12 replies to this topic

[mAydAy]

Are you sure the car is actually fluctuating and its not perhaps the bus noise resonating through the car or somethign of the like? I don't know, but perhaps the tachometer is sensitive to noise or something, it's definitely pretty odd.
Also - Glad to see another Richmond-er! Hang around man, got to join up for the meets and things. =D

-Andy

[aikitect]

I have noticed something similar. There is an oscillation, but the idle speed itself is not affected. However, whenever it happens I tend to look at all my guages to make sure nothing is amiss.

If it is what is happening to me, then it may be oscillations due to the difference in frequency of your engine and the deisel engine or a muscle car - both of which have a low frequency "note" from the exhaust.

When tunning a guitar one of the things to listen for is the oscillations of sound. When there is no oscillation, the two strings are in tune. Slow oscillations means that they are almost in tune. Fast oscillations means that they are not close to being in tune. This is all relative - the frequencies have to be similar to each other else the oscillations are not apparent, which may be way it is evident only around diesels.

Next time this happens, try gently increasing your RPM and see if the frequency of oscillations is altered.

Edited by aikitect, 03 May 2004 - 08:10 PM.

[volvolove]

Dirty throttle unit.

[Chopper]

The eletronic throttle body might have a problem with shielding? That's bizzare.

[850 Stylzz]

my guess would just be that they are matching the resonant frequency of your car. I dont find that at all implausible. However, I have never experienced it, so that must be taken with a grain of salt.

[kborneby]

Next time it happens, turn your a/c off. I have found that the T60 has a noticeable resonance from the exhast at idle but only as the a/c cycles on (which also explains your rpm fluctuations)

[volvolove]

Does your throttle unit have a yellow or white sticker on it? If the sticker is white I would suspect the throttle unit is the problem. All the new ETM's have yellow stickers.

[jross]

I'm curious about this, I read your post. (I'm a physicist, so this is the sort of thing I like) One, next time it happens, try turning off your A/C, and then increasing your revs. I'm not sure those will help, but might as well test both of those hypotheses. My first guess is that the vibration may be a resonant mode of your tach (there are springs and the like in those, usually). However, you said you can feel the idle changing, which may rule that out (another reason to turn off the AC, isolation of variables). Another possibility is that moving an electronic part will cause small pertibations on the electronic signal ("signal noise", essentially), and it's potentially possible that the outside vibration is inducing an oscillitory cycle that creates increased signal noise in your electronic management unit. Is the behaviour you observer periodic or aperiodic (ie, is it even timed, is it "random", is there a discrenible pattern in the oscillation?).

[edit: I don't mean it to read like "there's a chance moving electronics create signal noise". Moving electronics does cause signal noise, the possibility is that there's either louder noise or some sort of periodic noise affecting your engine management system.]

Edited by jross, 07 May 2004 - 05:08 PM.