would the crank pos. sensor tell if flywheel...

MarkE36 · 10-28-2003, 08:56 AM

would the crank position senor tell if the flywheel is off balance? im so mad.... my car vibrates at 900rpm then again at1700rpm in gear or out, its fine through out the rest of the rpms, my friend swears its the flywheel off balance or something in my bell housing.... the car has 60000miles. so why would the car drive/shift fine but has a vibration at those rpms? i have been hearing a whinning sound when i press the gas down enough to stay at a speed or speed up a little.but i think this is the fuel pump or filter.

blkongry · 10-28-2003, 10:27 AM

The crank position sensor does just that... sense position, or in other words it just senses the location. It can't tell you any dynamic information. As far as vibes, they are weird things. It sounds like, from what you have described, you are getting some resonance at 900 and 1700 rpm. Resonance is when the vibration waves interfere with each other and amplify each other (which is why you can feel it at those speeds), much like the right frequency of sound can shatter glass... it's the resonance developed in the glass. In your case, your frequency is your RPM. It does sound like a balance problem, but it could be anywhere on your crank 'system': the harmonic balancer (if damaged), the flywheel, the clutch assembly, etc.

Alternately, does it feel like a shudder, rather than a vibration? It could be a fuel delivery/ignition issue. If you hold the engine at those RPMs and it vibrates like mad, that is definitely a balance issue. If you are coming up to those RPMs and it shudders and vibrates as you pass (under acceleration) it could be a fuel delivery system.

One trick you can try is using a long thin dowel rod (of wood). With the engine running, open the hood and have a friend run the engine up to the problematic RPMs. place the end of the rod on different locations of the engine (front to back) down by the crank (where ever you can get it to fit) and see if you can feel where the vibrations are the strongest. That will help you pinpoint the origin. From there you'll have to start inspecting components.

Sorry to hear about your troubles. Always hate to hear about funky stuff like this... it's hard to troubleshoot and track down.

Good Luck!!!

MarkE36 · 10-28-2003, 10:48 AM

if it was a balance problem why would it happen ONLY at those rpms? wouldnt it do it all the time? and by shudder do you mean shake? it kindof like that...

blkongry · 10-28-2003, 12:38 PM

There is vibration all the time, but at certain rpms you get the resonance. It's much like a tire being out of balance. You ride on it and it feels relatively smooth, but then at certain speeds it really shakes/vibrates... but if you go a little faster it goes away. It's not so much the strick out of balance of the part, but more of the interaction that part has with systems around it. When it shakes, it shakes the housing/bearings and everything else it rides in or is mounted to... that piece oscillates at its own frequency... its when they interact you can get the resonance (all depends on the mass, how closesly they interact, etc). If your flywheel was mounted to a fixed, non-vibrating system, then you would feel the vibration all the time, and it would increase linearly as the rotational speed increased (presuming the problem was with the flywheel). Let's presume it is the clutch plate. The clutch plate spins within the pressure plate, which spins on the flywheel. Each discrete piece has its own harmonic signature, and when spinning together they create an interacting system. If one is off enough in balance, you can create reasonance. In general, resonance is a bad thing... it will rip things apart if the amplitude is large enough.
There are also internal vibrations in your engine that will rip itself apart, which is the logic behind the harmonic balancer... harmonic as in harmonic vibrations. The most inately balanced motor design is the in-line six. The V-configuration of engines actually creates all sorts of harmonic vibrations, and they do a lot of crazy stuff to counter act it.

There are so many things that could have happened in your system. Let's say your clutch plate had a flaw in it, and over the course of 60K miles, a section of it chunked off. Now it's unbalanced, but it still engages okay. What does that do to the whole clutch/flywheel/pressure-plate system? Don't know... maybe nothing... maybe if it is in just the wrong spot it will shake the heck outta the car. Hard to say for sure. Vibrations are a pain in the ass to work with. You have amplitude, frequency, phase, wave shape, and other characteristics. For your sake, I hope when you start looking, that you find a clear-cut source for this vibration... otherwise it may be hard to locate.

