Vibration during idle (theory for mine), Not like other threads, this is my theory on my situation. Options

[HectortheRican]

Correct me if I'm wrong.

I know STs are kind of notorious for having a rough idle, but I only experience a rough idle when the motor dips below about 900rpm. I've heard of bad motor mounts being the problem, but why would my motor have a super smooth idle before 900rpm? Seems to me that the engine works less at lower rpms, thus making less vibrations. So what I think might be happening is possibly a clogged/semi-clogged catalytic convertor. When I thought about it, it made sense. When my motor is at higher idle rpms (1,000-1,200), the exhaust velocity is high enough to smoothly overcome the clogged cat, but once it dips below the threshold of my rough idle, the exhaust velocity isn't enough to overcome the clogged cat, and it struggles to get through. It seems pretty reasonable and likely to me, but I'm not a master mechanic, I just have a basic, above-average knowledge of cars. Any input would be great.

This post has been edited by HectortheRican: Mar 25, 2011 - 1:56 AM

[opie_7afe]

mine has a little rough idle and i have no cat.......stock resonator and muffler though....been like this since i got the car...

[SwissFerdi]

Same here, BUT, only in drive. It rarely likes to dip in neutral. I experience the same drop in RPM sometimes, usually when it's hot or cold and some accessories are on.

This post has been edited by SwissFerdi: Mar 25, 2011 - 10:56 AM

[4-eyed-freek]

yup same here in the gt, i have no cat and it still shakes a bit. more with accessories on like swiss said.

[HectortheRican]

Because IIRC, Kyle (96stgreendemon) had a rough idle like me, replaced his cat, and it went away.

[4-eyed-freek]

in your case it could be, all cars are different and mabey it is just the cat in your. only one way to find out right lol

[HectortheRican]

Exactly. I'll find out in the summer when I go back to Texas. I'll be doing my header then, that way I actually have a place to put the stock manifold in. Lol

[stephen_lee]

add a 330 ohm resistor inline with the ecus coolant temp sensor. its the 2 wire CTS on the water neck kinda pointing towards the throttle body.

this will raise idle to about 1000 in neutral, and 800-900 in gear. if you think thats a bit much try the next smallest resistor.

[HectortheRican]

add a 330 ohm resistor inline with the ecus coolant temp sensor. its the 2 wire CTS on the water neck kinda pointing towards the throttle body.

this will raise idle to about 1000 in neutral, and 800-900 in gear. if you think thats a bit much try the next smallest resistor.

I'll hold off on doing that until I replace my cat in late May.