C5 Overheating BAD!! Help!
Le Mans Master
Joined: Feb 2021
Posts: 6,503
Likes: 4,351
From: Florida
2025 Corvette of the Year Finalist - Unmodified
2024 C5 of the Year Winner- Unmodified
2022 C5 of the Year Finalist - Unmodified
You post is what I thought the sequence was, then in another thread I was told in no uncertain terms that both fans ran together, never a single fan, and the only variables being beginning and ending temp and fan speed.
Thread Starter
Racer
Joined: Apr 2022
Posts: 258
Likes: 35
From: Boring Green, Kentucky
Cool Info (Literally)
Is that 215° with ac on?
I was driving around today in the 90’s with the ac on and the water temp was holding about 200°. You may indeed have some blockage.
Check out this post…
https://www.corvetteforum.com/forums...post1605291551
I was driving around today in the 90’s with the ac on and the water temp was holding about 200°. You may indeed have some blockage.
Check out this post…
https://www.corvetteforum.com/forums...post1605291551
Thread Starter
Racer
Joined: Apr 2022
Posts: 258
Likes: 35
From: Boring Green, Kentucky
Drifting
Joined: Sep 2009
Posts: 1,862
Likes: 594
From: Prescott, Arizona
Go over to the PS pump side and look down, you should be able to see the back of the driver side fan motor.
Here is the description of how the fan system works:
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).
People are always running around worrying about their temps because they see the numbers on the gauge. These cars are designed to work with high coolant temps and have no problems operating at the temps you indicate. As you can see the fans don't even get turned onto low speed until the temp hits 226 and don't go to high speed until the temp hits 235 degrees. At an autocross where we were running in heats and cycling through the runs quite fast I saw a 242 degree coolant temp on my C5Z for most of an event.
Here is the description of how the fan system works:
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).
People are always running around worrying about their temps because they see the numbers on the gauge. These cars are designed to work with high coolant temps and have no problems operating at the temps you indicate. As you can see the fans don't even get turned onto low speed until the temp hits 226 and don't go to high speed until the temp hits 235 degrees. At an autocross where we were running in heats and cycling through the runs quite fast I saw a 242 degree coolant temp on my C5Z for most of an event.
Thread Starter
Racer
Joined: Apr 2022
Posts: 258
Likes: 35
From: Boring Green, Kentucky
Go over to the PS pump side and look down, you should be able to see the back of the driver side fan motor.
Here is the description of how the fan system works:
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).
People are always running around worrying about their temps because they see the numbers on the gauge. These cars are designed to work with high coolant temps and have no problems operating at the temps you indicate. As you can see the fans don't even get turned onto low speed until the temp hits 226 and don't go to high speed until the temp hits 235 degrees. At an autocross where we were running in heats and cycling through the runs quite fast I saw a 242 degree coolant temp on my C5Z for most of an event.
Here is the description of how the fan system works:
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).
People are always running around worrying about their temps because they see the numbers on the gauge. These cars are designed to work with high coolant temps and have no problems operating at the temps you indicate. As you can see the fans don't even get turned onto low speed until the temp hits 226 and don't go to high speed until the temp hits 235 degrees. At an autocross where we were running in heats and cycling through the runs quite fast I saw a 242 degree coolant temp on my C5Z for most of an event.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Burning Brakes
Joined: Feb 2022
Posts: 897
Likes: 987
From: NW Burbs of Chi-Town
2025 C5 of the Year Finalist - Modified
As a new C5 Corvette owner this is golden information and should be a Sticky imo (jim 933):
"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)."
Dang these things run HOT!
I was running 80ish mph for an hour or so on the way home and did some "spirited" driving before hand and was wondering what the "Norm" was other than the analog temp sensor. OAT was 87 and I had the AC (off for rowing) on. 195° on the coolant and 212° on oil temps after rowing some gears and hitting the interstate.
Dang, these are really nice cars!!!
"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)."
Dang these things run HOT!
