Engine Cooling System & Erratic Idling

[Reylito]

Last week I noticed that several times throughout the week, whenever I'm stopped at a light and place the transmission on Neutral (while the A/C is on), the idle increases drastically and drops and races again. Since this 2014 Ford C-Max Hybrid does not have a tachometer, I am guessing that it is possibly goes up to 3,000RPM on its own. Several days after while on the Freeway I heard a noise coming from the front of the car, which sounds like a metal bolt twirling around and around. A few seconds later, antifreeze suddenly burst from the hood of the car and I decided to make an emergency exit. After I opened the hood I noticed that the plastic coolant reservoir expanded and there's a hole on the front side it (see attached picture)? I'm not sure why did it build up so much pressure on the reservoir like that and blew a hole on the plastic, and why didn't the reservoir cap relieve some of the pressure?  

 

After I replaced the coolant reservoir with a new one, I filled it with orange antifreeze for North American cars. After installing the new part,  I started the engine, and the engine's idle was erratic. It races up and down throughout the 5 minutes that I had the motor running. There are No white smoke nor any other colored smoke coming out of the tail pipe. I didn't see any message on the dashboard other than an orange lit up engine icon. 

 

If this scenario has ever happened to you, please Advise what you did to fix this problem.  Thank you! 

 

Pls. See attached pictures. 

Edited August 3, 2023 by Reylito

[homestead]

WOW!  Hope nothing was damaged.  I would guess, take a look at the coolant thermostat.  

If that was stuck shut such that the coolant couldn't circulate maybe the pressure could built up.

 

[Lacienega]

Maybe the reservoir Cap is defect? Should not let the pressure get so high that the tank pops. Did you replace the Cap as well?

[nogoodbum]

Sorry, but from my limited knowledge & experience, signs are pointed to a head gasket failure. Cap & tank are made to deal with pressures in the 15psi range & temps in the 200f range. With a blown head gasket pressure can easily reach 100+psi & temps 400+f very suddenly depending on the cause. With the metal bolt noise, I suspect a broken valve, spark plug, or even a possible broken piston ring. As the broken part rattled around in the combustion chamber it may have got caught between the head & piston & opened enough of a gap between head & block to break the seal between them. You may be not getting the tell-tell white smoke of a coolant leak because of where the crack in gasket may be [intake, compression, or exhaust sides]. Also in order to see water vapor depends on volume, temp of vapor & outside temp. Engine light on indicates a repair code[s]. I'd check that first, Then check each of the plugs for which one is mis/not/firing. If after finding their condition & none of then are the source of damage, have or do a compression test to pinpoint the possible bad cylinder. An oily plug may indicate ring failure. Head gasket replacement is not very hard but is labor intensive. Really hope I'm speaking to a worst case scenario & you find a much less intrusive cause. Thanks for the read.