need some help diagnosing

[03mgtermi]

the short story:

was driving in stop and start, slow, heavy traffic (20 MPH, second gear, coasting) when i heard a loud rattle and felt some weird vibrations. were intermittent and varied in feel and volume with changing RPM's.

got off at next exit and examined and found nothing major to be visibly wrong. car runs smoothly at idle, hesitates and runs rough from 500-1200 RPM's and then runs smooth again. there is a noticeably different tone to the exhaust and is somewhat louder, this difference is present throughout the RPM range. already have leaking cats, but now there is a more pronounced exhaust leak (as far as my ears can tell), obvious at idle.

nursed it home staying below 2k RPM's and 60 MPH the whole way. car is currently cooling in my garage and i will inspect more closely this evening.

my theories:

something came loose or fell off and created a larger exhaust leak close enough to the O2 sensors to cause the rough engine characteristics.

OR

i dropped/stuck a valve somehow and one of the exhaust ports is plugged or full time open.

any ideas or things to try? i plan on going over the exhaust tips to manifolds and doing a compression test when i can get the tools to do so.

i am at a loss right now as i just sold my normal DD and am dependent on the cobra until my new truck gets here. i have one day to fix this thing or i'll be rockin' a rental.

thanks guys,
brandon

more details:

this is the second week of DD'n the car and i replaced the alternator last weekend. scangauge i have in the car at all times reports good vitals and no codes were thrown

car had ample time to warm up since i idled a bit in the lot leaving work to get the a/c going. also, roads are rough around my office so i was driving very gingerly. got to the highway and cars were backed up on the entrance ramp and into the feeder, so no abuse there either.

MODS in my SIG

 

[lgonz051]

id bet its cat related. Or try swapping out ur 02's and see if the problem goes away.

 

[03mgtermi]

got it up in the air and could not detect anything else wrong with it besides the leak in the passenger side cat. it appears this is the only one leaking.

turns out there is now a second hole in it thus leaking a considerable amount more.

i will have the leaking one or both replaced and see what it does.

 

[03yellow]

Fix leaks. If you still have issues unplug FRPS and see how it accelerates; I won't run well when you back off gas but a. Good way to test frps withou buying a new one (may have a slight tear in diaphragm). Next check would be plugs or a bad coil.
Doubt you have a bad/stuck valve.

 

[03yellow]

Have you had the accufab tb for a while?

 

[03mgtermi]

yea, accufab was on the car when i got it

 

[03yellow]

Any luck? Usually will throw codes with a bad cat. Did you check your FRPS? Also check vacuum lines.

 

[03mgtermi]

checked FRPS and all lines and hoses and did not detect a problem anywhere beyond the leaking cat. have not had a chance to make that repair yet...it's my weekend to work:cuss:

 

[03mgtermi]

had the leaky cat replaced.

turns out it was hollow inside, so i guess all that rattling i heard was the innards getting pushed out of the exhaust. bassani mufflers are a straight flow through design so it is possible it all came out just fine.

the cat is part of a bassani x-pipe and was welded in so it had to be cut out. no way to inspect the other cat without cutting it out as well.

my issue now is that when i punch it, instead of a smooth shrill scream from the eaton, i am getting a more choppy note. same scream, just some undulation to the sound if that makes any sense. much like the sound of belt slip or the motor is under more resistance. vehicle is definitely down on power as well. i am guessing i ought to have the other cat replaced as well so its balanced flow? not sure what the deal is now. totally lost

 

[03mgtermi]

ttt any ideas?

 

[04SolidSnake]

I'd check out the other cat first, since you now have one hollow cat, and the other one not I'd say that would cause some strange sensations... The undulating sound could be from belt slip, or a slight misfire, or other things..

 

[03mgtermi]

ok. i'm fighting with the exhaust right now to see of it is plugged up. only thing i can figure is that some of those cat pieces are stuck somewhere clogging it up. just feels like the motor is choking........problem is ONE NUT is seized (cross threaded and rusted) and now the corners are stripped off. FML

im so lost!

 

[03mgtermi]

UPDATE: got the cat-back off and can see into the 'x' and it all looks free and clear. mufflers are free and clear. working on breaking the tail pipes off to check the corner bends. if those are free of debris i will really be confused!