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Discussion Starter #1
cam home today and i turned off my car, heard a bubbleing sound. i checked my engine bay and the water in the resevoir bottle was boiling? does this mean im low on water or is something else the problem? i filled the bottle yesterday after the same thing had happened. should i be conserned? car doesn't overheat at all and no other problems are noticable. any help would be appreciated.
 

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I'd certainly keep a close eye on it for awhile, could be just low on coolant, but it could also be signs of a headgasket letting looose...
 

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good indicator that your cap isnt holding pressure. the cap is the easist thing to check, ahd cheapest to replace. if the rubber in the cap is dry and brittle at all, replace it asap.
 

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Keep a close eye on your water pump. If the coolant is boiling in your tank then your pump is not circulating the coolant effectively. Your pump is probally letting enough coolant flow by to keep the car from overheating completely.
 

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are your radiator fans kicking on? how's the radiator itself. On my mk1 almost all the fins are gone from age/road debris, definately affects cooling. If the fans aren't kicking on then you're certain to boil. Also if you have too much water and not enough coolant it'll boil sooner.

Mike
 

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Any one thing here could be a problem, but kbox's theory is what I've experienced, but it's due to air being in the system. Now you have to figure out why... My past experience points to a leak, usually at the water pump that isn't seen just idling. It's at pressure and speed where coolant works out the weephole on the water pump. Sometimes it leaves residue which can be seen. It's telling you that the seal is dying on the water pump.

So, are you loosing coolant slowly? I was loosing enough to cause this in 500-1000 miles straight highway running.

Like others said, the cap could be allowing pressure to flow backwards. Boiling happens at lower temperatures without pressure. Still, this is probably exceeding normal temperatures with proper flow.
 

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try replace the o-rings on the bleeding valve on top of thermostat housing or replace the plug with toyota part #90910-09093.
 

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Discussion Starter #9
thanks for the advice. much appreaciated. ill most likely replace the cap and flush the system since ive been only putting pure water since the last time i flush the radiater. the waterpump if brand new so im hoping that is not the problem, and with an oversized pully it would be spinning faster then normal. am i right to think it is most likely not the pump?
 

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If the pump is brand new, I also would hope it isn't that. Make sure you've bled everything 100%.

I've found leaks by driving the car until at normal temp, then stopping and revving it to 3-4K for a few seconds. Under the higher pressure they seem to have appeared whereas idling they'd stop. That's one way I've gotten air in the system after a repair.
 

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this happened to me about a 3 weeks after i bought my mk1 i just replaced the radiator cap and it never happened again
 
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