C5 overheating
Thread Starter
Instructor
Joined: Sep 2016
Posts: 233
Likes: 59
From: Bowling Green, Ky
C5 overheating
Hope you all can give me some advice.
Symptoms:
Sits around around 226-238 at idle with ac on (93 degree days lately)
While driving to work. At 70mph approx. 2k rpm, with AC OFF car stayed at 244 & would jump to 248 uphill then right back down to 244 on level road.
If I turn the AC ON it would jump over 250 within seconds.
What I've tried:
"Burping" the system
Replacing thermostat with 160 degree replacement.
Cleaning all the grass clippings & crap from the radiator with an air nozzle & brush.
None of these have changed the outcome.
Symptoms:
Sits around around 226-238 at idle with ac on (93 degree days lately)
While driving to work. At 70mph approx. 2k rpm, with AC OFF car stayed at 244 & would jump to 248 uphill then right back down to 244 on level road.
If I turn the AC ON it would jump over 250 within seconds.
What I've tried:
"Burping" the system
Replacing thermostat with 160 degree replacement.
Cleaning all the grass clippings & crap from the radiator with an air nozzle & brush.
None of these have changed the outcome.
Last edited by Stephen Meredith; Jul 11, 2018 at 09:15 PM.
Melting Slicks
Joined: Nov 2005
Posts: 2,074
Likes: 309
Hope you all can give me some advice.
Symptoms:
Sits around around 226-238 at idle with ac on (93 degree days lately)
While driving to work. At 70mph approx. 2k rpm, with AC OFF car stayed at 244 & would jump to 248 uphill then right back down to 244 on level road.
If I turn the AC ON it would jump over 250 within seconds.
What I've tried:
"Burping" the system
Replacing thermostat with 160 degree replacement.
Cleaning all the grass clippings & crap from the radiator with an air nozzle & brush.
None of these have changed the outcome.
Symptoms:
Sits around around 226-238 at idle with ac on (93 degree days lately)
While driving to work. At 70mph approx. 2k rpm, with AC OFF car stayed at 244 & would jump to 248 uphill then right back down to 244 on level road.
If I turn the AC ON it would jump over 250 within seconds.
What I've tried:
"Burping" the system
Replacing thermostat with 160 degree replacement.
Cleaning all the grass clippings & crap from the radiator with an air nozzle & brush.
None of these have changed the outcome.
Acct Suspended APR 2026 by request
Joined: Dec 2010
Posts: 10,367
Likes: 1,272
2020 Corvette of the Year Finalist (performance mods)
C5 of Year Winner (performance mods) 2019
there are actually 3 fan relays but the fans should not be an issue at cruising speeds... do you still have the lower air dam installed?... you do need it but I'm not sure if it being missing could cause that big of an increase in temps while cruising
Thread Starter
Instructor
Joined: Sep 2016
Posts: 233
Likes: 59
From: Bowling Green, Ky
The factory lower air dam is intact.
The hi speed fans works as it should (coming on around 230), but the EBCM is behind the low speed fan, so I can't see it to tell if it's working correctly.
No coolant leaks that I see. I would think the water pump is working because both radiator hoses get hot & i've got strong heat blowing out the vents when I turn them on.
It's hard to diagnose it at idle & sitting still because it won't jump to 240+ unless i'm driving & under load (at least 1800 rpm or so). If I'm going down the road doing 35 there's no problem.
Now I did notice when I got back into town tonight, getting underneath it, the ac condenser is bowed out just a little. Like maybe the previous owner started to jack it up on that flimsy bar directly underneath it then stopped as soon as it started bending. But there's no damage to the ac condenser, just a slight curve to it & the ac has never lost any refrigerant.
The hi speed fans works as it should (coming on around 230), but the EBCM is behind the low speed fan, so I can't see it to tell if it's working correctly.
No coolant leaks that I see. I would think the water pump is working because both radiator hoses get hot & i've got strong heat blowing out the vents when I turn them on.
It's hard to diagnose it at idle & sitting still because it won't jump to 240+ unless i'm driving & under load (at least 1800 rpm or so). If I'm going down the road doing 35 there's no problem.
Now I did notice when I got back into town tonight, getting underneath it, the ac condenser is bowed out just a little. Like maybe the previous owner started to jack it up on that flimsy bar directly underneath it then stopped as soon as it started bending. But there's no damage to the ac condenser, just a slight curve to it & the ac has never lost any refrigerant.
