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Coolant blowing out reservoir

Old 02-25-2019
Thread Starter
Join Date: Feb 2019
Location: Lawton
Posts: 1
Coolant blowing out reservoir

Hey all. 1999 Ford Ranger. Water pump and radiator cap replaced last year, radiator flushed about a year ago too. I believe the thermostat was replaced about 2 years ago. For a few weeks it's been running a little "cool". Temp reads normal while giving it gas but ticks down a little when idling. Coolant makes a loud bubbling into reservoir right after shutting engine off for the last few days that I've noticed. Was driving today and temp gauge shot way up, read overheating. I pulled over and popped the hood, coolant was overflowing the reservoir, blasting out of the little pressure relief pinole in the cap. I released pressure on the radiator cap (radiator level was WAY down) and gently hobbled her home. Let it cool down then filled up radiator and turned it on to check head gasket. Water would gently pulse out of the top of the radiator, but not shoot out or bubble. Any ideas? Thanks so much
Old 02-25-2019
RF Veteran
iTrader: (1)
Join Date: Jan 2014
Location: Vancouver, BC
Posts: 12,744
Welcome to the forum

With cold engine Cooling system has on internal pressure
So with rad cap off after cold start then should be no coolant coming out of rad cap opening
Might be one surge at start up when water pump starts to circulate coolant

But no pulsing after that, there nothing in the cooling system that "pulses", water pump circulates at a constant speed

A cylinder leak however does "pulse", on each compression/ignition stroke of that cylinder

You can do the Glove Test, its free and 100% accurate, to see if you have a head gasket failure or cracked head

Instructions in post #5 here:

What happens is the leaking cylinder pumps air/exhaust into the cooling system, this displaces coolant in the heads and pressure in the system goes above 15psi, rad cap rating
This sends too much coolant over to the overflow tank
And this air in the system stays at higher points in the heads(and radiator), this lessen cooling so engine starts to overheat
When you shut off the engine water pump stops circulating coolant and any coolant in the heads will get hotter which cause pressure to go up above 15psi if it wasn't already
Since there is now air in the radiator, and its at the top then it is sent over to the overflow tank and bubbles up.

Its possible you just have or had a leaking in the cooling system and that sucked in air when engine was cooling off each time over the last week or so, and that caused the over heating
Refilling should be done with 1 heater hose unhooked at the firewall so air behind thermostat can get out and system is purged of air.

But that wouldn't account for the pulsing and coolant overflowing when cold engine is running
Old 02-26-2019
Join Date: Mar 2012
Location: Hampton,VA
Posts: 12
My old 97 ranger did that when the thermostat stuck shut. Back pressure forced everything into the expansion tank. Even with an almost empty radiator, the cap was still pressurized from pockets of water boiling. Sadly it won't always show on the gauge because the sending unit is between the radiator and the thermostat- the one place cool water would be trapped. Take the thermostat off and boil it. If it doesn't open, that's your problem.
Old 03-08-2019
Join Date: Mar 2019
Location: Longs, SC
Posts: 35
Sure sounds like a thermostat. I would swap it for a new one for just a couple of bucks. If you still have the issue I would have to agree with RonD above. Head gasket or cracked cylinder head.
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