Engine Cooling problem
2001 Pontiac Montana 6 cyl Front Wheel Drive Automatic 108000 miles
The story goes like this. I have noticed that the brakes are " mushy" feeling on my Montana. Changed pads and shoes, replaced both wheel cylindars and bled the lines. Refilled the resevoir as needed during bleeding and after. Also, have had an ongoing CEL code for an O2 sensor, and have replaced both O2 sensors on the van. Took vehicle to mechanic to have the customary oil change at 3k miles. This by the way is a new mechanic whom I have not used before. While checking other fluids in the van, he directs me to come and look at the radiator to witness all the sludge after he removed the radiator cap. This is a dark brown color and thick as red clay mud you would find in the south. He then goes on to tell me that this is typical of Pontiac V6's having a cracked intake manifold gasket. I understand that both the 02 sensor and mushy brakes could be due to a vacuum leak of a cracked intake manifold gasket; but I didn't mention either scenario with the mechanic. Can the intake manifold leak cause a problem with the cooling system like I have described. There has been no issues with power loss or poor running of the engine. I plan on flushing the system first thing in the morning, but should I seriously look into having the intake manifold plenum gasket replaced?
The intake gaskets went through several design changes to correct a coolant and oil mixing for the last ten years. Sludge in the coolant may be a symptom of this but is extremely rare.
The intake manifold gaskets leak coolant into the valley between the cylinder heads causing the most common symptom of coolant mixing in with the oil in the crankcase, not the radiator.
The sludge buildup in your system is caued by the original coolant engineered by GM called Dex Cool that was found to be caustic to several things in the engine. You guessed it, one was intake gaskets. But back to the side effect, after sitting for few years the Dex Cool coagultes like and becomes that brown color in you radiator. GM even settled with the public last year to pay or reimburse part of the cost for these repairs. Unfortunately the settlement ended October 27, 2008.
Unless there is other evidence besides the coolant the vehicle should have the coolant thoroughly flushed and replaced with another long life brand. If you have owned this vehicle for at least half its life, and never had to replace the intake gaskets it is very likely they will fail in the next year or two. This engine has its share of internal issues with piston slap and sunk lifters, which can get worse much faster with coolant in the engie.