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Engine running too hot on hwy, overheated when parked

J

jonward786

Member
Hey i was wondering if anyone would have any ideas about an overheating issue i had yesterday. the engine is a new rebuild with only 300 miles on it and it was the first long distance (150 miles) trip i had done. it wasnt a hot day and the air temperature was cool. noticed the temperature gauge sitting at about 80-90% the whole trip, then when i got to the garage and turned it off it overheated and coolant overflowed.

my first thought was that the thermostat wasnt opening all the way, but after doing a little more research, i read about air pockets in the cooling system and having to "burp" the system to remove the air. so the following morning i ran down to the garage, replaced the coolant that had overflowed, and started the engine with the radiator cap off. i let it idle for about 5 minutes and watched as the coolant flowed from passenger side to driver side as it should. however, the coolant began flowing IMMEDIATELY following engine starting, and i was under the impression that the coolant should only start flowing after the engine has reached operating temperature because that is when the coolant is directed towards the radiator. i did also notice a good amount of bubbles on the surface of the coolant as it flowed by. after about 5 minutes i shut it off, and replaced the cap.

i feel like seeing bubbles meant i could perhaps have removed some air in the system. but is it normal for the coolant to start flowing as soon as the engine starts?
 
Was the engine cool when you added the coolant or was it still warm from your trip....it takes some time to cool that much....I bet still warm and the thermostat was still open. Also, running hot on the highway could be a circulation issue. If the thermostat never closes, the coolant will not have a chance to cool in the radiator.

Also, hook up an after market gauge to see what the temp really is as the factory guage is a guess until you cross reference it's position with a better gauge.

The bubbles could be trapped air but they should stop soon. Another air source like that is a compression leak into the coolant.
 
Yeah the car sat overnight in san jose, so it had plenty of time to cool off. thats why i was confused as to why the coolant would start flowing as soon as i started the engine haha. ive never seen my temperature gauge go above middle while on the highway, the only time it gets hot is when i sit in traffic on a warm day most likely because i have no shroud. i kind of want to replace the thermostat just to rule it out, i have a new one and the gasket/sealant on hand

on a side note, i did this little trip with the vacuum advance line blocked off. maybe more advance while cruising will be less heat?
 
I would do the thermostat since you have all the things to do it. Since it was cool, it sounds like yours is bad.
 
Your thermostat is in stuck open position.
Coolant flow to the rad should be after it has reached operating temp.
To ensure complete air removal from system, Place heater control in heat temp high position fill rad with coolant till it reaches 1 inch below cap.
Start engine and let reach operating temp with coolant freely flowing and slowly fill with coolant to bring level to just below the filler neck and replace cap.
 
Ok but why would a thermostat stuck open cause the engine to run hot, seems the exact opposite should happen and it should run cold
 
The thermostat has two functions.

It holds the water in the engine to help it warm up to operating temps faster. Really minor function.

Then, it opens and allows water transfer. Once closed again, it must stay closed long enough for the water in the radiator to cool from the air flow. This is a balance of time based on how fast the engine heat is absorbed by the coolant in the block vs the cooling effect of the air flow.

If the thermostat is stuck open, the water continues to circulate thru the engine picking up engine heat but not being able to shed that same heat in the radiator. The coolant keeps getting hotter until you come to a overheat problem.
 
A couple of things my dad taught me
when the thermostat is open & the water is circulating too fast to cool down in the radiator, turn the heater to hot and get the water flowing through the heater box as well.
And dont sit there idling, get some speed up and some air flowing through the radiator
It should bring the engine temperature down
 
sorry, im not understanding what you're talking about with the whole opening and closing thing of the thermostat. i was under the impression that the thermostat opens when the coolant in the engine reaches a certain temp and then stays open to allow circulation through the radiator. i guess i just do not understand how a stuck open thermostat could cause overheating. it opens, letting the hot coolant flow into the radiator to be cooled. once this happens the thing stays open until the engine is cooled by being shut off overnight, am i wrong?
 
"jonward786" said:
Ok but why would a thermostat stuck open cause the engine to run hot, seems the exact opposite should happen and it should run cold
Click to expand...

