2000 Pontiac Montana Tried everything, what now?

2000 PONTIAC MONTANA
125,000 MILES • 6 CYL • FWD • AUTOMATIC
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AJRING
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I changed intake gasket last July. Everything worked fine until about Oct. Occasionally it would overheat, and the heater blow cold. Always if the engine would even start to get over normal temp, the heat will blow cold. I have changed the water pump, serpentine belt, thermostat 2x, radiator cap. For a while I could bleed the air from the water pump valve and have no problems for that trip, but now that doesn't work anymore. Now the heat is always cold, and if I use low gear to keep the rpms up the temp stays down. usually at first idle the temp takes a while to go up then stays about half. If I drive then the temp shoots up fast to the red and somtimes drops but not always. I thought it might be the gauge so I drove with it hot, and it boiled over through the overflow jug. The oil has always been normal looking and smelling, and I have never seen any steam or abnormal exhaust. Any ideas?
Jan 10, 2009 at 6:18 AM
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SERVICE WRITER
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Sounds like a head gasket or cracked head problem.

They tend to crack between the valves. After a couple of drives, air begins to enter the cooling system. The breech happens when it it cools back down. The operating temp of the engine seals it up.
Jan 10, 2009 at 6:54 AM
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AJRING
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Thanks. Thats what I was afraid of but didn't want to hear. I'm still going to try flushing everything first just to make sure, and then I am going to change to green antifreeze to hopefully cut down on these problems in the future. Thanks again.
Jan 10, 2009 at 12:43 PM
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AJRING
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Well I changed the head gaskets and had the heads checked. They said the heads were good. I put it back together and everything works fine now. I even drove it to florida and back with no problems. It has been 4000 miles now and hasn't lost a drop of water "yet". Knock on wood.
Apr 16, 2009 at 7:17 AM
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SERVICE WRITER
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Looks like it was either a faulty installation on the intake or the head gaskets were bad huh?

As long as it works!
Apr 20, 2009 at 5:08 PM
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AJRING
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I forgot to say the head gakets did have rotten places down to the metal rings. Oh well. As long as it works now huh. thanks
Apr 23, 2009 at 9:40 AM