My C5 is still overheating as high as 250 in summer heat, in town. I've already replaced the OEM rad with a dual core DeWitts, and installed twin fans, and added Super Cool.
What do I do now? Not drive the car? That seems to defeat the purpose of having a Vette Conv.
Should I try a stronger concentration of Supercool or Water Wetter?
Should I convert to a Waterless Coolant, like the Evans product? No one I have talked to here in Hawaii as ever used a Waterless product so I'm a bit concerned about being the first guinea pig.
Any help or ideas would be appreciated.

 

Does it overheat on the highway? The stock C5 has dual fans that are supposed to both turn on around 226 in low speed and then go to high speed at 235. Have you checked that the fans are coming on, and did you burp the cooling system when you filled it with coolant.

 

MMartinez; Thanks for your thoughts. No issue above 35 mph. Was not aware that 02 base C5 had two speed fans when I started. The Dewitts package (dual core rad and dual fans at over 3200cfm) is twice the stock airflow and with more than a gallon more fluid, so I was pretty sure I was ok. And I was for several years, up until this year when the ambient bumped up almost 10 degrees. We are getting most days over 92 degrees. Yes, the rad has been burped. Actually a couple of times, thinking that running low was a problem. It wasn't. And yes, the fans come on. First at 185, the second at about 205 indicated on guage.
Seems you have addressed this problem in the past. Any other ideas? Thanks, jim

 

Are you running an aftermarket front lip?

 

Both fans run at the same time, either at low speed or at high speed. Pretty sure that’s true for all years/variants. If you have one fan running at some points you have a control issue.

 

ALL C5s have dual two speed fans. A few things:

- Has the engine ever actually overheated and puked coolant, or are you just going by the gauge?
- Have you verifed temperatures and circulation with an IR thermometer?
- What temps does the engine run over 35 mph?
- Are your fans operating in low and high?
- You are saying ONE fan comes on at 185, and then BOTH fans run at 205? That is not correct operation. Both fans run at the same time whether in low or high speed 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)

If your fans are not operating as above, then the parameters have been changed in the tune....regardless of the fan set points, they both run together.

 

Professor just as a point of reference we have many days over 100 down here in Texas and I can idle around town all day without overheating. And I'm running a stock cooling setup as far as I know.

Based on your comment that you're not overheating on the highway I think this points to your fans not operating properly. They turn off at 35mph or above anyway.
Also check that your air dam is in place, although you would probably have issues at highway speed if it were not.

 

No probably about it.

 

Lucky; Thanks for the correction to my understanding of how the fans are supposed to work. I'll recheck both actual operation and the ECM.

 

Lucky and Dads: No front splitter addon, just stock nose. The side airdams are intact and the center does move back, presumably at speed.

 

Lucky; the running temp on roadway is 195-205 depending on how hot the ambient. That's at any speed over 35. When it does get hot (240+) in town, it cools down rather quickly, approximately 5 minutes on the highway. And all these are with the AC running and the top up.
The engine has never pukked or boiled over, though it has reached 250 a number of times (which is the reason for my concern). I never ran a race motor (LT1 iron block) that hot but I know an all aluminum LS1 is capable of a bit more.
But I must admit I am a bit confused as to when the fans run. At the end of the bold copy, you indicate that both fans run all of the time. That is consistent with a couple of other respondents have said.
But that is not consistent with the tech settings in the ECM. Even in the body you indicate that the low speed fan turns on at one temp and the high speed at another. So, one runs when a low temp is reached, then when a higher temp is reached the second turns on. I can tell you that when I open the hood when the temp has passed 220, both fans are running and the air flow seems quite substantial. In fact, they are creating enough negative pressure that the relief flaps on the fan shroud is fluttering.
Another point that might help is that the Dewitts Fans are set to use the stock electrical plugins. And I've never checked with an IR guage or temp probe. I'll do that to confirm that my guage is reasonably close.
And my HPTuners map shows Fan 1 (First Stage) turning on at 200 degree, and off at 180. Fan 2 (second stage) on at 215 and off at 195. So from that spec, below 180 no fans are running allowing for faster engine warm-up, and over 215 both are on. These settings should be ok.
So, where do I go now?
Thanks, you have been very helpful in forcing me to take a closer look at all the possibilities.

