On my 4th C5, 2004 Z06, 65k miles. Haven’t had it more than two weeks and I noticed in traffic the other day the coolant temp needle crept past the middle and the digital gauge was reading 238. This is in Vegas and we’re not even past 85° yet so I know it’s going to be a problem in summer. Ive replaced all the relays, thermostat, coolant, flushed, and checked for blockage’s in all the hoses. When it reaches around 230 I see both fans are on and it does drop to 217, but it’s like watching a pendulum of up and down. What should I look into next?

 

Blockage at the radiator coming from the bottom can be an issue. These C5 are so low to the ground they tend to suck up debris like grass and it gradually blocks airflow. Small vacuum, some compressed air usually will dislodge most of it.

 

Normal operation (provided nothing has been changed in tuning)..., the low speed fan does not come on until 226 F, and turns off at 219 F.
The high speed cooling fan comes on at 235 F, and turns off at 226 F.

 

The fans which are powered by relay contacts are configured in series for low speed operation, and configured in parallel for high speed operation seem to be operating correctly. As previously mentioned check your condenser and radiator for debris, you haven't by chance added a front splitter as those can raise engine temperatures.

 

Concur with the above. Also doesnt hurt to confirm system is properly bled

 

Thanks for the replies, I’ve already cleaned the radiator for debris and bled the system. I lm familiar with the operating temps and fans but like I said the amount this needle moves and it doesn’t stay relatively in the middle like all my previous C5s worries me as I approach summer.

 

test for compression? Pressure check?

 

Sounds like you have already checked, but I have had overheating tendency from the following:
Dirty radiator (external)
Fan motor failure (several times)
Long uphill in Death Valley on a hot day (probably near design limit)

 

Replace water temp sensor? Cheap / easy

 

I had intermittent issues with the car getting hot. I changed the thermostat... problem solved.

 

I’have installed a Coolit V3 from Sac city Corvette in mine so I can control my fans remotely anytime I need to.

 

Are you seeing these coolant temperatures at idle/city traffic? at 30mph and up road speeds? both conditions?

 

Street driving is when I initially noticed it. Seems to be okay on the highway but like I said it’s not even hot out yet. I can get it run these tempts in my garage on a 75° day. Looking for ways to troubleshoot before I change out a 65k mile radiator, fan, or water pump.

 

i used to have a 2003 z06. Every december i would remove the rad and clean with soap & water

 

Lower the fan turn on temps

 

The OPs first post indicates pretty much "normal" ops with A/C turned OFF. If turning on the A/C makes the high ECT issue go away, a "tune" for fan temps should help. That's what I did 2 weeks ago and so-far/so-good. Hasn't gone above 200 degs F in traffic.

 

In your hot environment I'd lower the fan activation temps along with a lower temp thermostat...
Don't do a 160* as your stock cooling system can't maintain and it's too low.. Use a 180*-185* different brands
sometimes have a bit diff number.
Since you are already a stick shift car we know that it's not trans related.. And a "tune" wouldn't give you
that much benefit over just setting diff fan temps..'
So the sac city coolit is a pretty good device to be able to control your fans yourself.
Other than the radiator itself maybe being an issue, I'd try the fan controls first.
Just because it has low mileage mean very little. It's still over 20 years old and if the coolant hasn't been
changed out every 2-3 years (over 20 years) you could have crap built up in the cores/etc

 

The reason I asked about idle/traffic versus down the road was because of the splitter question another member asked. Seems like there are a few threads a year where a member writes about hot temperatures on the highway but good at low speed. Invariably, if they have no mechanical issues or actual cooling system issues, it turns out that they removed the 3 piece air dam and installed a splitter.

Your past experience with C5’s suggests that you know what normal cooling system operation looks like. If you have a reasonably stock vehicle that is mechanically sound, in the 75 to 85 degree ambient you have now, normal temperatures in traffic and on the road will be very close to a stock thermostat setting. Our cooling systems are designed to operate under thermostat control under normal driving conditions. That means in the 190’s with ram air and then slowly creeping up with extended idle and moderate stop and go until the fan turns on.

So back to your question about trouble shooting.

The air side should be pretty straight forward. Some of these questions you have already answered. Are your fans running and do the blow the air in the correct direction? Have you checked for junk like plastic bags fouling up your fans or stuffed between the condenser and radiator? Have you or a previous owner disturbed the shape or structure of the air dams and inlet air flow ducting?

The coolant side, as you have suggested, is more difficult and potentially more expensive. Trapped air? Plugged radiator (this goes way back but I remember my first car had a radiator where at least half of it never got hot because the radiator tubes were so plugged with corrosion)? Badly corroded or plugged engine cooling passages (There are people who claim mixing so e coolants with Dexcool coolant can form gel. I have no idea if this is true)? Bad water pump (it is such a simple mechanical device that it is hard to imagine where it fails to pump unless it is seized, badly worn bearing, or broken impeller. Seems like spinning the water pump pulley by hand would be a pretty easy subjective check to verify it feels OK)? Bad coolant temperature sensor (should be very easy to verify this one. When you are at highway speeds, are you running in the lower 190’s)? Bad thermostat (perhaps it only opens partially)

 

Very first thing to check is that the fans come on at low speed, if a fan fails both fans will not work at the lower temperature set point as they are configured in series. Once the higher set point is reached only the one good fan will come on. The fans will be turned off once the car exceeds 35mph

 

After all my troubleshooting I have been able to get it to run generally cooler,but still creeps past 235 in traffic. Did notice now that when the A/C is on it does not go past 220 now. While highway driving it stay below 200 even on the hot days we've been having. So I broke down and bought a radiator, Dewitts Fan, and waterpump.