Alright, so I have a 2004 C5, and noticed that it was overheating. (Would be at normal driving temp then rapidly shoot up to 240-245, and then rapidly shoot down back to operating temp 198 ish)

So in all I have replaced thermostat, cleaned condenser and radiator, Drained coolant, burped the system, and replaced the coolant temp sensor.

Now It does not overheat any longer, but I believe that it’s running hotter then it did than before.

It seems no matter what during any sort of driving it will sit around 208 -212 degrees, whether it be 68 degrees, 88 degrees. it doesn’t matter.

I’m concerned because it seems higher than it was before as well as when it idles it will creep up to 226-229, no matter what temp out

What left is there for me to try other than Water pump. Or should I bite the bullet and replace it already

 

208-212 seems pretty normal for regular driving to be honest. Keep in mind the fan doesn't kick on until like 226 or something like that.

 

I appreciate the quick response, and yes I have seen that on multiple places, i’m fairly certain even the owners manual.

ironically I got a memory on my phone from a year ago today, After a 30 min drive from work the coolant temp was 190-198.

Maybe i’m paranoid, however the last thing I’d like is a lingering coolant issue lol

 

If the Temps stay around 200 now and don't spike unless you've been sitting and idling, I wouldn't worry too much about it. The difference could be from the new thermostat, manufacturing variance, maybe old one was leaking, etc etc etc.

 

Those temps are completely normal.

I've owned 4 new C5s

 

I had an experience similar to this last fall when I replaced all my hoses- and on a whim replaced the thermostat since everything was already apart. After this, my temps ranged also from 208-214- higher than I previously recalled before the hose swaps. My issue was that i replaced a one-piece thermostat with a two-piece (not knowing that apparently the 2 piece is compatible with a newer style water pump).


Once I swapped back to a new, one piece thermostat, and burped the system per the FSM, I’ve been running consistently at 198-200 when driving. The 2 piece thermostat, while tight, didn’t seal correctly and was letting air into the system. I only found this out because it eventually manifested into a small coolant leak (I was troubleshooting this as time allowed over a couple weeks. The leak didn’t come about immediately). If not a thermostat issue in your case, maybe a hose is slightly loose allowing air into the system??

 

Hmmm you say 2 piece As in the thermostat and housing separate from each other?


I have an 04 and simply replaced the Thermostat itself. Maybe it could be a loose hose. but I don’t believe so. I can double check.


Im assuming if it isn’t these things and I am certain that I used the right thermostat. It either had to be the radiator but I saw no visible damage. Or the water pump.

 

You absolutely have to use the FSM procedure to remove air from the heads. Especially an '04 which does not have rear coolant air bleed piping.

 

One thing I thought of that could also play a role in slightly higher temps: Is your coolant level slightly low? At one point working through my temp issue, I saw my coolant was slightly low when I initially thought I filled it all the way.

Keep us posted on what gets you back to the temps you use to see!

 

I could be wrong but I believe in 2004 all LS1/LS6 motor water pumps had two piece thermostats.

Also did you measure the opening temperature of your new thermostat? Ive always come across a lot of new thermostats opening later then their designated temperatures. Best to be safe, throw the stat in a pan of water, boil and use a food thermometer and measure the temp of the water as the thermostat starts to open. As thermostats slowly open at the start of a temperature but won’t be fully open until a X degrees amount later.

 

Yes mine was a two piece, I was confused at first at what was meant by “one piece/two piece”…. Brain fog moment.

I actually did check the t-stat for fun, threw it in a pot of water and it opened fairly quickly in 190 ish degree water, can’t really remember off of the top of my head.

Im pretty convinced that the slightly above average temps have to be caused by some factor, I know tolerances are a thing but, it just seems like something is off.

 

The stock T-stat rated temperature is 187.

You could have an air pocket in the cooling system, if your temp is always above 200F even cruising.

Everytime I’ve changed out the coolant in my Vette, with the engine cool I’ve filled the coolant to the line, then parked at an angle on a steep driveway or hill with the coolant reservoir tank as the highest point of the cooling system with the car at an angle. Make sure car is pretty cool, coolant temp <100F, Turn on the car with Heater on high with max heat, remove coolant reservoir pressure cap, let it idle, monitor coolant temperature on DIC, once temp hits 200-205F degrees, quickly reinstall coolant reservoir pressure cap. Park Vette on a flat leveled surface, let car/engine fully cool. Monitor coolant level on tank if below line, fill too the line. The coolant in the tank went down because the coolant replaced the air that was trapped in the system.

Doing this process I’ve never had any trapped air or overheating issues, when I was bone stock or to my now current setup (LS3 water pump, 180 tstat, and dewitts radiator with spal fans).

By the way how did you clean your condenser and radiator?

