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2009 Impala LTZ, 3.9 V6, 4 speed auto transmission. 167k miles.

During past few months or so I noticed increased vibration when I stop with the engine idling with the transmission in drive and foot on the brake. When I place the transmission in neutral under identical conditions, vibration disappears.

Before the car was vibration free in either mode. Can this mean a bad lower engine or transmission mount or some problem with the transmission torque converter? I checked upper engine mounts and they seem OK. Did not check the lower ones yet. How many are there?
 

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2009 Impala LTZ, 3.9 V6, 4 speed auto transmission. 167k miles.

During past few months or so I noticed increased vibration when I stop with the engine idling with the transmission in drive and foot on the brake. When I place the transmission in neutral under identical conditions, vibration disappears.

Before the car was vibration free in either mode. Can this mean a bad lower engine or transmission mount or some problem with the transmission torque converter? I checked upper engine mounts and they seem OK. Did not check the lower ones yet. How many are there?


I’d bet the inside of your throttle body is dirty, causing the rough idle. Clean the inside with throttle body spray cleaner, and use a bottle of Techron Fuel System cleaner according to label directions.
 

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Because I have an oil leak I suspect that the lower engine mounts have disintegrated to some extent, causing some engine vibration under load. The vibration disappears once the transmission is placed in neutral or park. There is no engine check light on, so I do not suspect any engine misfire issue or ignition problem.
 

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Because I have an oil leak I suspect that the lower engine mounts have disintegrated to some extent, causing some engine vibration under load. The vibration disappears once the transmission is placed in neutral or park. There is no engine check light on, so I do not suspect any engine misfire issue or ignition problem.

When you place the transmission in park or neutral, there is virtually no load on the engine - thus the smooth idle.

Inspect the inside of the throttle body. A small amount of “gunk” on the throttle body bore and the edge of the butterfly valve will be enough to reduce the airflow at idle, and cause a rough idle with a small load on the engine.

As you drive, the throttle butterfly valve is open and the engine gets enough airflow.

You may want to ensure the engine air filter is not extremely dirty also.

Good luck.
 

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Because I have an oil leak I suspect that the lower engine mounts have disintegrated to some extent, causing some engine vibration under load. The vibration disappears once the transmission is placed in neutral or park. There is no engine check light on, so I do not suspect any engine misfire issue or ignition problem.
I was going to say its the trans or motor mounts. Sounds like your on the right path.
 

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Discussion Starter #7
Well, the throttle body was very clean inside except for a thin layer of soot on the inner casting surface and none on the butterfly valve. Nothing has changed after cleaning.
 

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It is a damaged lower engine mount as I suspected. I looked at it today and it is crumbling apart. The trnamsission mount ont he other side of the subframe is in good shape.
 

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Because I have an oil leak I suspect that the lower engine mounts have disintegrated to some extent, causing some engine vibration under load. The vibration disappears once the transmission is placed in neutral or park. There is no engine check light on, so I do not suspect any engine misfire issue or ignition problem.
Oil leakage would tend to disintegrate the passenger side one first, it's easy to check it's just inside the wheel well right behind and below the a/c compressor and behind but in front of the brake assembly. Come to think of it the driver's side transmission mount isn't much further off, sits right behind the brake assembly I believe, just on top of the frame and both of the mounts are just in front of the CV axle.

Definitely would check those, one easy way to do it is to open the hood and get in the car... Start it and apply the brakes and keep the brake pedal depressed or even apply the parking brake. Now watch the top of the engine cover from inside the car and put it in drive, wait until it engages. Once engaged put it in reverse and again wait for it to engage. Once engaged if you still haven't seen anything go back to Drive with the selector.

What you are looking for is a sudden move of the entire engine block.
If a mount (or two) are broken the engine will move considerably and suddenly (it's like a WHOA thing, you can't miss it) when going between D and R or R and D...
If the mounts are whole the engine should barely nudge a little.

That or you can visually inspect them, for this I would recommend jacking up the car and pulling the wheel off each side, you might be able to get a good look also by just turning the steering wheel all the way to one end or the other.

I would do this soon, you're not doing yourself any favors driving around with broken mounts, you will also need to take care of that oil leak... Usually it's either the oil drain plug gasket or seal gone bad (yes these can go bad in as little as 2-3 oil changes, I've learned to replace this rubber gasket on the drain plug each oil change). Another oil leak location would be the oil pressure sending unit (or o-ring), the oil filter adapter gasket (the adapter is also the housing for the oil pressure sending unit), the front and/or rear valve cover gaskets... And last but not least it could be the oil pan gasket (this is a royal one so I'd save it for last).
Generally speaking the order of replacement here is oil drain plug seal or gasket first, then move on to the other stuff (usually the oil pressure sending unit and oil filter adapter gasket are next), the front valve cover gasket isn't too bad but the rear one takes an afternoon and requires replacing the coolant crossover pipe gasket as well).

Images 1 and 2: Motor Mount, passenger side before / after
Images 3 and 4: Trans Mount, Driver side
 

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Thanks for the advice. I change the oil pan drain plug gasket at each oil change, so this is not the source of oil leak. Neither are the valve covers, or oil filter housing gasket. Where is the oil pressure sensor on this engine? I need to check this next. Aside from the sensor, it is most likely oil pan gasket leak, or rear main seal leak. What special tool do you need to access the oil pan bolts hidden by the transaxle?
 
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