Car moves forward when revved in park - PeachParts Mercedes-Benz Forum

muleears · #1 05-06-2007, 08:50 PM

Not much but a little bit. Sitting still, in park, if I rev and release the accelerator, the car moves forward about an inch or two then rolls back when the RPM's die down. What would cause this? It is not annoying or anything but the car has other tranny issues (upshifting WAY to soon), I'm wondering if the two are related somehow. Any thoughts?

Fimum Fit · #2 05-06-2007, 09:13 PM

but I've only seen a few, relative to the professional mechanics here; my F-150 ('91) does it too.

300SD81 · #3 05-06-2007, 09:15 PM

Does it do it in neutral?

MBeige · #4 05-06-2007, 09:31 PM

I too would like to know...

My guess would be worn engine mounts, but I'm not sure really...

dukegrad98 · #5 05-06-2007, 09:33 PM

I'm beating up my brain trying to remember what the physics principle is called which causes this. Basically even in neutral, the engine torque can cause the car to progress forward. Effect will vary with things like vehicle weight, resistance, rotational mass, etc. The name for this phenomenon is on the tip of my brain, and I just can't pull it out! Will update if/when it comes to me.

Cheers, John

300SD81 · #6 05-06-2007, 09:40 PM

Quote:

Originally Posted by dukegrad98

I'm beating up my brain trying to remember what the physics principle is called which causes this. Basically even in neutral, the engine torque can cause the car to progress forward. Effect will vary with things like vehicle weight, resistance, rotational mass, etc. The name for this phenomenon is on the tip of my brain, and I just can't pull it out! Will update if/when it comes to me.

Cheers, John

How exactly does that work? Don't even auto trannies have a clutch for neutral? Read about them having clutch packs a few times..

pleiades · #7 05-06-2007, 09:56 PM

Probably due to the crankshaft torque, which is pointing directly forward... rev it fast enough and it will move the car.

TheDon · #8 05-06-2007, 10:13 PM

^ thats what Im going with.. a rotational force will sometimes or all the time cause something to move forward or backwards depending on rotation of the mass

I sounded smart for a second there

#9 05-06-2007, 10:23 PM

It's moving because your parking pawl is not holding.

Mustang_man298 · #10 05-06-2007, 10:29 PM

Quote:

Originally Posted by 300SD81

How exactly does that work? Don't even auto trannies have a clutch for neutral? Read about them having clutch packs a few times..

They have several clutch packs, they resemble motorcycle clutches, and are called "wet" clutches since they are bathed in oil. (Whereas a manual trans has a dry type clutch). An object rotating, such as a clutch disk, will cause anything touching it to want to move with it due to the friction of it's surface, be it air or liquid, you get a sort of fluid coupling effect between the discs when they arent engaged. A spinning disk in a computer's hard drive picks up the air around it and "flies" the heads, a disc crash is when they stop flying when they shouldn't...

On the rotational torque, I don't know the name of it but I do know what is being referred to, though I could see that being an effect moreso in a transverse engine setup over longitudinal, as the crank would spin in a way to pull the car, not to make it want to roll sideways.

79300sdtd · #11 05-06-2007, 10:42 PM

my 80 300SD did this...now the tranny is kaput... not saying this will happen to you just saying.

When the new tranny is in i will see if it still does this.

Palangi · #12 05-06-2007, 10:54 PM

Which way does a car with a transverse engine move?

pawoSD · #13 05-06-2007, 11:11 PM

Its because the torque converter gets real rev'd up when you do that, and it slightly moves the innards of the tranny (which are clutched and the parking prawl is engaged but has 1-2" of play that the car can move)

Reving increases torque converter pressure, so the car lurches a bit. Completely normal, mine does it too.

pleiades · #14 05-07-2007, 12:12 AM

crankshaft torque would pull a transverse engine to the right, assuming the crankshaft spins in the same direction as the wheels moving forward, and also assuming the effect is due to crankshaft torque in the first place

it's a physics "right hand rule" that operates the same way a bolt turns... if you are looking at the head of a bolt turning clockwise, it moves away from you.

PanzerSD · #15 05-07-2007, 12:26 AM

It's a combination of soft motor, tranny and axle mounts coupled with the torque from the engine. Changed my motor mounts, and it doesn't do it anymore, solid as a rock. as for your upshifts, a new trans modulator ( or cap) should be considered, as well as a going thru of your transmission vaccum system..