I'm a new, old guy, to this forum and I've experienced coolant loss with this 4.6l 2v engine in a 2003 Grand Marquis.
A while back, while driving, I detected the odor of antifreeze. Ah, couldn't be me, maybe it's the guy in front. This continued for a while until the time came when my temperature gauge started indicating a low temperature. This was considered the fault of the thermostat. I bought a new one and took the mounting apart. When I took the thermostat out I noticed 2 little chunks of plastic resting at the bottom of the housing. What are they, and where did these come from? I tossed them in the trash. I also noticed there was a bulge in a flat 'O' ring protruding into the housing area. I tried pushing it back in, but no success. I left it as such.
I flushed out the system and put in a cleaner, ran the car for the suggested time and then drained the system. I opened the thermostat housing, and when I removed the thermostat, I saw ANOTHER small piece of plastic chunk. This is the third one. Where in the hell are they coming from?
After having a new replacement manifold and coolant crossover installed, I decided to investigate the old manifold.
Removing the crossover from the manifold I found where those pieces of plastic were coming from.
There are 2 channels designed into the manifold that accepts a flat 'O' ring to seal.
The inner wall of this channel is very thin.
The stresses of the heat from the engine, the cold of the natural elements and the basic vibration stresses on the engine itself, and/or improperly torquing of the mounting screws, over time, caused this inner wall of the channel to fail and break off into little pieces, thereby allowing the gasket or 'O' ring to squeeze out and into the passage of the coolant, allowing a leak.
This failure happened on both sides of the crossover. There is no repair to this plastic channel of the manifold that will withstand the rigors of the what the engine goes through.
There are no replacement gaskets for this unless you buy a complete, new replacement manifold, and because the inner wall of the channel was broken, the new gasket would be useless anyway.
I'm guessing these small chunks of plastic, or a single one, got stuck in the thermostat, somehow holding it open.
With the new manifold and crossover on the engine, my temperature gauge now indicates proper operating temperature, and there are no more leaks.
So for this tiny, little, teenzy weenzy leak, the entire top of the engine had to be removed and the manifold replaced causing the loss of a weeks pay.
Fords 'better idea' sometimes suck.