Heater/Cooling System Mystery on 1995 Buick Century

I've asked a number of knowledgeable mechanics and we're all stumped. Figure this one out and you win the cookie. :) Symptoms: At normal driving temperature, the temp gauge fluctuates between "Normal" and "Red Zone." When the temp gauge goes UP, the heater blows COLD. Rev up the engine a bit and the temp gauge goes back down to "normal" and the heater blows HOT. We've replaced: The Radiator, The Thermostat, The Water Pump, We've flushed and back-flushed the block, the Radiator and the heater Core. The Coolant is new 50/50 mix and we've properly bled the air out of the system. Any Takers?

by in Fairview, TN on December 30, 2010
8 answers 3 comments
ANSWER by on December 30, 2010
It sounds like you may have a blown head gasket or cracked cylinder head. Combustion getting in the cooling system can cause an intermittent air pocket, causing the temperature to rise and the heater to blow cold. If I am right how do I collect my cookie! :)
COMMENT by on December 30, 2010
Makes sense, but if that were the case, wouldn't we be seeing oil in the coolant or coolant in the oil? How would we check for this? I do remember seeing some brownish, liquid-like goo in the PCV Valve when checking for vacuum leaks. Would this also point to a cracked head or blown gasket?
COMMENT by on December 30, 2010
You may be seeing the very first signs of a head or head gasket issue. The best test would be to check for CO or HC in the cooling system. You can use a "Block-Check tool or an exhaust gas analyzer held over an open radiator - "sniffing" the hot fumes from the radiator. Please read this RepairPal article - http://repairpal.com/testing-for-head-gasket-failure.
ANSWER by on December 30, 2010
ck vac leak go to inside dash i thank the vac hose u looking for is on driver side heater door is not opening
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COMMENT by on December 30, 2010
But how would a vac leak cause the temp gauge fluctuation?
ANSWER by on January 26, 2011
beginning of head gasket.I had same symptoms.Air in system.So when you rev.it the pump is spinning fast enough to push coolant.Gives you heat.when i was at red light it would blow cold.my temp gauge all over the place.I would bleed system ,add coolant.everything fine.Then coolant level low again more air.I would repeat bleeding process.Eventually the head gasket blew completely.oil in radiator and coolant in oil.Maybe you can just bleed it and add coolant.But if it happens again and no coolant on ground or engine then its going somewhere.
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ANSWER by on September 11, 2011
I agree with Jim. you have a head gasket starting to show sugns. have a shop do a leak down test and a hydrocarbon test to verify. Roy
ANSWER by on February 09, 2012
same thing for my 2001 saturn sc1 it was a cracked block, since i had just bought the car the dealer just put a new motor in it.
ANSWER by on April 10, 2012
WHAT ABOUT THE HEATER CONTROL VALVE MIGHT BE PARTIALLY OPEN AND NOT FUNCTIONING WHEN YOU REV THE ENGINE IT COULD BE FORCING MORE CIRCULATION THRU THE VALVE AND CAUSING IT TO BE HOT INSTEAD OF COLD AIR ???????????
ANSWER by on January 24, 2013
bottom radiator hose collapsing. sucking together. not letting water circulate. it's making your enginr run hot . when you rev engine it sucks to gether. replace hose. your systems are exactly what happens when a bottom radiator hose collapses.
ANSWER by on March 10, 2013
The problem is lower radiator hose. It is dual walled and the inner wall is collapsing . Revving engine increases pressure pushing past "bubble". Remove lower hose and inspect. Good luck