Vibration that comes and goes

[AKADriver]

Just starting to characterize this after getting my car back together back in the spring and driving it more. Under particular conditions, I would say when the engine is above 'lugging' RPM but still below the powerband and I tip into the throttle, I get a lot of vibration through the floor. Noticeable when, say, I'm cruising at 55mph and going uphill.

It's a high frequency vibration that seems to follow engine RPM, not the low-frequency vibration of an unbalanced wheel or bad axle or suspension issue. I've tried downshifting to see if it shifts the frequency upward but that just tends to make it subside.

It's most noticeable in the floor/seat. Not much in the shifter, nothing through the steering.

My first thought is that my header or exhaust are contacting the floorpan when the engine tips over towards the right under load. I'm using OEM-spec mounts so I expect some movement. But that would be a lot. But that's the first thing I'm going to check, I guess if there are fresh witness marks on the floorpan or cat heat shield. Header is an ISR 4-2-1, is hitting the floor a known issue with those? In my Miata days a lot of header brands were known for being either too close to the tunnel or too close to the bellhousing and sometimes needed a precision dent.

It's not an engine, harmonic balancer, or flywheel problem, I don't think, because I can rev it out in neutral and feel nothing.

Second thought is a driveshaft problem, but I don't get any clunking and there wasn't any unusual play in the u-joints and the center support bearing turns freely and quietly. The driveshaft is OEM. But it's in the right frequency range to be a driveshaft issue.

I actually wish I had a transducer and oscilloscope so I could record the vibration and match it to engine rpm or driveshaft rpm. Much easier to track it down

[tuzzio]

First places i'd look are the header / cat area, and second would be transmission mount.

[AKADriver]

Good point on the trans mount.

One thing I don't have right now is any bracket connecting the exhaust to the trans crossmember. The factory exhaust has this as well as most downpipes. But my bracket rusted off (and wouldn't have fit over the larger flange on my aftermarket header/cat anyway).

[tuzzio]

Originally Posted by AKADriver

One thing I don't have right now is any bracket connecting the exhaust to the trans crossmember.

I don't have, and never have had this, but I also have a fully custom exhaust so its more than likely not applicable for this situation. I would also look for any broken / loose exhaust hangers. I'd even go as far as saying just replace them all anyways.

[AKADriver]

I have all the hangers in place except that front/transmission mount one, they're all either new or the nicest non-torn ones in my hoard of parts.

The exhaust itself is a cheapie and might be bent. I do have another exhaust I can try.

[burnsauto]

If you still have a two piece driveshaft, you may want to check the carrier bearing. I chased a very similar issue, just in a different car. The car I was working on actually had a 3 piece driveshaft with two carrier bearings. When I got a new OEM one, the factory had redesigned it to eliminate one of the bearings...guess it was a common failure point.

[AKADriver]

The bearing is good, verified before I put it in. The rubber is intact but soft. I might buy a new one, a Febest bearing is pretty cheap.

If I can't find any evidence of exhaust contact with the floor I'm going to try clocking the halves of the driveshaft. I marked them before taking it apart but the marks rubbed off so I guessed.. might be wrong.

[AKADriver]

Finally got off my ass to solve this. The driver's side inboard rear bolt for the seat was contacting the midpipe. Dropped the exhaust and whacked the witness mark with a hammer.

Edit: It's not solved lol. It's getting worse! It's the center support bearing. I've got a new NTN one lying around that I'm going to throw in but I should probably just go one piece.