Engine coolant bubbling over inside coolant overflow tank

VICMUN100

MEMBER
2002 MAZDA TRIBUTE
3.0L
6 CYL
4WD
AUTOMATIC
110,000 MILES

I have a cooling problem where my engine coolant becomes hot and and 'boils over' within the plastic coolant overflow tank and discharges out of the overflow tube. This condition happens after I have driven about 5 kms from a cold engine condition. At that point, I observe that the 2 (two) radiator cooling fans at the front have not even kicked in and not spinning at all. In this situation, I suspect two possible causes (although there could be more): (a) the thermostat is faulty and/or (b) the 2 radiator fans are spilt. I need to use the car today and is the weekend now so getting a new thermostat is a couple if days away. Is there a temporary fix that I can do today to get it on the road without harming the car engine? For example, is it okay to take out the thermostat, reseal the joint properly and drive the car without a thermostat for a couple of days while waiting for a new thermostat to arrive? And how do I hot wire the electrical wiring to test if each or both of these two radiator fans are working? Any urgent advice will be most appreciated. Also, if these two fans are indeed working, how to I put in a ‘all the time switched on’ condition as a temporary measure whenever I drive this car over these couple of days just so that I do not inadvertently overheat the engine while I sort out this car coolant overheating problem permanently? Of course, I am taking this time to also look for any leakage of engine coolant, leakage of built-up internal pressure and discharging captured internal air bubbles through proper ‘burping’ of the engine coolant. Your advice or feedback please. Many thanks in advance.

1 Reply

MOTOR MASTER

MECHANIC
279 POSTS

Hello,

It sounds like you have several possible causes to your issue. My first suspect would be the coolant cap. It has a pressure rating and what the cap does is allows a preset amount of pressure to remain in the cooling system which actually raises the boiling temperature of the coolant. When the cap looses the ability to keep that pressure the coolant boils at a lower temperature, lower than the opening temperature of the thermostat and also lower than the temperature that the sensor that the fan coolant sensor is triggered to turn the fans on. The end result is that the coolant boils, expands and "boils over" into the expansion tank, and most likely raises in temperature higher rate than the system is designed to handle, and it also looses coolant and gets air into the system and air does not get rid of heat as effectively as water, yo causing the temperature to rise. If the problem is the cooling fans the easiest way to determine this issue is to observe if the temperature goes down with speed, if it does and tends to overheat at lower speeds and lowers with speed then that would indicate that there is an issue with the cooling fan system, this is where I would start and then there are further steps to follow up after those steps. An important fact to keep in mind while diagnosing cooling system issues is that the sensors, for both the gauge and the fans, are designed to detect liquid coolant temperatures and if there is a low coolant situation they will not function at 100% accuracy and this also includes the thermostat. Usually the thermostat will cause abrupt and sudden changes in temperature. I hope this helps and thanks for using 2CarPros! If you need any more assistance please feel free to contact us again!