fluctuating engine vibration when at a steady RPM

[marckula]

not sure if this might be timing related or fuel delivery issue or maybe something else...

when i rev the engine in my 540 and keep it steady around 2500 rpm, i get a fluctuating (varying intensity) vibration - somewhat like if I were to turn on the AC - the engine seems to be losing varying amounts of power. this vibration occurs even when I have the clutch pedal pushed down (transmission disengaged) so I can eliminate the transmission as a cause.

i would describe the varying vibration intensity as a regular sine curve - the intensity slowly increases then decreases then increases over and over. and the 'vibration' is much like

I'm not getting any error codes right now. I recently replaced the PCV cover, 02 sensors, intake manifold gaskets, oil change, oil filter and fuel filter. and there don't appear to be any vacuum leaks as far as I know.

can anyone make any suggestions? could irregularities in fuel pressure cause this? what about coolant or air intake temperature? or even power steering pump perhaps?

[dt8068]

My car does exactly that while at idle. I don't know if its a problem or normal for the M60.

[brosher]

Is it fine other wise? Does it idle smoothly and drive smoothly when on the highway at the same RPM?

[TC535i]

Does it feel almost as if you have a center bearing that's failing as you accelerate? I get this feeling too at that exact RPM range, most noticable in 1st and 2nd gear... since it's RPM related, I'm leaning towards engine.

[marckula]

Is it fine other wise? Does it idle smoothly and drive smoothly when on the highway at the same RPM?

it idles pretty smoothly (kind of low rpm sometimes) but this vibration also occurs while underway - always around the same RPM range 2500 whether applying the accellerator, coasting or braking - at other rpms it's fine.

whats really strange is that the vibration fades in and out at 2500 - if it stayed consistent I'd say it's the engine for sure.

[marckula]

Does it feel almost as if you have a center bearing that's failing as you accelerate? I get this feeling too at that exact RPM range, most noticable in 1st and 2nd gear... since it's RPM related, I'm leaning towards engine.


not really, there's no mechanical feel (grinding clunking or otherwise) it just feels like theres a load slowly being applied then removed. like it's losing power then gaining the power back over and over. kind of like turning on the A/C when you're going up a hill but worse

[marckula]

regarding the fluctuating idle reply by dt8068 -

i was also having this problem at idle but I'd get a check engine light as well. solved it by checking the pcv plate at the real of the intake manifold - one of the vacuum line nipples had become unplugged. I plugged that up and the problem went away. one nipple should have a hose to the fuel pressure regulator, the other can be closed up. you might also get away with running a T joint from the FPR to each nipple. don't know what this would do...

[BillionPa]

i'd put my finger on the motor mounts. harmonic amplification at a specific RPM. try it at 5000 rpms and see if there is a similar but not as pronounced vibration.

even replacing just one on mine made a massive difference.

as for low idle speed, my idle speed is about 480-530rpms when warm depending on how i was driving before it hit idle.

[632 Regal]

the M60 3.0 and 4.0 are balanced best as can be with some oscillations at 1800 and 2400 rpms (appx) above that threshold there will be no dramatic off balance areas as these were internaly balanced for longevity.

all too common gentlemen, with bad motor/trans mounts this vibration can seem excessive but no actual physical problem.