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Discussion Starter #1
I just replaced the cracked heads and blown head gasket on my step moms Buick Rendezvous. She got it hot in some Christmas mall traffic and couldn't get off to the side to shut it down. It was bumper to bumper. Anyway, I towed it home and did the necessary repairs. It runs great with no codes or anything.

But here is the problem. When driving it down the road the temp gauge reads below 1/2 then shoots up fast to 3/4 and then back down to where it was for a while then repeats. Could this be an air pocket or something?
 

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Yes. Check for air bleeds in the highest point of the coolant system.
 

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sounds like an air pocket. i got burned on one when i used to work at gm. ever since id always use there vacum fill system. dont rember if the 3400 had that air bleed screw on the coolant pipe in the front?
 

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Yeah, I did check the air bleeds. It has two of them and I THINK all the air is out because it had a great stream flowing out both bleeds. I did change the thermostat when I did it.
 

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Discussion Starter #9
Just re-bled the system and now the temp gauge has a steady reading. It was definitely an air pocket.

Here is the new problem. After it gets up to operating temperature and you go put it in park it'll die. In fact, at stoplights it will die. It always starts right back up and runs fine with no codes of any kind. When put it in the garage I could smell the converter (rotten eggs).

Now, I've been told that a badly blown head gasket could allow enough antifreeze get into the cat and mess it up. I wonder if this is causing some sort of back pressure and killing the engine. Seems like it would misfire though if this was happening. Any thoughts?
 

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Not sure when they quit exactly but a lot of GM stuff had to do an idle relearn after the battery was unhooked. Been awhile at a GM dealer but mainly remeber doing it on a lot of W-bodied cars and a few N-bodies with V6's. Maybe somebody who remembers more than me. May not even pertain to your car. Being a 3.4 I'm sure it's 95-up. And this was done on OBD1 cars.
 
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