Engine Coolant boiling in hose?

Teen'93stang5.0 · Jun 3, 2026

Evening all,
I took my project around the block after work today to try to recreate an odd stutter it seemed to develope the other day. Let it warm up in the driveway for ~ 5 minutes. Drove it about 1.5 miles total to and from the gas station. (Shut it off while filing up). Maybe 20-25 minutes runtime total.

It had a little trouble starting after I filled up, which I think has more to do with the charging system. But when I got home I could tell it was hot and went to check under the hood. You could audibly hear the coolant bubbling inside the upper rad hose.

I’m thinking there’s a blockage? Or a stuck thermostat? Any help is appreciated

Teen'93stang5.0 · Jun 3, 2026

For context the dash is non-functional aside from the tach. So I had no way to see what the coolant was reading. It’s on my to-do list

AeroCoupe · Jun 3, 2026

So I would not drive that car until you get an oil pressure gauge and a coolant temp gauge.

Have you checked the coolant level and do you have an overflow tank?

Noobz347 · Jun 3, 2026

Wut He Sed

Teen'93stang5.0 · Jun 3, 2026

AeroCoupe said:
So I would not drive that car until you get an oil pressure gauge and a coolant temp gauge.

Have you checked the coolant level and do you have an overflow tank?

Yep. It’s parked until I can get some gauges, or more ideally the dash to start working again.

To answer your questions yes and yes. I noticed the coolant was a bit low in the radiator when it was cold. I topped it up before driving.

KRUISR · Jun 3, 2026

Do you know all the air is out of the system? If not, cold start the car (on level ground) with the rad cap off and let it idle until the t-stat opens (you will see coolant flow coming through the rad tubes). Then let it circulate for another 5 mins or so. Then top up the rad and put the cap back on quickly. Shut the car down and after it cools top up the overflow tank to the proper level.

Air should be out of the system and you will have the correct coolant level.

Teen'93stang5.0 · Jun 3, 2026

KRUISR said:
Do you know all the air is out of the system? If not, cold start the car (on level ground) with the rad cap off and let it idle until the t-stat opens (you will see coolant flow coming through the rad tubes). Then let it circulate for another 5 mins or so. Then top up the rad and put the cap back on quickly. Shut the car down and after it cools top up the overflow tank to the proper level.

Air should be out of the system and you will have the correct coolant level.

I did this last time I flushed the coolant, but I definitely lost some coolant after that (lower rad hose wasn’t tight enough)

Mustang5L5 · Jun 8, 2026

To boil coolant with a 16psi rad cap, you'd have to be hitting 250degrees or more. If the cap has failed, this number will be lower.

It's likely air bubbles coming out in the system. I've generally had decent luck with these systems self-purging. The air bubbles to the radiator, pushes out into the overflow, and draws coolant back in as it cools. Key is to top the radiator and overflow up when it cools down to remove the air from the system.

Of course, you need to have a coolant gauge to make sure you are not overheating. But as long as you aren't, should take 2-3 driving cycles to really purge the air out.

This took will usually do it in the first cycle.

https://www.amazon.com/Thorstone-Radiator-Automotive-Antifreeze-Universal/dp/B097BHJJPS/ref=asc_df_B097BHJJPS?tag=bingshoppinga-20&linkCode=df0&hvadid=80127120445294&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=104035&hvtargid=pla-4583726601493712&psc=1&msclkid=7fb3913823451994c159fb1ee6e23677

Engine Coolant boiling in hose?

Teen'93stang5.0

Teen'93stang5.0

AeroCoupe

lube between the nut and the face. I know my lubes

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

Teen'93stang5.0

KRUISR

Teen'93stang5.0

Mustang5L5

That is…until I whipped out my Bissell

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Engine Coolant boiling in hose?

lube between the nut and the face. I know my lubes

Stangnet Facilities Maint Tech... Er... Janitor

That is…until I whipped out my Bissell

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