jamren

Sometimes when I stop at a light, the car vibrates slightly. Clutch is always fully depressed. When I look over at the passenger seat, I can see the seat vibrating. It is intermittent and does not happen all the time. Engine is not making any weird noises. Could it be that engine mounts are shot and engine vibrations are not being dampened enough? Thanks in advance.

bornrich

Possibly motor mounts. This is a simple fix with the right tools.

KrazyK

I think I would start with checking the mounts. Is the engine "hanging" lower than normal. The stock mounts dont seem very strong.

jamren

If it doesn't get worse, I may just wait until I need an oil change and just do motor mounts and water pump at the same time. My water pump has 6 years on it currently.

jamren

Just got the car so have no idea what normal hang is to be honest.

Dale Gribble

With the AC off the car idles approx 100-200RPM lower than with it on. My car has always vibrated as you're describing with the AC off and I believe my motor mounts to be in good shape.

Barn996

Some would suggest the tailpipes or engine position may indicate bad motor mounts. My passenger seat vibrates when empty as well. How many miles on your Porsche?

M3BimmerBilly

Seems normal to me. My car does that as well. I have the same exact model you do. Mine has 55k miles on the original mounts.

KrazyK

Your car's are 13+ years old if their OEM. Couldnt hurt to change them.

Macster

IIRC the '99 MY has an idle control valve (ICV). Not sure if this item is serviceable (aka it might need cleaning) or if you should just go ahead and replace it. It depends upon how much it costs and how much work it takes to replace it. You should find out and if the cost/labor is within your comfort zone... sometimes if the part is not too expensive or labor intensive throwing a part at a symptom is a reasonable course of action.

Might mention both of my cars sometimes idle a bit rough. Nothing gruesome, it is just that I notice by the empty passenger seat or something else.

Oh, and both are e-Gas equipped.

They both do this at seemingly random times. I can't attribute it to say the extra load the alternator generates when the radiator fans come on -- it does only happen when the engine is hot -- or possibly when the radiator fans and engine compartment fan are on at the same time.

Both cars have recently had their batteries replaced and it could have been a side effect the alternator having to work harder due to a going bad battery.

Less likely to be an engine mount but you can check these just in case. They can fail to be sure and you do not want to let them go. Based on my experience with my Boxster I never knew the mount was bad and afterwards I couldn't really notice any change in the car.

groovzilla

when i purchased my 99 996 2.5 yeas ago i posted the same question regarding slight vibration at idle - i was told it was normal by many RL' ers who responded because of the cam that is used on the stock engine. my mechanic said the same thing that many 996's vibrate at idle.

if you search this forum you will see many having a remedy to stop the vibration by pressing the a/c button and keeping that on.

Invius

+1 for Motor Mounts, I had a pretty solid vibration at idle that would slightly move the passenger seat. After replacing them, the car is smooth at idle and the passenger seat no longer vibrates.

DS

Dale Gribble

Girlfriend must be upset now...

Invius

Well played!

jamren

Thanks for all the responses. I'm glad that it's normal. The car is still new to me and I am learning little by little every day. With that said, I may do motor mounts anyway. Couldn't hurt since previous owner said that they have never been replaced. To answer Barn996's question, the car has 77,664 miles on it. It had 77,400 when I bought it a month ago.