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darrinsmith

996 surging under part throttle lowish RPM

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Hi everyone.

My 1999 3.4l Tiptronic cab has started having on odd problem.

When cruising along at steady speeds, (just feathering the throttle) the engine is surging (ie the tachometer moves up and down across a band of about 300 rpm or so), you can feel it as well.

The car is fine under moderate to heavy throttle, and idles perfectly.

It got serviced yesterday and the dealer reprogrammed/re-adapted the ECU and said it was fixed, but it's not.

The car is definately not mis-firing and there were no error codes in the ECU (they initially suspected a faulty MAF).

I suspect it is a transmission problem (ouch) car has (only!) 130,000 km (about 81,000 miles) as the shifting is sometimes a little hesitant and sometimes it's can't seem to decide which gear it should be in.

I've seen others with similar complaints, but no real resolution.

Unfortunately the dealer here is a bit of a sausage factory, they don't really give the techs time to work on the cars, they don't even test-drive them any more, and charge a fortune.

I have had the transmission fluid checked about 50K km ago, but I might get it changed to be sure.

I'd appreciate any iother deas from the group.

Thanks

Darrin Smith

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Hi,

I have the same car as you ( Tiptronic - but it's a Cabriolet). When I first purchased the car I had noticed a similar change in rpm where you wouldn't expect it under light throttle. The lights that indicate the gear show that no change in gear has taken place. I purchased my car 2.25 years ago from the original owner, and have noticed a dramatic improvement in the car since I have owned it (when I bought the car I had no idea that the car was not performing up to normal Porsche standards).

I attribute this improvement in performance to the fact that I drive the car 3-4 days per week (in each case getting the car up to full operating temperature), use high quality fuels, etc. My car drives dramatically better than it did when I purchased it. I suspect the original owner used low quality fuels and/or drove the car infrequently. I don't know how to explain such an improvement in performance of the vehicle since the day I purchased it. (I added Granitize De-Carb once about a year ago at the suggestion of the dealer). For the first time since I have owned the car, just last week, I added 3 gallons of 100 octane to 9 gallons of 91 octane, and this has also resulted in additional improvement.

The surging rpm issue is less frequent and almost never occurs now.

John

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Clean the throttle body; search for DIY on this site, it's been covered many times

+1

I was skeptical that cleaning the throttle body would really do anything... until I finally took a few minutes to do it.

best return on time spent with the 996 so far. do it tonight!

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Try cleaning the idle valve as well. My 99 was surging at idle. Not exactly the same symptoms as your but it's an easy DIY and worth a try. It cleared up my problem in my non e gas 99 C2 and didn't cost a dime.

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Try cleaning the idle valve as well. My 99 was surging at idle. Not exactly the same symptoms as your but it's an easy DIY and worth a try. It cleared up my problem in my non e gas 99 C2 and didn't cost a dime.

Hi cleaned the throttle body (it wasn't that dirty anyway). No change I'm afraid.

Any other ideas?

the dealer checked the MAF sensor and said it was working fine.

Thanks

Darrin Smith

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Try cleaning the idle valve as well. My 99 was surging at idle. Not exactly the same symptoms as your but it's an easy DIY and worth a try. It cleared up my problem in my non e gas 99 C2 and didn't cost a dime.

Hi cleaned the throttle body (it wasn't that dirty anyway). No change I'm afraid.

Any other ideas?

the dealer checked the MAF sensor and said it was working fine.

Thanks

Darrin Smith

Idle valve. Refer to my last post

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Try cleaning the idle valve as well. My 99 was surging at idle. Not exactly the same symptoms as your but it's an easy DIY and worth a try. It cleared up my problem in my non e gas 99 C2 and didn't cost a dime.

Hi cleaned the throttle body (it wasn't that dirty anyway). No change I'm afraid.

Any other ideas?

the dealer checked the MAF sensor and said it was working fine.

Thanks

Darrin Smith

Idle valve. Refer to my last post

Hi

The car idles perfectly. However I will give it a go anyway.

Where is the valve located? Do you just use throttle body cleaner?

Thanks

DS

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Try cleaning the idle valve as well. My 99 was surging at idle. Not exactly the same symptoms as your but it's an easy DIY and worth a try. It cleared up my problem in my non e gas 99 C2 and didn't cost a dime.

Hi cleaned the throttle body (it wasn't that dirty anyway). No change I'm afraid.

Any other ideas?

the dealer checked the MAF sensor and said it was working fine.

Thanks

Darrin Smith

Idle valve. Refer to my last post

Hi

The car idles perfectly. However I will give it a go anyway.

Where is the valve located? Do you just use throttle body cleaner?

Thanks

DS

http://www.renntech.org/forums/index.php?showtopic=6828

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Try cleaning the idle valve as well. My 99 was surging at idle. Not exactly the same symptoms as your but it's an easy DIY and worth a try. It cleared up my problem in my non e gas 99 C2 and didn't cost a dime.

Hi cleaned the throttle body (it wasn't that dirty anyway). No change I'm afraid.

Any other ideas?

the dealer checked the MAF sensor and said it was working fine.

Thanks

Darrin Smith

Idle valve. Refer to my last post

Hi

The car idles perfectly. However I will give it a go anyway.

Where is the valve located? Do you just use throttle body cleaner?

Thanks

DS

http://www.renntech.org/forums/index.php?showtopic=6828

What kind of gas do you buy? This may be why you have this problem in the first place (bargain basement fuel is a bad idea).

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Hi

Well I cleaned the idle valve, it was quite dirty. I noticed that at rest (ie disconnected and removed from the car) the valve is about 1/4 to 1/3rd open.

When you push the valve open even more with a screwdriver, you can definately feel the resistance of it's (I guess) internal spring.

When you release the valve it closes (snaps back) all the way, but there is no resistance to opening the valve back to about 1/4ish open. You can just shake the valve at it opens by itself.

Also the valve ball itself appears quite worn, it has kind of groove like scoring marks in the metal half-ball inside the valve (visible by just looking at the right-hand port).

This definantely seems like it might be faulty.

Can anyone comment on whether the valve should be fully closed at rest and whether this lack of valve tension to the 1/4ish opening is normal.

Is there a way to test it electrically?

Also in answer to the fuel question, I only use 98RON fuel. If it was a fuel problem I would have expected it to show under hard accelleration, but the car goees like the wind with the foot to the boards!

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I would just re install the cleaned idle valve and see if there is any improvement. When I cleaned mine the car had 70k miles on it. It was very dirty as well. Ididn't analize whether it was worn out.

Put it back together and see what happens.

success :D

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Sorry I forgot to mention, I did re-install the valve and there was no improvement at all.

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I'm sorry to hear that. At least you have eliminated the idle valve from possible culprits. I hope you can find the problem. I'm all out of ideas with my limited knowledge.

Good luck and keep us posted.

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I've ordered the durametric software. I might use the real-time data features to see if it can pin-point anything.

I still suspect the idle valve, I might see it I can find one at the wreckers to look at.

Thanks!

Happy Easter! I'm off camping in the Cayenne! no phones!

Cheers

DS

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Hi,

Just to finish off this topic I got my car back yesterday after a full rebuild on the transmission :-(

On the down side cost was about $7,400 AUD, which I didn't think was too bad in the scheme of things.

On the plus side it drives like new now.

There was nothing they could really put their finger on, just generally worn out all together. A little disappointing after only 140,000km.

Anyway all fixed now,

Thanks

DS

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Sorry to hear you had to dish out that much cash but happy to hear you got her back!

:drive: :cheers:

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