Thread Starter
Instructor
Joined: Sep 2016
Posts: 233
Likes: 59
From: Bowling Green, Ky
Good info for me to check after work tomorrow. I know the cap doesn't leak, but don't know if it's holding proper pressure.
I DO know that passenger side fan DID NOT come on at all until near the 230 degree range when I watched it today.
I DO know that passenger side fan DID NOT come on at all until near the 230 degree range when I watched it today.
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Melting Slicks
Joined: Sep 2011
Posts: 2,258
Likes: 543
From: Appleton WI
If you turn your heat on high, does your engine temp go down? If yes, that is a good indicator that your radiator is plugged. It is either still plugged up externally or could be crudded up on the inside.
My radiator was gunked up on the inside and once replaced, all was good. I had well over 200k miles when I changed it. Hope that helps
My radiator was gunked up on the inside and once replaced, all was good. I had well over 200k miles when I changed it. Hope that helps
Thread Starter
Instructor
Joined: Sep 2016
Posts: 233
Likes: 59
From: Bowling Green, Ky
Ok. Problem NOT fixed. It only starts overheating at highway speeds (65mph/approx 1,600rpm & above). The temps drops right down in town at low speeds & stoplights.
Here's what I've tried as of today.
I am completely out of ideas aside from throwing money at it (new radiator/water pump), But th
Here's what I've tried as of today.
- pulled radiator - pressure washed/foam cleaned/pressure washed again/brushed
- foam cleaned & pressure washed SC condenser
- flushed coolant system
- changed out thermostat again
- checked air dam to make sure it's not bent
- tested fans to make sure they are working properly
- thoroughly checked oil for any sign of coolant
- refilled with new coolant
- bled air with the steam vent tubes on the heads method
I am completely out of ideas aside from throwing money at it (new radiator/water pump), But th
Last edited by Stephen Meredith; Jul 21, 2018 at 08:39 PM.
Thread Starter
Instructor
Joined: Sep 2016
Posts: 233
Likes: 59
From: Bowling Green, Ky
With the symptom of heating at higher speed, you are generating heat greater than the cooling capacity of the cooling system.
Could be a head gasket leak.
Could be a water pump that is not flowing coolant adequately.
Could be a bad surge tank cap that is not keeping adequate pressure on the cooling system and therefore allowing the coolant to boil at a relatively low temp. Once coolant boils the effectiveness of the coolant to remove heat from the metal of the engine is hugely diminished.
Could be an internal blockage in the radiator that is reducing cooling capacity.
Could also be some other leak that is not allowing pressure to build in the cooling system.
Those would be places to start.
Could be a head gasket leak.
Could be a water pump that is not flowing coolant adequately.
Could be a bad surge tank cap that is not keeping adequate pressure on the cooling system and therefore allowing the coolant to boil at a relatively low temp. Once coolant boils the effectiveness of the coolant to remove heat from the metal of the engine is hugely diminished.
Could be an internal blockage in the radiator that is reducing cooling capacity.
Could also be some other leak that is not allowing pressure to build in the cooling system.
Those would be places to start.
I don't seem to be physically losing any coolant as their isn't any on the ground & the tank stays at the same level. But what is the best way to check for a leaking head gasket beside checking the oil for coolant? And I guess there isn't a way to test the water pump other than replacing it?
Thread Starter
Instructor
Joined: Sep 2016
Posts: 233
Likes: 59
From: Bowling Green, Ky
I hate to throw money that I can't really afford right now at this guessing, but this car is normally my DD & I'm forced to drive my escalade until it's fixed. And 400+ miles a week just to work & back has my dumping well over $100 in gas into it.
So I ordered a new ac delco water pump today. Hopefully that fixes it, but if not, a dual core radiator will be next. And pulling the heads will be my last resort.
Hopefully when I solve this eventually, it might help someone else in the future.
So I ordered a new ac delco water pump today. Hopefully that fixes it, but if not, a dual core radiator will be next. And pulling the heads will be my last resort.
Hopefully when I solve this eventually, it might help someone else in the future.
