Out kit today. Also, in addition to cleaning the radiator it's probably time for a flush/new coolant and I will probably install a new thermostat while it is drained.
Thank you for the cleaning suggestion! I will order the Blow Out/Wash Out kit today. Also, in addition to cleaning the radiator it's probably time for a flush/new coolant and I will probably install a new thermostat while it is drained.
You are not wrong. I have had more than one thermostat simply fail as it aged. Not on my C5, but thermostat technology has been weirdly constant.
Just be careful if you use water! When using water with a nozzle on a garden hose, even at normal hose pressure, will produce enough pressure to damage/bend the delicate fins. For this reason, I only use air.
I highly recommend a performance tune for any stock C5 owner. There are hp increases, and lower coolant temps to be had. I think many C5 owners would be very surprised how much performance GM left on the table with the stock tune. Of course, they had emissions and other parameters that had to be satisfied.....
Just be careful if you use water! When using water with a nozzle on a garden hose, even at normal hose pressure, will produce enough pressure to damage/bend the delicate fins. For this reason, I only use air.
I highly recommend a performance tune for any stock C5 owner. There are hp increases, and lower coolant temps to be had. I think many C5 owners would be very surprised how much performance GM left on the table with the stock tune. Of course, they had emissions and other parameters that had to be satisfied.....
Thanks for the tune advice. I will look into that.
Water Temperature
