Light vibration around 1900 RPM, through gas pedal *only*

[dkvasnicka]

OK, probably the last thing I now feel needs to be adressed on my "new" 9-7X is this annoying buzz I'm getting through the gas pedal...

It seems to me that somewhere around 1700 - 1900 RPM the I6 has some kind of resonant frequency and that it's most noisy in this range. That might be natural and normal but I have a bonus in the form of unsolicited foot massage through the gas pedal. It seems to be most apparent in lower speeds, under light and steady acceleration (1900 on the highway usually means a lot more wind noise & foot off the gas pedal, because cruise control). The thing is that no physical vibration can be felt on other places... floor, dashboard, center console. Also, in some specific cases (3rd or 2nd gear, steady acceleration, just before upshift) I can hear a brief metal-to-metal rattle from under the car, like two metal pieces vibrating against each other.

I have repeatedly checked engine and trans mounts and they seem to be intact. No tears, no damage. Maybe the rubber may have already become worse but that would result in more ubiquitous vibration, not just pedal buzz, I believe.

I've read on the OS that it might be the heat shield between the trans and the exhaust or that it could be some metal lines being too close to something. Next week I plan to have my wife sit in the car, slowly going up with RPM and me playing cat & mice with sounds and vibrations under the car... I can only do that in P and maybe N, though

Any other tips and places to look? And please don't tell me it's inside the trans
I was expecting the I6 to be a very smooth powertrain so this is kinda annoying, even though it's probably not a life-death issue.

 

[Chickenhawk]

The exhaust system does have a resonant frequency around 1700 RPM, but if you are feeling vibration through the pedal, there is likely another issue just starting. The next time you feel it at steady speed, look carefully at your outside mirrors. If they are vibrating even slightly, it is likely a U-joint.

The other cause, which is more common on our platforms, is a wheel bearing starting to go.

Both problems manifest themselves as just a "feeling" on the part of the driver in the very early stages that something is not quite right. If it's a wheel bearing, there will also be a very slight growl sound that is hard to notice at first until it gradually gets worse. Wheel bearings are pretty simple to replace yourself if you have some mechanical experience and a couple of cheap specialty tools. Mine took about 4 hours on the left side because I had never done one before and I took the time to proudly look at my work, torque everything down perfectly and THEN realize I had the brake shield on backwards, so it all had to come off.

The second side was faster. I still put the brake shield on backwards but I had the whole thing done in less than an hour. I ended up doing both sides because I guessed wrong which was the worst. The growl noise got worse when I turned right and disappeared when I turned left. I concluded the left side was bad. I was wrong; it turned out to be the right side. When I spun both bearings once they were off, the right side was obviously bad, but the left side was also starting to get a bit rough too. That's why lots of use recommend doing both at once. I used Timken.

As for the rattle, it could be related to the vibration, or it might be two issues. If you get it up on a lift or on wheel ramps, make sure it cools off and then look at the heat shields. Another possibility is a broken baffle inside the cat converter. Hit the muffler and the converter with your fist, and a broken baffle will be immediately obvious. This could also cause the vibration in the pedal if you have ruled out U-joints and wheel bearings. While you are underneath, take a look at the U-joints to see if there is any play in them, or any rust-colored powder coming from them.

Basically, a slight vibration in the pedal is not normal but is only the very beginning stages of a possible exhaust problem, bad U-joint or bad wheel bearing. Those are the most likely.

 

[dkvasnicka]

Thanks for the elaborate answer!

Interesting, I must admit I wasn't thinking about things like U-joints and bearings at all because it seemed to me that those would cause more widespread vibrations (e.g. steering wheel) and sounds when going bad...

Exhaust and everything around it is definitely on my list

 

[dkvasnicka]

So I did the "test" today. In P and N. I couldn't find anything though... the trans vibrated much more when in idle than under load. Once my wife started pushing the pedal the trans calmed down. I don't think there is anything odd or bad about this, seems like normal physics to me. The exhaust manifold also didn't sound bad, the cat included. I tried to hit it lightly with a rubber hammer and there doesn't seem to be anything loose in there.

