I remember reading somewhere that the natural frequency of the NA S4 engines was right around 3000 rpm. Well, whenever I go past about 1500 RPMs at low throttle opening/low load, the engine and car start shaking very noticably, and the car starts jerking, not accelerating or decelerating smoothly. This is most noticable in 2nd gear when i engine-brake with no throttle (but also no brakes applied). My working theory is that the engine/trans mounts are shot, reducing their effective spring constant (by approximately a quarter if the nat freq dropped by a half), lowering the natural frequency to 1500 rpms and causing the vibration. To fix this, Ive got new engine and trans mounts to go in with the new engine. Does this sound like it could be the cause? What other possible causes could there be?

 

I know that driveshafts can cause RPM-related vibrations.
Not in any way saying that is what it is, just saying its a possibility

 

You may be overthinking this. The only time i've felt this resonance was with an open exhaust or a huge exhaust leak. my open exhaust used to drone on the interstate at 3000, vibrating the car, but not bucking... my huge exhaust leak right after the header made the car feel like it was shaking itself to pieces. sounds more like a sensor problem like the tps or maf. Your tps may have a dead spot at your low throttle position. just some more possible causes. if you have the mounts already change them out and let us know.

 

I actually drove mine for a day without any exhaust past my (at the time) 1st and 2nd cat replacement pipe (for testing purposes only ) Man, it was freakin' loud.... it shook my nutz....


hahaha

 

mine was an open RB header. I didn't get my exhaust back on in time and couldn't find a ride to work. It sounded terrible and even at idle it was rediculously loud.

 

I remember that.. didn't hear it... but I've been there. It's like a weed eater on meth and steroids, having sex with an amazon woman. Not that I've done that. But I know a guy.....

 

i think this should be taken to the "wildest sex stories" thread in the lounge...

Ive driven an 88 saab that had the exhaust fall off, and its nothing like that. Yea, that made the whole car shake, but it shook no matter what i did, even at idle. My vert drives fine all the time, except at that RPM when it starts bucking and vibrating (but nowhere near as bad as that old saab). Ill try re-adjusting my TPS, and the driveshaft is getting replaced in a few weeks with the turbo stuff, so we'll see.

I was thinking it might be the mounts/spring rate thing, cause if the effective "k" value were to decrease to approx 1/4 its original value, at whatever the new resonance is, the engine would shake a lot more than at resonance originally

 

Sounds more like a TPS issue to me..

 

Id think so too, but it happens at around that specific RPM (+/- 250) almost regardless of throttle position, but not outside of that range

 

Maybe a dead spot in the TPS could cause that.

Have you tried looking for one?