And yes, by shudder I mean shake. The fuel delivery system could have a problem at a certain rpm and cause..... HMMM... now that I think of it, my Fiat Spider had a shudder problem at certain RPMs. It turned out to be a rupture in one of the vacuum lines around the induction system. You mention hearing a whining sound under acceleration. It could be a vacuum leak, and it gets louder when you accelerate because you are drawing in more air. That vacuum leak could lead to pulsations in the intake system, hence causing your shuddering/vibration. I would first check all vacuum lines under the hood. If you're not sure what you are looking for, then you may want a qualified tech to give a quick overview of your intake system.

Good luck.

Wayne 962 · Wayne 962's Garage

Which car do you have? The dual-mass flywheels can deteriorate over time, and cause vibrations in the drivetrain...

-Wayne

MarkE36 · 11-09-2003, 05:16 AM

Quote:

Originally posted by Wayne at Pelican Parts
Which car do you have? The dual-mass flywheels can deteriorate over time, and cause vibrations in the drivetrain...

-Wayne

I have a 97 328is. and the vibration only happens a certian rpms....

Wayne 962 · Wayne 962's Garage

Auto or manual transmission?

-Wayne

MarkE36 · 11-09-2003, 11:51 AM

97 328is manual 60,500miles

Wayne 962 · Wayne 962's Garage

"in gear or out" - okay, I guess I could have assumed it was a manual transmission.

How are your engine mounts? Is it only when driving? blkongry is right on his assessment of vibrations, but it may be a global (engine as a unit) type of thing. I like to do tests that change the mass (and harmonics) of the engine. For example, if the engine vibrates while not moving (and revving it through those RPMs), then place a very heavy object (not your butt) on the top of the engine and rev it again. See if the vibration changes or disappears. This would be a good test to see if the mounting of the engine was causing these problems (engine or tranny mounts). If not, then it would be something internal. I would say some 50-lb free weights or a concrete block or two would make a difference in the harmonics of the engine mounting system - enough to tell you if the problem is internal or external. Plus, it's a very easy test...

Your last comment about the fuel pump or filter - the fuel pump runs in a continuous circle, whether your car is idling or whether it's under full acceleration. It shouldn't change sound at any time when the car is running. Are you sure it's not a belt?

-Wayne

MarkE36 · 11-09-2003, 01:41 PM

thx for taking the time to help. it vibrates in gear or out, its not noticable while the engine is warming up(start up till rpms level off at 700).

Wayne 962 · Wayne 962's Garage

Check the motor mounts, and do the weight test - I'm curious to see what you find...

-Wayne

Jared at Pelican Parts · 11-10-2003, 08:14 AM

All you need to do is put the parking brake on, put the vehicle in gear and gently disengage the clutch, if you see the motor jerk or rock, then you have a bad motor mount.

You can also jack the front of the motor up, using a block of wood to distribute the weight. If the engine moves up an excessive amount with out also lifting the car, then you have a bad motor mount.

I DO NOT advocate placing a heavy object on the top of your engine which can slip off and damage other components.

blkongry · 11-10-2003, 09:10 AM

I agree with Jared's advice on how to check for a broken motor mount. Although, the problem doesn't sound like a broken motor mount. I've had broken mounts before, and they tend to make more of a 'clunk' when driving (since the torque side is usually the one to let go, and then the engine lifts up and drops between acceleration and shifting, which is why you can see it by doing Jared's test). Solid motor mounts would transfer more vibration as a whole, not at key rpms. It really sounds like a balance problem somewhere.

As to the adding of weight/mass onto the engine... well, it is a well know fact that the best way to quell vibration is to add mass. If you had vibration traveling up your shifter and you sleeved it with a huge lead weight, you wouldn't feel that vibration as much. It's not really going to help you isolate/pinpoint your vibration source. From what you have said, the vibration definitely seems to be in the engine or clutch/flywheel/pressure-plate system.

Wayne 962 · Wayne 962's Garage

Quote:

I DO NOT advocate placing a heavy object on the top of your engine which can slip off and damage other components.

I'm not talking about driving the car around with this on the engine, obviously.

Sometimes the motor mounts are cracked in such a way that the lift test reveals nothing...

-Wayne