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Hi, my 04 c5z sat for a few years & back driving now, got a overheat light & high temp read. Before I go nuts with diagnostics, I am guessing stuck fan relay. Where is it, can I rap on it and see if it unsticks? TKS Froggy back. Pictures are awesome.
Under what conditions does it overheat? Have to provide a better description of it overheating, just sitting with engine running, slow driving in traffic, or overheating at highway speeds?
Hi, my 04 c5z sat for a few years & back driving now, got a overheat light & high temp read. Before I go nuts with diagnostics, I am guessing stuck fan relay. Where is it, can I rap on it and see if it unsticks? TKS Froggy back. Pictures are awesome.
Your owners manual has a map of the fuse boxes, and there is a diagram under the lid of each fusebox.
Good luck rapping on all 3 relays.
Last edited by lucky131969; Nov 5, 2025 at 10:51 PM.
Was this at above or below 30MPH?
If it's above, it's not a fan issue. At speed, more CFM of air are forced through the radiator than any fan could provide
Have you blown out the radiator? C5's are famous for clogging up their radiators with crud.
need more info. is there water in the engine? Does the rad system hold pressure when hot or is the hose soft? Any leaking? How hot exactly does it get? Driving or sitting still? You would need an actual gauge or IR thermometer or something to get a realistic number *F or *C to tell whether its actually overheating or not. unless its blatantly obvious like, hose exploding, water pushing out, engine smoking running funny, burning smell of oil and coolant, etc... The light for example, coming on, it really doesn't tell you temp. Is the light actually correct or not? is what you should ask
The fans will come on in low speed mode when the temperature reach 226, and turn off when the temperature lowers to 219. The high speed fan operation will happen when the temperature reaches 235 and turns off at 226. The fans turn off at speeds above 35mph. You will have to let the car warm up and verify that the fans are or are not turning on. You can turn the fans on by turning the AC on.
Thanks all, good info. The needle gauge did peg and bright red engine overheating showed on dash. This in 5 miles of local slow stop n go. Under the hood no sign of leaks. I will inspect further soon, crazy busy with other stuff. Need a new ir gauge mine got toasted in the garage fire. Along with most other tools, Ugh. NOT THIS VETTE THE 2004 is overheating, this one gone. Li-ion battery on power tool exploded
Last edited by froggy47; Nov 6, 2025 at 05:29 PM.
Reason: info
Hi, Full transparency, I fudged up and missed the basics. What are the basics for overheating? Coolant level, clogged radiator (debris ), fan not coming on. Well my coolant was low by a lot, there was never any apparent leak but it did sit for some years unused. I had not checked it b4 I put it up, so probably low when I parked it, now it's running about 210F fan cycle on / off all seems good, Thanks all.
That coolant went somewhere. In a sealed system, it won't evaporate. If I were you I'd keep a close eye on the coolant level.....
well. water molecules are extremely small.
I promise you no cooling system is 100% sealed.
Every day, some water molecules leave the cooling system. Especially more so when its under heated pressurized.
Every cooling system in the world has this sort of... gradual loss.
We can extend this to gasoline in the presence of water, to illustrate another practical point about fuel systems - that is
Fick's law, concerning effusion
There is always some rate of permeation, and effusion, of fuel and water that goes right through the fuel tank's walls. Just like water will undoubtedly effuse right through the hoses and probably even the aluminum fixtures of an engine
We have to be careful when a cooling system is left alone for long periods, because of general losses, evaporation, effusion, permeation, and keep in mind the coolant overflow tank is exposed to the atmosphere which means it is constantly acting as an evaporative emission e.g. water pushed into the coolant over flow tank is constantly being lost to the atmosphere based on the surface area and access to the little vent at the top.
I tend to agree, Where there is no obvious leak, there is a very small/gradual loss over time, mine was probably put away years ago already low due to autox/track use and then sat so long it got lower. Still ok now after top off but not driven daily so will keep an eye, better than I previously did.
Oh boy.......I'll buy maybe a 10 or 15 year period of time sitting. But over a few (define few?) years time it shouldn't go from full to so low it will overheat due to the loss of coolant. I had a coolant loss over one summer. No water in oil. No coolant under the car. One day right after a short, 30 mile trip, I had the hood open, and saw a drip from 1 heater hose where it connects near the T-stat housing. It evaporated in a second or two, leaving no trace of a leak. The spring type OEM clamp had lost some tension, and when the engine heat sinked, it seeped coolant. Put on a plain Jane worm gear type band clamp that worked great for over 70 years-until GM decided they needed to save 10 cents a car by changing the basic design-and never had a problem since......
I am estimating it was put in storage already low (maybe quite low - not sure how low) and sat 5 years while I had the c6z (maybe 7 years). Bottom line is I WILL MONITOR for leaks going forward, The past is sort of speculation. It's pretty remarkable I just stuck a new battery in and started it up. Goes to the quality (for the most part) of the powertrain.
Overheating (fan not coming on?)
