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Lucky: correct, no issue at any speed above 35+/-. Only in town driving where speeds are normally <35.
So I'll reset the fans on/off using your low speed on at 226+ and off at <219; and high speed on at 235+ and off at <226.
You indicated that the low speed turns on at 185 below 35mph. Is that built into the circuitry? I ask because there is no table or switch in the HPTuner program that I can find to set that condition.
If I can't establish that low mph speed condition, then wouldn't it make sense to set the low speed on at 185? Since I live in such a year round warm climate I don't have to worry about cold weather warmup.
Thanks so much for your continued support.
jim
Lucky: correct, no issue at any speed above 35+/-. Only in town driving where speeds are normally <35.
So I'll reset the fans on/off using your low speed on at 226+ and off at <219; and high speed on at 235+ and off at <226.
You indicated that the low speed turns on at 185 below 35mph. Is that built into the circuitry?
Already provided.....and I explained that the text was directly from the service manual for your year.
Originally Posted by professorjim
I ask because there is no table or switch in the HPTuner program that I can find to set that condition.
If I can't establish that low mph speed condition, then wouldn't it make sense to set the low speed on at 185? Since I live in such a year round warm climate I don't have to worry about cold weather warmup.
Apologies, but it's hard for me to wrap my brain around using software to manipulate the PCM settings without even having a basic understanding how the engine/PCM is designed to operate. I'm not in the business of convincing people to do the right thing, or take a methodical approach to troubleshooting. With that, I will leave you with the the design intent description and operation.
I'm out. Good luck.
Last edited by lucky131969; Aug 17, 2025 at 11:46 AM.
Lucky: correct, no issue at any speed above 35+/-. Only in town driving where speeds are normally <35.
So I'll reset the fans on/off using your low speed on at 226+ and off at <219; and high speed on at 235+ and off at <226.
You indicated that the low speed turns on at 185 below 35mph. Is that built into the circuitry? I ask because there is no table or switch in the HPTuner program that I can find to set that condition.
If I can't establish that low mph speed condition, then wouldn't it make sense to set the low speed on at 185? Since I live in such a year round warm climate I don't have to worry about cold weather warmup.
Thanks so much for your continued support.
jim
To the OP, the PCM/ECM recives a signal from the VSS vehicle speed sensor and uses it to turn on the fans at speeds below 35mph, and of course to turn them off at speeds above 35mph. I do not believe that that is a configurable parameter.
To the OP, the PCM/ECM recives a signal from the VSS vehicle speed sensor and uses it to turn on the fans at speeds below 35mph, and of course to turn them off at speeds above 35mph. I do not believe that that is a configurable parameter.
Hptuner you adjust all parameters. Speed, temp, A/C psi.
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?
It reads like if you drive at 36mph, you have no concerning coolant temperature issue. If you drive at 34mph, you experience high coolant temperatures. So what happens at 35mph? The fans in a stock configuration are turned off.
Here is an experiment for you to consider: Disconnect your fans (or pull fuses perhaps), and with the a/c off, drive your city traffic and see what happens.
I had trouble ascertaining what my fans were doing after fighting with the DeWitt radiator / stock fan installation, so as a test I eliminated all the variables and replaced the high speed relay with a toggle switch.
As a test. Flipped the toggle and both fans ran at high speed.
So just a thought: For a few bucks you can test the system’s ability to cool, sitting in the driveway with the fans on high speed. Control module won’t matter. Programming won’t matter. A/C won’t matter. Heck, even the ignition switch won’t matter. Test the fans with the engine off and no belts being a danger.
Forgive me this has been mentioned but the factory fan on/off settings are pretty high. If you lower those and add a lower high flow 185 stat, you will see lower temps. You might also consider running a high water (distilled) content, like 70/30 with ww additive. Water cools better than antifreeze and you could see a 10-15 degree drop with this ratio. Conversely, the waterless coolant will actually run hotter.
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.
Running Evans coolant will slightly elevate coolant temperature, NOT run cooler. It is propylene glycol, rather than ethylene glycol. The only thing Evans is good for is for eliminating steam pockets, because it takes more heat to boil it as compared to ethylene glycol. I ran it in a Darton 427 years ago. My engine coolant reached 195° within 1 mile on the first start of the day! It doesnt transfer heat nearly as well as ethylene glycol. Most of the freezer ice packs are filled with propylene glycol. Because they don't thaw as quickly as plain ice....
The problem, as described by the Forum member, is running near 250 degrees at speeds below 35 mph but temperature is OK at idle. Above 35 mph, he states the temperatures are OK too. Running 250 degrees in traffic at 90 degree ambient is so off from normal that I expect that no variety of coolant or coolant concentration will address whatever the actual cause of the problem is.
One of the best recommendations I read within this thread was to get the fan settings back to stock. I would further recommend that he reinstall the stock fans/shroud, a new thermostat, and 50/50 Dexcool as a baseline. Then continue the problem solving from the near stock baseline.
Years ago, when I first built my 427 Darton engine, I went to a DeWitts radiator. When I later installed the LS7, the tuner asked me what fans I was running. He was happy to hear I was running (and still do) the stock C5 factory fans. They work fine in 90° heat, though I'm still running the DeWitts. I doubt OP has serious blockages, especially plastic garbage bags in the A/C condenser and radiator, or it wouldn't cool well at over 35mph, either. Sounds to me like the fans just aren't doing their job, for whatever reason...
