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'03 Chevy Malibu, v6, 3.1, coolant loss, loud knock, loss of power, missfire?

I have a 2003 Chevy Malibu, V6, 3.1, auto, 140k miles. Car started out with code 0420 with no noticeable symptoms, but then added code 0303 (miss fire cyl #3) with loud knock (mostly after warmup) and loss of power under load. Now internal loss of coolant (no puddles or external drips). Changed plugs and wires - no change - all plugs looked good with no sign of contamination. Questions - if this is a valve/piston/lifter problem (which would explain the knock, miss-fire and loss of power), why would I get the loss in coolant? If the loss of coolant is fouling plug #3 (which would explain the coolant loss, miss fire and loss of power), could that give me the loud knocking? I don’t see how a valve/piston/lifter issue would cause a leak in coolant but I also don’t see how a coolant leak into cyl #3 would give me such a loud knock. Do I have two different problems? If it is a leak into cyl #3, is there any way to tell if it is the intake manifold gasket (which leaked before and was replaced five-six years ago and which I could do again if I had to) or a blown head gasket (which I don’t think I could fix or afford to get fixed)? Engine will occasionally get hot (but not overheat).

I’d check the head gasket by pressurizing each cylinder in turn, with a close look at #3.
What does the oil look like? Milky?, clear?
Many 3.1 manifold gaskets were replaced with the OE gasket, which guaranteed a repeat failure. The latest replacement gasket has been redesigned, and is much better.

Oil looks fine. If it is the intake manifold gasket again, would this give me the loud knock? (when the I.M.G. leaked bad before I never had the knock).

My daughter-in-law had this same car and it was a 3.1 automatic. When she left our home one morning she left a cloud of white smoke for blocks. I called her and told her to drop the car off at my brother’s shop and my wife took her on to work. My brother called her the next day and told her that both head gaskets were bad along with the intake gasket. He replaced the gaskets and water pump as well as the alternator. She got the family rate because my brother still owed me for a Toyota Tacoma that I had “sold” him. We called it even which made him happy and my daughter-in-law was too. She even baked her famous Duncan Hines Classic Devil’s Food cake made with fresh coffee and dropped it off after she picked up her car. It’s so good it doesn’t even need icing. My wife and I can make it ourselves but it always tastes better when someone else makes it.

I agree with Oliver70 on the new design intake manifold gasket. Been there. Done that! Try the FelPro PermaDry.

Replacing the gasket will not cure a “loud knock”. If you are not going to fix a head gasket, you certainly will not fix the knock.

I’m wondering if the coolant has been leaking into the crankcase so long, that one or more bearings are now shot . . .

I would remove the oil pan drain plug, and collect all the stuff into a tub. I wouldn’t be at all surprised if the missing coolant comes out, along with the oil

+1 to db’s post.

If coolant’s been getting into the oil, you may have more serious problems than a blown headgasket (or intake manifold gasket). Coolant is unable to keep the crankshaft wear surfaces separated from the crank bearings (& rods separated from their bearings), and damage becomes inevitable. Every time a cylinder fires adjacent to a bearing prone to damage, the surfaces that should be separated by a pressurized oil barrier bang against one another. The only way to check this out is by dropping the pan and pulling some caps.

If this turns out to be the case, the only options are a rebuild or a new, used, or rebuilt motor.

Sorry.