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TL;DR - overheated on idle; replaced thermostat, pump, cleaned radiator...same issue on cool night idling in garage even with fan blowing air into scoop
I have scoured the forums for other options, but haven't come up with anything.
A couple days ago, drove home after work, parked in street at house to run into garage. Spent a couple minutes.
When I went back to the car, first thing I noticed was exhaust was shaking...thought it was odd.
Got into the car and immediately noticed the air was blowing HOT. DIC was chiming and showing "Reduced Engine Power." Coolant gauge completely maxed out (>260 F). Turned off engine then popped hood and took a look around. Noticed that the street was covered in coolant. After top and bottom inspection, all the coolant came from the pressure cap on the surge tank. No leaks from hoses or otherwise.
Next steps I performed
Pulled in garage later that night, topped off coolant. Ran car on idle and temp climbed up to 245 before I shut it off
Drained coolant (did break metal thingy on the petcock...)
Coolant looked normal
Spent an inordinate amount of time getting radiator out
Radiator wasn't really that dirty, but cleaned radiator fins anyway; radiator also seemed to have normal flow from inlet to outlet
Removed thermostat; tested on stove and it was operating normally
Removed water pump and replaced
Even though thermostat was fine, still replaced it
Replaced pressure cap
Reassembled everything
Refilled with with pre-mixed 50/50 DEXCOOL
Burped exactly according the service manual
Following all this, I started the car back up in the garage at night (81 F) and let it idle. Temp climbed all the way to 255 F before I shut it off. Let it cool and tried again with a box fan blowing air into the scoop. Same issue. No coolant came out this time, though.
During this time, I was able to verify that both driver and passenger fans were being turned on by the ECU.
The one thing that I found odd during the idling process is I can't get the A/C to blow hot, even with the temp up to 90 F on the controls.
I would say it might be a faulty ECT, but that doesn't explain the original coolant spillage.
I thought my HVAC system needed no new actuators, no heat car was running fine for 2 weeks. Pulled into work temp skyrocketed to hot, I shut down at 240. Replaced thermostat and everything went back to normal.
Last edited by RIKKI Z-06; Jul 14, 2019 at 01:07 PM.
How long to get from from cold start to 255 idling?
I actually went out and recorded the temp and fans at every 15 second interval. This is idling in the driveway with a box fan blowing at the scoop.
See attached graph.
It took 9 minutes for the temp to reach 235 and the fans to kick on.
It took until 12 minutes 30 seconds for the temp to reach 252, when I turned it off.
No, I should have done that went it was all apart. Wish I would have, seems like a likely candidate for the problem.
There's not much in the block anyways so it don't matter. If your radiator don't have leaks and your thermostat was swapped, check for leaks around the heads.
After all that I would bypass the heater core. Do you have any signs of loss of coolant anywhere?
Check the oil and the cap for any signs of coolant leaks.
Inspect the surge tank on the bottom t connection
Inspect the top and bottom corners of the rad
Get under the car to look for signs of dried coolant which will look chalky white.
Last edited by My Vette Life; Jul 15, 2019 at 10:36 AM.
Very interesting. That's not good behavior, that's for sure! And you've done pretty much all the steps we usually recommend.
One thing that sticks out to me is that you said the fans kicked on at 235F. The high-speed fans, yes. But what about the low-speed fan setting?
The one thing that I found odd during the idling process is I can't get the A/C to blow hot, even with the temp up to 90 F on the controls.
No hot air from HVAC system even when coolant is hot? That's a classic sign of a problem with either the heater core, or often, the coolant being low. Since your coolant is decidedly not low ... maybe it's worth looking into the heater core. As mentioned above, one thing to do is to bypass it by rerouting the coolant hoses around it - take the "to heater core" line and just attach it to the "from heater core" port.
Is there air flowing THROUGH the AC Condenser and Radiator?????
There is a very common problem with our C5s. There is a 1" space between the AC Condenser and Radiator. It commonly gets clogged with grass/trash/sand/dirt. You need to pull the air bridge and the plastic cover over the radiator and inspect that space.
BACK Flush the cooling fins with a garden hose on direct stream and see what all comes out. After that procedure see if it returns back to normal.
Also look at the cool air intake in front of the car and make sure its clean and not blocked with trash/leaves/dirt.
...It took 9 minutes for the temp to reach 235 and the fans to kick on...
Originally Posted by gimp
...One thing that sticks out to me is that you said the fans kicked on at 235F. The high-speed fans, yes. But what about the low-speed fan setting?...
It seems the Low Speed mode is not kicking in. That would explain what's happening!
Both fans are supposed to start in low speed at 226°F. If temperature continues to raise and reaches 235°F then they change to high speed. Once temperature drops back to 226°F they change to low speed again, and if temperature reaches 219°F they are turned off.
Take out the thermostat and put the housing back on..;;;;Fill it back up. Fire up the motor and see what happens. If it still overheats, then you have a restriction somewhere. Did you verify that the radiator would flow collant?.....Hose in one end and watch flow at other end?? Run water through the block with a garden hose. Hose in the top hose and watch water come out the bottom. It would be nice if you could put an A/C thermometer in the radiator somewhere and run the motor. Compare block heat to radiator heat. Im leaning towards that radiator,,,, As they get old, they loose their ability to transfer heat from the tubes to the fins. Usually they dont go bad instantly.. It takes a period of time. Try these things and see what happens. My bet is on that 16 year old radiator.
There's not much in the block anyways so it don't matter. If your radiator don't have leaks and your thermostat was swapped, check for leaks around the heads.
After all that I would bypass the heater core. Do you have any signs of loss of coolant anywhere?
Check the oil and the cap for any signs of coolant leaks.
Inspect the surge tank on the bottom t connection
Inspect the top and bottom corners of the rad
Get under the car to look for signs of dried coolant which will look chalky white.
Absolutely no coolant leaks I could detect anywhere
Very interesting. That's not good behavior, that's for sure! And you've done pretty much all the steps we usually recommend.
One thing that sticks out to me is that you said the fans kicked on at 235F. The high-speed fans, yes. But what about the low-speed fan setting?
When I monitored the temps and time, I was sitting in the car with the hood closed. I didn't hear them come on, but the sound could have just been suppressed. I'll check again standing close to engine.
Originally Posted by gimp
No hot air from HVAC system even when coolant is hot? That's a classic sign of a problem with either the heater core, or often, the coolant being low. Since your coolant is decidedly not low ... maybe it's worth looking into the heater core. As mentioned above, one thing to do is to bypass it by rerouting the coolant hoses around it - take the "to heater core" line and just attach it to the "from heater core" port.
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HELP! Overheating coolant
It seems the Low Speed mode is not kicking in. That would explain what's happening!
