Pontiac Repair: re:1996 pontiac bonneville - anti-freeze

Question
Hello Mr Puccio- I am having a problem with my car.  The anti-freeze seems to disappear.  Last summer, my husband filled the overfill with what I believe was just water.  a few days later we had a huge spill on garage floor.  mechanics fixed the "plug".  recently at oil change they said they had to put 5 gallons in radiator and I told them I had filled up overflow just night before.  they also said the previous oil change they had to add about a gallon on anti freeze.  My husband took it to a shop that said the plastic part on the right hand side had a crack.........replaced it for $200.   It was fine for a week or 2.  I seem to be able to a noise like a "radiator" and I notice issues seem to happen if the "heat" or "defrost" have been on.  My husband filled up the radiator then took it to the shop next day.......they found no issues but also filled up the anti-freeze.  We just don't know where all this anti-freeze is disappearing to?  Any idea?

Thank you so much,
Natalie Fitzgerald

Answer
Hi Natalie:

those 3800 V6 engines were noted for coolant leaks due to the plastic parts the put on the engine. the part you were referring to was an coolant bypass elbow for the heater. very common. the other things that leak on that engine are the intake manifold gaskets (lower) intake the can leak either externally on to the ground or worse into the engine (very bad) the other this that was common for that engine was the upper intake manifold it's self. over time the plastic distorts and it will leak either external or internal to the intake and then it will burn in the engine the issue with this is they uaually leak when the engine is cooling back down from a hot heat cycle. if the leak is bad enough it can actually fill up one of the cylinders in the engine and when you try to crank the car cold it will actually Hydro-lock the engine meaning the cylinder is so full of coolant the starter motor can't turn the engine over because  you can't compress a liquid. that is bad as well. obviously I can't tell you what the issue here is with out seeing it I'm just giving you possible causes. hope that helps. Good luck :-}