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Thought I'd share what I learned firsthand on this thing. I had a P2204 engine code on the '03 I just bought, which is supposedly "intake manifold control stuck open".
A vacuum line comes from the drivers side of the intake, goes through a control solenoid on the lower back driveres side of the intake, and then up to an actuating diaphragm on the back drivers side of the intake, this diaphragm actuates a butterfly valve in each intake runner, they are right before the intake transitions to the head.
My vacuum line was cracked in half which was causing the code, but I pulled my intake off to check this thing out (thinking it might also be stuck). These valves look exactly like throttle valves in each runner, except they don't cover the top fourth of the runner, ever. When fully acuated (which is when the engine is under light load and vacuum is strong) they're just like a throttle body that is closed, except for not blocking the top fourth of the runner like I mentioned above; the top fourth of the runner is still open, which apparently turbulates the air and looks like it puts the airstream at the top of the intake port where the injector is. Strong intake vacuum is what keeps them closed/partially closed. When the engine is under acceleration load (i.e. intake vacuum drops off) then spring action cause the butterflys to fully open for maximum airflow and the butterflys are considered to be NOT actuated, which is their default position. If the unit is not working due to broken vacuum line or ruptured actuating diaphragm then the butterflys will be fully open, so you do not lose acceleration performance, which is good to know. Now I suppose they could get stuck in the closed (i.e. "actuated") position due to intake crud buildup, you can test by moving the actuating crank arm on the back of the intake, under the diaphragm, but it is a bear to get your finger on because it's crammed up against the firewall and a huge wiring harness, you can make it work by sucking on the diaphram canister's hose (isolate it from the soenoid though) but you can't hear/tell if the butterflys are moving, so you gotta get your finger back there.
A vacuum line comes from the drivers side of the intake, goes through a control solenoid on the lower back driveres side of the intake, and then up to an actuating diaphragm on the back drivers side of the intake, this diaphragm actuates a butterfly valve in each intake runner, they are right before the intake transitions to the head.
My vacuum line was cracked in half which was causing the code, but I pulled my intake off to check this thing out (thinking it might also be stuck). These valves look exactly like throttle valves in each runner, except they don't cover the top fourth of the runner, ever. When fully acuated (which is when the engine is under light load and vacuum is strong) they're just like a throttle body that is closed, except for not blocking the top fourth of the runner like I mentioned above; the top fourth of the runner is still open, which apparently turbulates the air and looks like it puts the airstream at the top of the intake port where the injector is. Strong intake vacuum is what keeps them closed/partially closed. When the engine is under acceleration load (i.e. intake vacuum drops off) then spring action cause the butterflys to fully open for maximum airflow and the butterflys are considered to be NOT actuated, which is their default position. If the unit is not working due to broken vacuum line or ruptured actuating diaphragm then the butterflys will be fully open, so you do not lose acceleration performance, which is good to know. Now I suppose they could get stuck in the closed (i.e. "actuated") position due to intake crud buildup, you can test by moving the actuating crank arm on the back of the intake, under the diaphragm, but it is a bear to get your finger on because it's crammed up against the firewall and a huge wiring harness, you can make it work by sucking on the diaphram canister's hose (isolate it from the soenoid though) but you can't hear/tell if the butterflys are moving, so you gotta get your finger back there.
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