It does run colder for a longer period of time till it warms up. No restriction of coolant flow.
The typical thermostat 180* is not fully in the open position till temp reaches about 187-188*
As the rad cools off the fluid as it passes thru it the thermostat opens and closes to keep the engine temp even.
It will only be fully open when the temp rises past the full open point and remains there, Now the fluid going into the rad has less time to cool and returns back to the block heated causing the temp guage to rise.

You would be surprised on how fast the stat will close with just a small amount colder coolant introduced into the system.
Try burping a 24 in rad sometime, pouring as little as 8oz of cold water into the rad top will shut the stat down to half and trapping air in the system
prolonging the air to purge till the coolant temp rises.
 
Replace the thermostat, making sure it is installed correctly, and re-install coolant package and expel air as suggested in thread. Install fan shroud.

I also found an aftermarket clutch fan to outperform the stock fixed blade fan substantially, and is quieter to boot.

If all else fails, presuming you have an OEM radiator, replace with an aftermarket model. Ever since I put one of those Northern aluminum ones in the D-coder, it runs like an iceberg. It used to run a lot like you're describing with the OEM rad.
 
Do a stove top test of the new stat before you install it. As the water reaches the opening temp, watch the stat to ensure it does infact open near the correct temp.

Yes, if you don't have a shroud, install one.
 
Ok i installed the new stat this morning, started the engine, no coolant flow through the radiator, ok good. i let it idle with the cap off for about 6 minutes and the temperature gauge slowly climbed to about 40%, just before midway between cold and hot. but as it neared the 6 minute mark i was seeing the coolant level in the radiator slowly rise, very slowly, until it ended up trickling out the top of the neck and then came out pretty violently. so i shut the car off and watched the coolant level recede back down inside the radiator to at or maybe just below the top fins. that is normal correct? the engine didnt overheat or anything, it never even passed midway, just seemed like pressure was forcing the coolant level up, and if i had the cap on it seemed like it might have overflowed through the overflow hose.
 
That was possibly just an air bubble trying to escape. You may have had the radiator to full to begin with for the burping to work. Try it again. Just as it is, start it and watch for the coolant to rise and then an air bubble or two to pass.
 
Have you pressure tested the cooling system? If you are having an unusual amount of pressure build up in a short amount of time it could be a cylinder leaking pressure into the cooling system. I hope this isnt it but you should keep it in mind if it continues to behave like this
 
Prob air bubble like Pete said, But if your getting a repeated air bubbles after coolant flow is regular then you may have compression gases being pumped into the cooling system.

As air can be compressed and fluids can't that sudden purge may have been air looking to get out and pushed its way up forcing the coolant in front of it out. This is why letting it idle and a slow refill of rad of coolant is important, small air bubbles can be trapped just waiting for the right amount of coolant to force them out. You'll know when they are all out when you see the coolant flowing thru the rad like a small steady stream past the cap opening,
And the level stays steady.
 
Now on a side note, If your getting a repeated coolant purging or small air bubbles in the coolant stream after warm up.
You have a seeping head gasket condition. Compression gases are being forced into the cooling passages and will purge the coolant.

You can eather get the test kit that uses a dye to reveal exh gases in coolant or try the ballon trick by closing off the overflow tube and strech a ballon over the filler neck while it idles. If the ballon inflates, there is gases being pushed in.
Another way is after you have the rad completely topped off, run the engine at 2500 rpm for 2 min and shut it down.
If the coolant starts purging from the overflow you have gases present.
 
Not to hijack, but on a related note, my Cuda has always run hot. I have just now got the temps mostly controlled now with the addition of a pusher fan.

Strangely, the radiator never really builds pressure. When its over full, it will push some over board via the over flow tube but thats it. You can open the system when its a nice and toasty 210* with no noticeable pressure release. New and correct 16psi cap. Radiator stays full, and am not losing coolant. Also, is not in my oil and I cant detect any via the exhaust. System pressure tests good.

Cooling issues can be a pain and I am always interested to hear of working solutions.
 
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