I was running 80ish mph for an hour or so on the way home and did some "spirited" driving before hand and was wondering what the "Norm" was other than the analog temp sensor. OAT was 87 and I had the AC (off for rowing) on. 195° on the coolant and 212° on oil temps after rowing some gears and hitting the interstate.
Dang, these are really nice cars!!!
Thread Starter
Racer
Joined: Apr 2022
Posts: 258
Likes: 35
From: Boring Green, Kentucky
As a new C5 Corvette owner this is golden information and should be a Sticky imo (jim 933):
"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)."
Dang these things run HOT!
I was running 80ish mph for an hour or so on the way home and did some "spirited" driving before hand and was wondering what the "Norm" was other than the analog temp sensor. OAT was 87 and I had the AC (off for rowing) on. 195° on the coolant and 212° on oil temps after rowing some gears and hitting the interstate.
Dang, these are really nice cars!!!
"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)."
Dang these things run HOT!
I was running 80ish mph for an hour or so on the way home and did some "spirited" driving before hand and was wondering what the "Norm" was other than the analog temp sensor. OAT was 87 and I had the AC (off for rowing) on. 195° on the coolant and 212° on oil temps after rowing some gears and hitting the interstate.
Dang, these are really nice cars!!!
I think my head to too big to go low enough to spot the driver's side fan, lol...Last edited by zztopo; Jun 20, 2022 at 01:39 AM.
Melting Slicks
Joined: Sep 2008
Posts: 3,138
Likes: 802
From: Espanola New Mexico
Cars do normally run hotter with the AC on because the air reaching the radiator is warmer due to the condenser coils. You should wash the condenser and radiator with a garden hose or use a low pressure air wand to blow out road debris that gets caught in the fins.
Thread Starter
Racer
Joined: Apr 2022
Posts: 258
Likes: 35
From: Boring Green, Kentucky
Thread Starter
Racer
Joined: Apr 2022
Posts: 258
Likes: 35
From: Boring Green, Kentucky
Thanks You Guys!
Overheating/AC problem solved!!! 
I took it to the shop and we put her up. Then we blew a shitload of debris out of the back of the radiator. The whole area was covered with dirt clods, dust, hay and a beehive. I could not believe my eyes.
So we fired her up. Cold air was blowing again, and she maintained 218* even with the AC on. I am a happy man. All that was needed was a good blow job.
Thanks again to all!
I took it to the shop and we put her up. Then we blew a shitload of debris out of the back of the radiator. The whole area was covered with dirt clods, dust, hay and a beehive. I could not believe my eyes.So we fired her up. Cold air was blowing again, and she maintained 218* even with the AC on. I am a happy man. All that was needed was a good blow job.
Thanks again to all!
Drifting
Joined: Jul 2015
Posts: 1,844
Likes: 375
From: San Jose, CA
Post #5 made 6/15, only took a week to do the thing 
I'm not trying to give you a hard time, but as the weather gets hot, people keep posting this question. Literally the first thing to do ... blow the crud out the fins with compressed air. The first. You can spend months or even years chasing down this issue thinking "nah it's not that." People on this forum have posted updates _after over a year_ when they finally did it, and it solved their overheating issues. Over a year! Any shop that can lift a C5 without screwing it up, can do this for you, for an hour of labor or less. Or you can do it yourself if you have compressed air. Fans, shmans - do this first.
I learned this lesson the hard way too, I spent probably 2 months not doing it, then I did it. I am here to pass on my hard lessons to everyone else so they can be smarter than I was.
I'm not trying to give you a hard time, but as the weather gets hot, people keep posting this question. Literally the first thing to do ... blow the crud out the fins with compressed air. The first. You can spend months or even years chasing down this issue thinking "nah it's not that." People on this forum have posted updates _after over a year_ when they finally did it, and it solved their overheating issues. Over a year! Any shop that can lift a C5 without screwing it up, can do this for you, for an hour of labor or less. Or you can do it yourself if you have compressed air. Fans, shmans - do this first.