 

To clean the radiator I took off the shroud, got it jacked high enough to get under it, took a homemade air nozzle with holes in the top of it and sprayed dust and gunk out of both the condenser and and radiator, as well as using some coil cleaner for extra measure.


As far as bleeding the system, I was pretty confident it was correct, doing cycling the engine, letting it cool, etc. If i wanted to basically “double check” could I not just open the reservoir cap now and do the same process over?

 

Thats good using Coil Cleaner. I always use a can, with low pressure water afterwards, I always do it every year before summer, to free up any dirt on the fins and keep air flow good for the condenser and radiator.


Yes you could double check by doing the procedure now, if theirs no air in the system and the temps don’t change.

Your tstat could be faulty, opening much later then intended. Do you still have your old tstat to pop in boiling water and check its temp compared to the new one that you measured?

Though the tstat only sets the floor for the temperature of the cooling system, its your cooling capacity that sets the ceiling of maintaining it. With your current tstat(stock temp) and stock radiator, your just cruising temps should be in 187-195 range thats what mine were when I was bone stock.

 

A few thoughts…

First, the condenser can get a lot of tiny stones in the fins that are almost impossible to see from the top or bottom. You can use all the air and water you want, but only a tiny screwdriver or pick with get those rock chips out. While I had my shroud out I climbed up inside the front and picked out rocks for an hour. Once I had enough of that I looked down and the floor was covered with them; shocking really. Must have been 50% clogged that “looked” clean of debris.

Second, the front coolant steam crossover tube is an ideal spot to bleed air from the system. I would first try the nose high method, mentioned above. If that doesn’t do the job just loosen the passenger side of the crossover a couple threads and let the engine run until fluid streams out.

Finally have a tuner turn on the first fan stage at 197° degrees when the ac is off. Made a world of difference on my ‘04.

 

okay okay, I am very thankful for all of these comments, it gives me a lot of different steps I can go through until it’s figured out.

First, The old thermostat was the one that I tested, I did not test the new one. Secondly, I do know coolant is full, when I drained and refilled system I triple checked that.

So I believe I will do the nose high method, if that doesn’t work, check the steam tube, and if that doesn’t work, I will get back under and search for more lodged rocks, pebbles etc,



also i would like to state I replaced the OEM stat with a duralast part.

 

I would change out the Tstat for another after testing. If it was a duralast tstat more then likely it was a repackaged Motorad tstat. Their quality control is so so….

For what its worth years ago when I first installed the LS3 water pump in my Vette with the already installed dewitts radiator I ran the car with the stock LS3 tstat it came with the waterpump(before my 174 tstat came in the mail) it ran too warm for my liking 198-208 cruising, weeks later I installed the 174 tstat, now it ran too cold, it would dip sometimes to the mid 160s but would cruise at 176-180 degrees, wasn’t a fan of potential coolant shock. I ended up buying 3 motorad #523-180 LS3 tstats, tested them in boiling water compared to my one piece OG LS1 187 tstat. Picked the one closest in opening similar to the original, and returned the other two. That tstat is still in the car today, it will cruise between 185-190 degrees and sometimes drop to 180 degrees coming to a stop at a light, a happy medium.

if the tstat is opening to late it will regulate the temperature higher then it should, sadly quality control when it comes to new parts isn’t always consistent.

 

You know I hate how the very first “rabbit hole project” in the two years of owning my c5 is a coolant problem.

What makes this interesting is that I never had issues until one random day, and like any issue i’d replace a part and then troubleshoot…… rinse and repeat. If the original t-stat I took out WAS opening and closing inside the boiling water pot at around 190 degrees.

The only time throughout this process I noticed a difference however was when I replaced it and drained coolant. To which the common response is “air in the system” or “low” coolant. But you would think that in the entirety of the two years of me having the vehicle, this issue would’ve been noticed by now.

What baffles me is, if the stat was opening normal in a pot of water, and the coolant was filled correctly, then why would I only see a difference in temps after replacing the two.

Anyways, that also has been a thought in my mind, replacing with another OEM stat if all the other tricks don’t work.

I have been monitoring it though, even today got up to idle temp, and for the entirety of a 45 min drive 208-210


Good news is, is that if everything else fails and I hire the bullet on a new water pump, I already have a set of 243’s, Cam, and an A&A kit,

 

^ i was having strange coolant issues about a year ago. a new water pump fixed it for me

 

After reading your situation it brought light to mine, Before replacing the T-stat and flushing the system, one of the hoses also felt collapsed, I verified it wasn’t normal by comparing with my fathers c5. Felt as if there was zero fluid going through, and sure enough it was true.

After replacing the T-stat I felt that fluid inside the hose. But temps are still a little higher than liking.