How far is the heat shield supposed to be from the trans pan? I took pliers and bent it a little bit to move it a bit farther... in one place it was like millimeters from the pan.

So it's virtually certain that the problem is only manifesting itself when the vehicle is moving. And it isn't related to speed of the vehicle, just speed of the engine. Maybe it could be a problem with one of the gears or something but that still invokes the same question in my head: why through the pedal only?

Also what's weird is that in one place there are two slim lines (trans cooling?) attached directly to the trans body, using just a metal clip. Metal trans, metal lines, metal clip...

 

[littleblazer]

Do you seem to notice more vibrations when cold? Like if I just started the truck and drove to the end of my block stopped and started to accelerate I get a pretty good vibration through the pedal. I also know with 140k on original motor mounts that they are on their way out. It goes away when warm but since yours doesn't I have no clue. Does it seem worse when cold?

 

[dkvasnicka]

Do you seem to notice more vibrations when cold? Like if I just started the truck and drove to the end of my block stopped and started to accelerate I get a pretty good vibration through the pedal. I also know with 140k on original motor mounts that they are on their way out. It goes away when warm but since yours doesn't I have no clue. Does it seem worse when cold?

No, I think it's there regardless of the temp.

 

[littleblazer]

Okay. I have it slightly when warm but it's really bad when it's cold still.

 

[dkvasnicka]

Had a few drives and was focusing on the symptoms. First of all I could basically feel those vibrations intermittently anywhere between 1500 - 2000 RPM. Not as localized as it might seem from my previous posts.

I was also focusing on what I previously descibed as "metal-to-metal rattle". I think it would be better described as "howling"... it is probably caused by some metal stuff but it's pretty high pitched and not just simple metal banging.
I think pinpointing this stupid thing could solve the problem. Sounds like something from the exhaust system and I think that the sound is simply the "climax" of the problem... And that really only occurs a tick before the 2k mark. If it wasn't for this culprit I'd have probably just bought a set of motor mounts and started from there. But I suspect the problem somewhere else.

I also tried touching the brake pedal but that doesn't vibrate at all. Maybe because it's bolted much more firmly to the metal (brass?) plate and the body. Also, there is some little side-to-side play in the gas pedal. But that itself can hardly be the main problem.
Gosh how I love simple problems like stuck-open thermostats...

 

[littleblazer]

I mean I have a metallic rattle under acceleration that I suspect is exhaust related... it sounds like ball bearings rattling around at about 1300 rpms. Appears to be coming from the muffler. It is possible you're getting higher resonate frequencies from your exhaust and it's translating to something it's directly bolted to, as in the pedal and you're only feeling it because that frequency happens to be the natural harmonic of the pedal.

 

[Mooseman]

LB, sounds like yours is possibly a heat shield that is either vibrating or hitting the exhaust. You pretty much have to get a helper rev the engine and you are under the truck to look, listen and feel (on non-hot parts )

 

[dkvasnicka]

An observation: after I changed the belt the vibrations calmed down quite a bit. They are still there but not as much. I guess the belt tightened things up. Also somthing tells me it could have something to do with this. Maybe it all just boils down to a dying water pump...

 

[dkvasnicka]

Solved.

 

[xavierny25]

Solved.

I'd like to know what solved it the belt change or did you change out the water pump? Don't hold out on us.

 

[dkvasnicka]

That "Solved" is a link I'm not that kind of a**hole to just write "solved"

 

[BlazingTrails]

Mine was doing the same thing earlier this year, I was convinced that it was a problem in the driveline, It turned out to be my fan clutch. I could not believe that I could feel that in the floor board like a bad universal joint!

I knew the bearing was starting to go out, so one day I just removed the fan and drove it, it was very smooth. So i ordered electric fans

 

[xavierny25]

That "Solved" is a link I'm not that kind of a**hole to just write "solved"

Oops... didn't now the light blue font meant "this is a link". That's my ignorance. I would of thought bold blue font.

 

[dkvasnicka]

Yeah, I think using a more contrasting colour for links wouldn't hurt