 

Then you do not have air in the system.

No I did not. I said they run together, because you made it sound like your fans were turning on one at a time. The fans run together, in low speed or high speed.

The bold text is a copy/paste from the service manual. Again, make sure that you understand that both fans run WHEN COMMANDED BY THE ECM either at low or high speed.

Faster engine warm up? This does not make sense. The stock thermostat does not open until 187 degrees, so if your fans kick on a 180, you are actually fighting steady state temperature. Why would you do this?

What is the intention of setting your fans like this for a street car? If you have a stock thermostat, your first fan setting is actually driving the temp below the thermostat temperature. Even if you put in a 160 thermostat, it still does not make sense. I don't get the logic. It's not a SBC....it's designed to run at higher temps.

 

I will ask the question no one has. Are your fans running backwards?

 

OK, it gets clearer to this slow mind. Dual fans with 2 speeds each, running in parallel. Will re-examine closely and report back.
Also, I thought the stock thermostat was 180 degree. So my ECM should have a higher turn-on setting. Can make that change easily.
One sure point is that they are turning correctly which is apparant when I open hood and air is blasting from behind radiator.
Thanks to all of you for helping!

 

Duel fans with two speeds low and high. The fans are controlled by the pcm/ecm by outputs which control three relays. The contacts of the relays configure the power to the fans so in low speed the fans are powered in series with each other, so each fan has 6volts. In high speed mode the relay contacts configure the fans in parallel so each fan has 12v.

 

MMartinez: got it! Thanks. I can take this to my shop and get it checked out properly. Will update post if problem is solved.

 

Not trying to give you a hard time here, but that understanding is even worse. If you thought you had a stock thermostat that opened at 180, and set your low speed fans to come on a 180, the engine would never reach operating temp.

 

Your problem is perplexing so I went back to you first post:

My C5 is still overheating as high as 250 in summer heat, in town. I've already replaced the OEM rad with a dual core DeWitts, and installed twin fans, and added Super Cool.

This sounds like you were having a high engine temperature issue in city traffic some time ago. Then you replaced the radiator and fans to try and fix the problem. Now you have a new Dewitt’s radiator and fans package and still have the same problem. Is this correct?

As a point of reference, a stock C5 with a properly functioning cooling system is designed to operate in city traffic under high ambient conditions (think desert southwest) without overheating. Whether testing in the Mesa AZ former GM Proving Grounds, or on the road testing in Las Vegas city traffic, GM and Delphi engineers would not accept 250 degrees in either test condition.

 

Lucky: I believe low speed turning on at 200 and off at 180, and then stepping up to hi speed at 215 and off at 195, isn't same as on at 180. Not sure what I said that misled you to that idea. The spread indicated are those that I used which were provided from the HPTuners maps.

Book: Your question has some validity for sure based on the inputs I made. But to be more clear: when I first brought the car to Hawaii after having no issues with it in Ca, I experienced overheating as I started making some mods not allowed on street cars in Ca. Did the normal hopup of headers, air intake, throttle body, larger intake manifold, highlift rockers, and custom tune. Worked very well, except in the higher temps in town. So I made the rad and fan changes. That dropped the running temp by 15 degrees. But intown was still too hot. Not always, but an issue when the temps were over 95 ambient. Another point is that the engine would not overheat just idling so it is an issue only when driving at low speeds.

Do these facts alter your thoughts on what I should be looking for, or what I might ask the shop to look at when she goes in?

 

So wait, what are you calling "low speed"? You indicated previously that you have no issue about 35 mph. Are you saying the engine only runs hot between 1 mph and 34 mph ? I would start by restoring the factory on/off points for low/high fans to establish a baseline.