Hi all,

I have a 97 4runner with the 3.4. After driving on the freeway today, I noticed the coolant was boiling. I checked the temp gauge and it was normal. The engine has never over heated on me. The over flow tank was completely full. I don't know what the deal is... Could it just be that the coolant system is losing pressure? Possibly a bad radiator cap?

Thanks

 

Could be a bad cap? Coolant boiling over out of the overflow tank?

 

It didn't boil over, but it was pretty full. Once the engine cooled down, the level went back to normal.

 

if your coolant system isn't holding pressure (1.2 bar is stock, I think), then it'll boil out. Increasing pressure on the system raises the boiling point. If it's not holding pressure you'll boil out. The radiator cap is the usually prime suspect. If you haven't changed it yet, just do it, they're cheap. Keep looking your coolant system over, noticing any cracked hoses, puddled coolant, etc

 

Boiling point at Orem UT is about 203 degrees (pure water, but still close enough).

We've had two times where my wife's 4Runner has boiled the coolant, both of them were high in the mountains in Colorado, working hard going up passes. In both cases, the culprit was a failed coolant cap. The first time the OEM cap had broken after 15 - 16 years. Short on options, we bought a replacement at Autozone. The next year, that one failed (there's a light spring that is supposed to pull the drain back valve shut that failed and left it open all the time). Bought an OEM replacement that time froma dealership and it's been fine since.

In both cases, it's likely that the cap failed some unknown amount of time prior in Missouri, but the lower altitude here kept it from boiling. Get up a little higher, though, and the lack of pressure plus the lowered boiling point gets you into trouble.

 

As others said, replace the cap. It is the easiest thing to do and cheap as already said above. I have talked to people over the years that act like Radiator caps do not go bad. Oh yeah, tell that to all those ford powerstroke owners that had the poor factory cap fail and ruin there expensive engine.

I actually had a cap fail on a Honda Civic before as well. To me it is one of those maintenance items, whenever it is time to replace coolant, replace the cap along with it.