Shaking at idle

I've just got a 98 996.
This mourning when I was waiting in a queue it started to vibrate, if i raise the revs slightyl it stops. I can really feel it through the drivers seat and the passenger seat is shaking. Anything to worry about?
 

Migration info. Legacy thread was 15289

Only if it stops shaking...

That means you've stalled it


Migration info. Legacy thread was 15290

I would look at the following first of all:

1. Have you driven it hard lately? (Too much idling in traffic?) It may need a few revs to "clear it out". (That's the excuse I use anyway)

2. Does it need a service? Maybe the plugs need changing.

3. Is there an Idle Stabliser Valve on the 996? If there is, it may need cleaning.

4. Running very low on fuel? Picking up dirt from the bottom of the tank?


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I bought the car on monday and drove it 150 miles back home. It's just had a 12,000 service done buy the independent dealer who I bought it from. I think they changed the plugs.

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I personally enjoy the vibrations in our 996 :wink:
Reminds you it's a sports car! Hope there's nothing wrong.


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Hi

Two things;-

1. If you have climate control and it's turned on a 996 will vibrate more on tick over...turn it off and there will be less vibration. The 3.4 996 is alot more lumpy on tick over than the 3.6 996.

2. Could be the mass air flow senser...this is a very expensive bit of plastic with a bit of metal one end and a two pin electrical connection the other end that reads the air flow being drawn into the induction manifold. Only around £295.00 from your happy Porsche Parts Department. They do smile when they swipe your card ...yet another mug!! This is a common part to fail on 996's and Boxster's.




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My previous car was a Boxster S which behaved exactly the same - at idle the rev needle would visibly move up and down a mm or so and this could be felt through the driver seat and the vibration showed on the passenger seat if empty.

When I had the car serviced at an OPC I asked them to investigate the cause and they told me there was no fault and that all boxster and 996 engines are built to within certain tolerances and mine just happened to be approaching a tolerance limit but was fine. I never had any engine problems with my car and it certainly didn't effect the performance.

Hopefully yours is nothing more than this in which case you'll soon forget all about it - especially if you keep your right foot planted!


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Hi DerbyLad

Sorry to hear about your idle problems - thankfully (or not!!) I've had the very same problem on a '99 C4 that is now well and truly cured. Turned out to be a real mixed bag of horrors so I'll list them all. Your problem could be one or a combination of all the following!

The 3.4 engine is capable of ticking over very smoothly (sorry to the poster above) and will idle better with the aircon switched on. This is because with the aircon off it should tick over at 700rpm; on it should be 810rpm - the higher rpms smooth the tickover slightly. Items that can affect the tickover are:

1) Air leaks - I'll come back to this one

2) Oxygen sensors (also known as lambda sensors) located near the catalysts. These measure the composition of the exhaust gasses (that are dependent on the air/fuel ratio being burnt in the engine) and feed this back to the engine electronics These, in turn, continuously vary the amount of fuel entering the engine via the injectors to keep the mixture correct and constant.

3) The MAF sensor (located in the inlet tract) that measures the mass of air entering the engine. This calculates the fuel requirement and is corrected by the output from the oxygen (lambda) sensors forming a closed-loop system.

4) Hydraulic tappets failing

5) Major mechanicla fault losing compression in 1 or more cylinders.

Thankfully #5 is very rare and we will assume your engine is basically sound. #4 will be an obvious metallic 'clack' every second revolution of the engine. Again let's assume this isn't happening for now

As will be obvious, any fault in the MAF or Lambda sensors will mean the mixture is not being correctly calculated or corrected and a lumpy tickover will immediatley result. Oxygen sensors are notorious for failing on early 996s and are about £115 each to replace. A Faulty MAF sensor (again common) is, I believe, around £150. All three sensors can be checked by a Porsche dealer with the PST-II diagnostic computer.

If you still have a lumpy idle once you are sure all the sensors are working then the most likely cause is an air leak. Again your dealer can help as any air leak should cause an abnormally high long-term fuel trim correction to be shown in the diagnostics. Sources of air leaks? Where do we start?! Oil filler pipes are known to split. The oil return bellows on the front of the nearside bank of cylinders is a favourite to fail (also depositing a fair trail of oil all over the bottom of your engine. The casing of the oil/water intercooler (think that's what it is) can split (with much the same symtoms as above) and the various breather pipes around the engine can perish. Cam cover seals can fail (again monster oil leaks) and the rear main seal on the crankshafts is another nightmare!!

Any 'breach' that allows oil out of the engine also allows air in and as this air is entering the system after the MAF sensor it is not accounted for by the electronics (except as showing as an erroneous figure by the oxygen sensors (if they are working) and a lumpy tickover is the result.

My car had ALL the problems described (albeit over a number of weeks, not all at once) so it does happen! Have them all checked and you should have a car with a perfectly smooth tickover.

Good luck

Ian W






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Hi Ian
thanks for the reply.
Since I first posted about this problem I've had a complete vehicle inspection done at an official porsche centre.
When I got there I explained the problem and a technichian came out and sat in the car with me and said the vibration level was fine, and any faults would show up on the computer.

The car passed the inspection fine (front disc's needed changing in 4000 miles, wheel brace missing etc). Hopefully they should have spotted any problems. They even offered me a Porsche centre warranty.

Simon

Migration info. Legacy thread was 15896

Hi Simon

sorry to jump on thread but I've also just bought a 996 C2 and it's got a few noises and things i'm not to sure about and would like to get it inspected, could you tell me is there a charge for the inspection and did they give you a cost for the warranty



Cheers

Andy


Migration info. Legacy thread was 15901

Andy
It cost's about £89 +vat. I think it's called a CTI or something like that.
I did not ask how much the warranty was as the car came with a 'off the shelf warranty', but they did say there offer will stand for a year and that it is a very comprehensive warranty.

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Thanks for that I'll get it booked in, Could you tell me which dealer you used I'm guesing from youre name you are in the midlands

Andy








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