Question about 2002 Buick Century

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Can a bad intake plenum leak antifreeze into the engine on a 2002 century 3.1?The computer code was misfire #1 cylinder. Complete tune-up done, same problem, new#1 injector, same problem. Now the coolant is gone, no leak on ground. Shop says blown head gasket but I have been told to have them check the intake plenum by a retired GM shop tech. They tell me it can't be the intake plenum as "that only handles air, no coolant involved with that." HELP!!! Scott

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  • Jonathan S
    Jonathan S Jun 01, 2010

    The plenum is referred to as the upper intake no coolant runs through this.The lower intake can definitely leak antifreeze into the engine.Check the oil level if intake is leaking will end up down in the oil pan and raise oil level. You can check the plenum for cracks especially around the bolts.This will cause a misfire as well.The lower intake leaking should not leak into the cylinders though a blown head gasket would.

  • Jonathan S
    Jonathan S Jun 01, 2010

    In addition if the upper intake was over torqued after it was removed it can very easily crack.

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I see a lot of lower intake gasket leaks on these but usually you get an external leak or you'll find coolant mixed with the oil, not a misfire. Upper plenum is dry on the 3.1 and just bolts onto the top of the lower intake. We usually apply pressure to the cooling system overnight with the spark plug removed from the misfiring cylinder. If there is coolant in the cylinder come morning then you only have to figure out where it is coming from. If the plenum is removed during the test you might be able to see where the coolant is or isn't coming from with a borescope by looking down the intake runner.

Posted on Jun 02, 2010

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