1999 S70 GLT, A/T with a "smidge" under 250K mi on the clock.

OK... so, when I am in "Park" or "Neutral", the engine idles as smooth as silk. Hot or cold, no issues.

When I am driving along and come to a stop-light, the idle starts out smoothly at 850 RPM. But after about 5 seconds, the engine starts sending big, vibrations all throughout the vehicle. Gas pedal, seat, dash...

I replaced the right front engine mount... Absolutely no change.

I tapped, throttled and listened for any exhaust leaks, but as the RPM's go up, even under load (if any of you have ever seen "Ace Ventura Pet Detective," that's how I listened for exhaust noise. Head out the window at 25 MPH), there is no excessive noise. Just at idle, and just in gear. = No downpipe issue.

Once the vibration starts, I've put the car in reverse. Absolutely no change. Opposite tranny torque direction, but same vibe, same intensity, with no delay associated with the counter-torqueing of the tranny itself when going back and forth from "Drive" to "Reverse" and back again. = Not a tranny mount.

So, thinking the vibes could be coming through contact with the firewall, I replaced both ends of the torque rod assembly this weekend. The rod itself was re-used, but the engine-mounted bushing and mount, along with the firewall-mounted bushing and mount are now both brand new. This helped to PROLONG the amount of time necessary to begin the vibration when I come to a stop. I'm now at 10 seconds, so I think I'm on to something, here.

Here's my theory: When I come to a stop, the engine, now at idle, is still running and exerting force against the transmission, in the opposite direrction of the crank rotation. One or more of my hydraulic mounts (down low) has failed, and is no longer cushioning against engine vibration. That engine vibration is now transmitted (as both radiating sound and physical buzzing) throughout the car. Though I would love to be the eternal optimist and recognize the potential of this issue as a possible marital enhancer, it's driving me nuts.

So help me out here... which direction does the engine rotate in, and will my likely culprits be the hydraulic mounts on the "upward" side of the circle drawn by that engine rotation? My assumption being that the "upward" side of engine rotation shows you where the most "downward" torque-weight is being applied? Installing those torque rod ends has made it take longer for the engine to reposition itself from "deceleration position" back to "idling position". I really think I'm getting close, here.

Oh, the most important part... If one or more of these mounts do turn out to be shot, where are they located on the vehicle and are they replaceable by somebody with 2 jacks, 2 ramps, 2 jackstands and a 2x10 to lift the engine gently from the oil-pan?

Cheers,
LG

 

My guess would be that it's missing. Misfiring. When was the last time the car was tuned up? do you have a cel?

 

No CEL. I know the CEL system is functioning properly because I seafomed the vehicle a few weeks ago. Now THAT caused a miss, and a pretty orange light on the dash, which I cleared immediately (with the UltraGauge of course). I currently have no codes active or pending.

No, this is definitely not a miss. I used to own a GT750 Water Buffalo that would need the center plug to be pulled and cleaned every 500 miles. Believe me, I know what a miss sounds and feels like. This is a tone-smooth, consistent metal-on-metal buzz...

Think of it this way: It's the difference between hearing the lady who lives above you walking in heels across the carpeted living room...

...then she makes it TO THE HARD WOOD FLOOR OF THE HALLWAY...

The sound itself... frequency, tone, rhythm, it's all identical. It's the medium through which the soundwaves are transmitted that is the difference. It really sounds and feels like the engine is resting directly on the frame of the car, but only when the car is at a complete stop and in gear.

 

Found the culprit...

There's a black rubber hockey-puck shaped "bump-stop" mounted to the top front of the driver's side control arm. The bottom of the engine appears to be resting against this "bump-stop." Here's a pic:



and here's a zoom-out showing the general location:



Is this (below, dead center) the mount I need to change? I think you call it the "Lower Torque Rod." What other mount is there that would keep the engine from resting on that hockey puck when the car is in gear, but float above it when in Neutral or Park?

 

there is the top mount that u replaced i think wont cause that also that torque mount wont case that.most likely your rear mount which is located by the steering rack is colapse and also your front engine mount. check for a black fluid substance around the mounts i think your right
check a,b,and g

 

I replaced "B" last weekend. It seems like "G" up front is the next easiest to get to.

What (and where?) is "J" on the driver's side? It almost looks like you can adjust the level of the driver's side by adjusting that crown nut...

 

I would think that the motor resting on the puck is a good thing. But the lower transmission mount does not support any weight. It muffles shifting points. If your car seems as if it shifts hard, that lower torque mount it bad/worn.

I'm at the opinion that a combination of all of these mounts are transmitting vibrations. I wish I could wave my had and replace them all ever 10 years.

If you are turely experiencing a mount vibration, some mount has suddenly failed, like the front mound does.

 

This is what happens when I work sober...

Lesson for the day: Whenever you remove/replace any motor mount, be sure to support the weight of the engine while you do so. Even if it's a torque rod.

Reason: When the upper torque rod is removed, the engine will shift its position to a forward-canted angle, causing the compression of the front engine mount, and subsequently allowing the engine to rest its weight on the subframe bump-stop. To fix this: Simply jack up the engine with a 2x6 under the oil pan. Loosen the bolt that secures the large bushing in the engine-mounted torque-rod housing. Push on that housing in the direction of the firewall a few times. Re-secure the bushing bolt, and you will notice that everything has tightened in a position about 1/2" rearward of where it was initially.

This also means that the engine's default position is now hovering about 1/8" ABOVE the subframe bump-stop, instead of resting upon it. Vibrations eliminated. Problem solved. Celebratory whiskey consumed.