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could a rusty exhaust cause the car to have rough idling?

A hole before the oxygen sensor or after?

it should be after the oxygen sensor. just bought this one a couple days ago and i want to figure out the idling problem. sprayed the intake manifold with carb cleaner and helped a bit. changed the TB but the car still seems to be idling very rough. could it be the IAC valve? Also needs a new master cylinder as the brake pedal sinks as you hold it.

can anyone give me a hand please. car is shaking very rough i'm not sure if its from the exhaust or some engine components. TIA

Did you spray on the outside of the intake manifold with carb cleaner and see an improvement?

You may have a vacuum leak. Try getting a can of engine starting fluid and spraying around the intake (intake tubing, manifold, throttle body, hoses to brake booster, etc) while the engine idles. See if any of it affects the engine speed. If it does, there's a leak there you need to fix.

When the engine idles roughly, is the engine speed unstable? Is it high or low?

Have you tried resetting the ECU? Or pulling codes from the ECU's memory (http://www.surrealmirage.com/subaru/engine.html)?

just reset the ecu with by removing battery cables. code 22(knock sensor popped out and went away a short while. The engine speed is not very stable at all kinda bounces, when the brakes are applied the engine speed drops and will almost stall.

Looking for a vacuum like won't take too long and has a good shot and finding the problem... I doubt the knock sensor has anything to do with the rough idle. My car idled kind of how you describe when my intake manifold gasket was leaky.

it could be the iac valve that's how my car was acting, does it die sometimes when you stop quickly? or does this only happen when the car has warmed up.

the car does not go to the extent of dying when the brakes are applied, but RPMs drop to about 250-300. I will try the vacumm test method as you described vrg3. I have a spare IAC valve in my garage and i may also try that too. question: Is the flap in the IAC valve suppose to be partialy opened or closed? Any other ideas to fix this problem will be greatly appreciated. Thanks

Forgot to mention, when the engine is cold the car is very stable(RPMs higher), however as soon as it starts to warm the engine begins to shake and idle funny.

I don't know much about the symptoms of a dying IAC valve as I've never seen it in person. The position of the valve should just depend on where it was when it was last installed on an engine, I think; the ECU can push it open or closed.

A vacuum leak usually makes much less difference when the engine is cold, because it's running rich anyway so that counteracts the unmetered air coming in. The higher speed also smooths out the roughness on its own.

Could there be any other possibilities? will a leaky exhaust cause all these problems though?

I can't think of a way a leaky exhaust could do it, except maybe if the leak were before the oxygen sensor. The only potential causes I can think of are a vacuum leak or some problem in engine electronics somewhere (bad IAC valve, clogged injector, etc).

the iac valves ive seen have been partially open. Sounds like what mine was doing, it started out just dropping the rpms really low, but then it started dieing sometimes, but again didn't do it while cold, so this could be the case. I say take your spare and clean it with brake cleaner (if its been sitting for a while or is kinda dirty) then put it in. If it's not that my next guess would be vacuum or something like that.

time to check something stupid- have you checked how well the battery is secured? If you have a bad battery connection, (dirty terminals or whatnot) you will get those results.

well just put in my spare valve and the car still idled the same as before,one difference though, the check engine light illuminated and a code 24,22 popped out. Battery connection is clean. i'm beggining to think vacumm leak as well. so do i just go out and buy a can of starting fluid and spray it in various places?

If you got the CEL code for the IAC valve then your swapping test probably wasn't a fair test. Something was wrong with the valve or something and the ECU wasn't trying to properly drive it...

But the vacuum leak check is pretty easy. Yeah, get a can of starting fluid (carb cleaner would probably work too but doesn't evaporate as quickly). Run the engine at idle. Spray maybe a half-second burst at a time onto parts of the intake. Then watch and listen for a second or two.

If there was a leak, the spray will simultaneously feed the engine a little more fuel and seal the leak temporarily, so you'll see the engine speed increase a little and the idle will smooth out a little. You can spray again with a little longer burst to verify.

Whether or not you see that happen, wait until the fluid's evaporated (only a couple of seconds) before trying again.

Check all over, including the intake tubing, all the various fittings on the intake manifold, the interfaces between the manifold and the heads, and the brake booster tubing.

well i also took my dads advice of checking the coil for a spark. all cylinders gave a spark so that was good. however, as i pulled the boot off of cylinder 4 the engine almost ran the same as before except a little rougher. Symptons from the car led me to believe the car is misfiring.But that doesnt seem like the case. i step on the brake pedals and the idle drops, i push the AC button and the car idles smoother but still shaky. still confused. i will do the vacumm test for sure.

Find the darkest place you can, then start the car and watch under the hood....

If you see nothing spray the engine with a water bottle (like an old windex bottle)

I have the same issues with idle speed after I had installed my exhaust system. The idle speed has dropped to about 650 rpm at idle. your rough/low idle could be a result of low back pressure after the turbo (although I don't know why or how. It just happens on my car)

well i tried vrg3's vacuum test with no sucess.i tried looking for ignition problems, came out empty. Then i started fiddling around the throttle body, bliping it here and there, got in the car and noticed it running even rougher. got out popped the hood and saw what looked like my cat(next section after where oxygen sensor is housed) glowing red. I immediately shut off the car. So my question is, can this be my problem, i never noticed it like this.

i've had this problem for a while now and have replaced just about everything i can think of. i had an exhaust leak too, when i got it patched it it seemed a little better but was still not perfect,

I was also wondering if you unplug the oxygen sensor does it subtitute the signal for a preprogrammed one or would it make things worse?

I wish someone knew what the hell it was it would save us all alot of trouble. Once thing i noticed though when i tried cleaning my idle control, when i adjusted it it would nto adjust evenly, sometimes it would get stuck at high rpm then drop and sorty of pick its own place to be instead of where i set it. Had to rev the engine a little to balance it out.

Check your throttle body gasket. Mine had nearly disintegrated and was causing a seriously crappy idle. Jumping all over the place, up to over 1k and down to nearly 500. It was horrible.

Replaced the gasket, and all my problems went away.

Is there any way to check the gaskets before replacing them? ie.. vacuum test?

Look at it I suppose.