Thread Starter
Instructor
Joined: Sep 2016
Posts: 233
Likes: 59
From: Bowling Green, Ky
If you are not already aware of the replacement C5 water pump issue here are a couple threads you might want to read through:
https://www.corvetteforum.com/forums...d-subject.html
https://www.corvetteforum.com/forums...t-opening.html
Bottom line you need to determine whether the WP on your car is the old style or the new style. Looking at the last picture in post # 5 (2nd link above) you can see that the old style pump can easily be recognized by the shape of the T'stat cover. If you have the old style and you ordered a "new" pump and not a rebuilt pump you will also need to order a new T'stat cover and new T'stat. The old cover and T'stat will NOT seal on the new style pump. The diameter of the opening in the new pump is different than that of the old style pump. Old pump opening is 2" and the new style opening is 1 7/8". NEW pumps will only be the new style. Rebuilt pumps are a crap shoot. There are plenty of the old style pumps in the rebuild stock pile so you have to look and see what the rebuilt is, old style or new style.
https://www.corvetteforum.com/forums...d-subject.html
https://www.corvetteforum.com/forums...t-opening.html
Bottom line you need to determine whether the WP on your car is the old style or the new style. Looking at the last picture in post # 5 (2nd link above) you can see that the old style pump can easily be recognized by the shape of the T'stat cover. If you have the old style and you ordered a "new" pump and not a rebuilt pump you will also need to order a new T'stat cover and new T'stat. The old cover and T'stat will NOT seal on the new style pump. The diameter of the opening in the new pump is different than that of the old style pump. Old pump opening is 2" and the new style opening is 1 7/8". NEW pumps will only be the new style. Rebuilt pumps are a crap shoot. There are plenty of the old style pumps in the rebuild stock pile so you have to look and see what the rebuilt is, old style or new style.
Either way, this one was ridiculously cheaper than the $139 for 01 pump at autozone & $200+ for 04 model pump!
Thread Starter
Instructor
Joined: Sep 2016
Posts: 233
Likes: 59
From: Bowling Green, Ky
The new style T'stat and T'stat cover DO NOT need a gasket to seal. The new Style T'stat comes with a rubber seal on the T'stat.
Again reference the top two pictures in post #5 of this thread:
https://www.corvetteforum.com/forums...t-opening.html
Good luck getting the system to seal if you use the old style T'stat cover and old style T'stat with this new style pump body. Several threads about those sealing issues on the Forum.
Again reference the top two pictures in post #5 of this thread:
https://www.corvetteforum.com/forums...t-opening.html
Good luck getting the system to seal if you use the old style T'stat cover and old style T'stat with this new style pump body. Several threads about those sealing issues on the Forum.
Thread Starter
Instructor
Joined: Sep 2016
Posts: 233
Likes: 59
From: Bowling Green, Ky
Got the new water pump on. It seems to have solved the problem. This new water pump DOES work with both the old style & new style thermostats. It even came with instructions for using either one. So this water pump is a great solution for anyone wanting a genuine acdelco.
Pro
Joined: Mar 2006
Posts: 519
Likes: 22
From: Golden CO
Radiator Cap! 2003 C5, 100k miles - Well I suddenly had similar overheating problems a couple weeks ago and discovered I was about a gallon low on coolant. Topped it off, better but still some overheating a bit, hitting 235F which it never did unless in stop and go on a hot day with AC etc.
I considered all the costly and complicated suggestions and fixes above (thanks all!) and then suddenly, when checking my coolant level again the other day noticed - duh- there was a white trail from under the cap down the side of the reservoir tank. And then I looked more closely at what I had thought on a previous 'quick look' was a perfectly good radiator cap, but it was not - its' plastic was cracked, clearly a bit warped, etc.
$10 on Amazon a new AC DELCO OEM replacement is on the way. I was not keeping a pressurized system and thus the temp was creeping up.
Start simple is my new mantra.
I considered all the costly and complicated suggestions and fixes above (thanks all!) and then suddenly, when checking my coolant level again the other day noticed - duh- there was a white trail from under the cap down the side of the reservoir tank. And then I looked more closely at what I had thought on a previous 'quick look' was a perfectly good radiator cap, but it was not - its' plastic was cracked, clearly a bit warped, etc.
$10 on Amazon a new AC DELCO OEM replacement is on the way. I was not keeping a pressurized system and thus the temp was creeping up.
Start simple is my new mantra.