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Hello,I will be glad to try and assist you with this,Welcome to Just Answer.
I just need a little more info to assist better,
Was the intake manifold gaskets replaced due to a coolant leak and were the intake manifolds themselves replaced as well?
Is the vehicle or has the vehicle been going through any coolant now or in the past?
Let me know,Thanks Pete.
one of the biggest problems these engines had in this year range is the lower intake manifold egr stove pipe sits to close to the edges of the upper plastic intake manifold therefore heats up and melts the upper intake manifold and causes coolant to leak from the upper manifold into the engine and can and will contaminate the engine oil and damsge the crankshaft bearings,GM is aware of this failure and advises to replace both manifolds as the new lower intake has a smaller diameter egr stove pipe that doesnt sit up against the upper manifold.In most cases when this failure happens it takes out or damages the crankshaft bearings and causes engine failures and rod or main bearing knocks,very common,I have had to replace allot of these engines for this failure before.GM doesnt cover this repair under warranty unless the vehicle is still under warranty but by looking at the year and mileage of this vehicle it is not under warranty anymore.This affected 1997 to 2001 3.8 liter vin K GM engines in all makes and models.Dormin makes an aftermarket repair kit that comes with a new upper intake and a new smaller diameter egr stove pipe to replace the old stove pipe in your existing lower intake for allot cheaper then GM's repair prices.You will have to check with aftermarket parts stores for this Dormin kit.This could be what is failing here on your vehicle and if the intake manifolds havent been replaced before most likely coolant has gotten to the crankshaft bearings and damaged them,this failure usually starts to appear at about 70,000 miles and above.
To answer your questions, Gaskets were replaced due to anti-freeze on top of engine
& the manifolds were replaced per my repair order copy.
As far as going thru anti-freeze, I had to add anti-freeze about every 2 weeks. This is when we took it repair shop and is what the mechanic relayed to me.
o.k.,the antifreeze on top of the engine was most likely due to the upper manifold being melted,this coolant could have and most likely damaged the crankshaft bearings which I have seen many times before,most of the coolant leaks inside the engine when this fails and some will leak outside of the intake.This is most likely where your knock is coming from,the only way to be certain would be by pulling the engine oil pan and removing the crankshaft rod and main bearing caps and inspecting the bearings,if they are scuffed or wore down then the bearings could be replaced if the crankshaft journals look ok with no scuffs or scrathes on the journals,if there are scuffs and scracthes on the journals then the crankshaft would need replaced or the entire engine.This is about the only reason I have heard this engines knock before,you could have a piston wrist pin slap as allot of GM engines develop this,if this is the case then it would be safe to drive this way as I havent seen one yet that has caused any damage from this,but like I was saying above most of the knock noises ive found on this engine is due to the crankshaft bearing failures.