idle issue and lower intake manifold

It's a 2000 LX 3.8. I have been having rough idling for a while and it's been very annoying. I can feel the idle varying through vibration on the steering wheel. There is no obvious fluctuation in the RPM needle when the rough idle happens but occationally it would drop and surge. Sometimes the car stalls when I pull into a parking space and have to steering much to one side or the other. There is no CEL. With the hood opened, I rev the engine by turning the throttle plate there is a brief screeching noise as if the car is sucking air in the back of the engine before it revs up. My mechanic said he thinks there is a vacuum leak because of this sign.

Here's the history of what's done:

-New engine <4000 miles (this problem occurs even prior to new engine)
-new plug and wire + coil <6000 miles
-isolator bolts replacement and EGR ports cleaned
-Fuel injector serviced, new fuel filter + new fuel regulator
-new O2 sensors <6000 miles
-checked and tested vacuum hoses

When I did the isolator bolt job I noticed that on one bank (the back) the butterfly plates of the IMRC do not close completely. Can this cause vacuum leak if they don't completely close at idle. Can I service these butterflies or do I have to replace the lower intake and whether the new lower intake comes with the IMRC plates? Please help if you know how these work!

Thanks for reading!

I would think that would cause a huge leak. It might not effect vacuum but those valves are supposed to be normally closed and only open at very high RPMs. If they're not closing all the way you'll be getting too much air into the mix.

You probably don't need a new lower intake, you can probably just take yours off and clean it really good (if you haven't) with some Sea Foam or Berryman's. It's probably getting stuck on the gunk built up in there.

Take or leave my opinion, I'm a complete amature.

You should be able to clean them..having them open (maybe even partially) will lower your low-RPM power because the resonation of the intake manifold won't be correct for the engine speed. Having them stuck open won't allow more air in than neccesary though...but a sticky idle-air control valve will.

I forgot to mention that I also had the IAC valve replaced recently while diagnosing this problem.

I can tell that these IMRC plates in the back are slightly open at idle because I can see the little thing that connects it to the linkage from the outside is not pressed all the way against the the stopper on the engine but the one in the front bank is.

Do I need to take the lower intake out to clean it? Will I damage the engine if I have fluid dropping into these port when cleaning if I don't remove the intake? What are these ports (covered with the butterfly plates) connected to?

Thanks!

Do I need to take the lower intake out to clean it?

It would be a lot easier to clean if you did. If you take it off some coolant will leak into the engine and you'll need to change the oil.

Will I damage the engine if I have fluid dropping into these port when cleaning if I don't remove the intake?

If you use something like Sea Foam or Berryman's that won't hurt the engine, but do you really want that gunk dripping into it?

What are these ports (covered with the butterfly plates) connected to?

That's the air intake into the cylinders.