I learned this lesson the hard way too, I spent probably 2 months not doing it, then I did it. I am here to pass on my hard lessons to everyone else so they can be smarter than I was.
Thread Starter
Racer
Joined: Apr 2022
Posts: 258
Likes: 35
From: Boring Green, Kentucky
Post #5 made 6/15, only took a week to do the thing
I'm not trying to give you a hard time, but as the weather gets hot, people keep posting this question. Literally the first thing to do ... blow the crud out the fins with compressed air. The first. You can spend months or even years chasing down this issue thinking "nah it's not that." People on this forum have posted updates _after over a year_ when they finally did it, and it solved their overheating issues. Over a year! Any shop that can lift a C5 without screwing it up, can do this for you, for an hour of labor or less. Or you can do it yourself if you have compressed air. Fans, shmans - do this first.
I learned this lesson the hard way too, I spent probably 2 months not doing it, then I did it. I am here to pass on my hard lessons to everyone else so they can be smarter than I was.
I'm not trying to give you a hard time, but as the weather gets hot, people keep posting this question. Literally the first thing to do ... blow the crud out the fins with compressed air. The first. You can spend months or even years chasing down this issue thinking "nah it's not that." People on this forum have posted updates _after over a year_ when they finally did it, and it solved their overheating issues. Over a year! Any shop that can lift a C5 without screwing it up, can do this for you, for an hour of labor or less. Or you can do it yourself if you have compressed air. Fans, shmans - do this first.
I learned this lesson the hard way too, I spent probably 2 months not doing it, then I did it. I am here to pass on my hard lessons to everyone else so they can be smarter than I was.
To boot, they did this for free and also did the anti venom mod since it was up in the air. I wasn't charged or that either.
It pays to know the owner. This will be my "go-to" shop when I break **** at the strip.
Tomorrow, I am buying pizza for the boys.
Le Mans Master
Joined: Feb 2021
Posts: 6,503
Likes: 4,351
From: Florida
2025 Corvette of the Year Finalist - Unmodified
2024 C5 of the Year Winner- Unmodified
2022 C5 of the Year Finalist - Unmodified
Good point, but the shops around here are booked out for weeks. Otherwise, I would have done it the very next day. A lift was needed. Plus, it gave me a good look under the C5 since I bought fairly recently. It was cool to see the dual exhaust with the X pipe.
To boot, they did this for free and also did the anti venom mod since it was up in the air. I wasn't charged or that either.
It pays to know the owner. This will be my "go-to" shop when I break **** at the strip.
Tomorrow, I am buying pizza for the boys.
To boot, they did this for free and also did the anti venom mod since it was up in the air. I wasn't charged or that either.
It pays to know the owner. This will be my "go-to" shop when I break **** at the strip.
Tomorrow, I am buying pizza for the boys.
BTW, buying the guys pizza is a very considerate gesture.
Thread Starter
Racer
Joined: Apr 2022
Posts: 258
Likes: 35
From: Boring Green, Kentucky
Thanks, yeah I'm lucky; these guys got my back. They understand "the hot-rod" mentality too; most mechanics don't. They just want to fix **** and go home. Some shops here won't even do mods.
Le Mans Master
Joined: Feb 2021
Posts: 6,503
Likes: 4,351
From: Florida
2025 Corvette of the Year Finalist - Unmodified
2024 C5 of the Year Winner- Unmodified
2022 C5 of the Year Finalist - Unmodified
Right after that I found a shop one mile from me that will do anything I need on my Vette.
Oh well, timing…
Thread Starter
Racer
Joined: Apr 2022
Posts: 258
Likes: 35
From: Boring Green, Kentucky
Yeah, had my car in the shop for a clutch and asked if they could change the rear gears while the driveline was out. They said they could do it, but when I went to pick it up they said they didn’t change the gears because that was a “high performance mod” and they didn’t want to get into that. WTF?
Right after that I found a shop one mile from me that will do anything I need on my Vette.
Oh well, timing…
Right after that I found a shop one mile from me that will do anything I need on my Vette.
Oh well, timing…