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AOS Replacement eliminates vibration


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I just replaced my Boxster's AOS on a 2003 with a 2.7L engine, 65,000 miles, 5 speed manual transmission. An unexpected byproduct is that an engine vibration at about 3200 rpm's that Porsche said was normal is completely gone. I also cleaned the MAF but surely that was not the fix. If you have a vibration that occurs after warm up when accelerating thru this rpm, driving on the highway at this rpm, and more pronounced decelerating in gear thru this rpm, and you have not replaced your AOS, you might just want to try this. I would welcome any dissertation anyone out there may have as to why this may be so, although I have my theory and told Porsche that in 2004.

Have at it.

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I just replaced my Boxster's AOS on a 2003 with a 2.7L engine, 65,000 miles, 5 speed manual transmission. An unexpected byproduct is that an engine vibration at about 3200 rpm's that Porsche said was normal is completely gone. I also cleaned the MAF but surely that was not the fix. If you have a vibration that occurs after warm up when accelerating thru this rpm, driving on the highway at this rpm, and more pronounced decelerating in gear thru this rpm, and you have not replaced your AOS, you might just want to try this. I would welcome any dissertation anyone out there may have as to why this may be so, although I have my theory and told Porsche that in 2004.

Have at it.

Great job! Did you find it a difficult repair? For $90 this must be a DIY correct?

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I just replaced my Boxster's AOS on a 2003 with a 2.7L engine, 65,000 miles, 5 speed manual transmission. An unexpected byproduct is that an engine vibration at about 3200 rpm's that Porsche said was normal is completely gone. I also cleaned the MAF but surely that was not the fix. If you have a vibration that occurs after warm up when accelerating thru this rpm, driving on the highway at this rpm, and more pronounced decelerating in gear thru this rpm, and you have not replaced your AOS, you might just want to try this. I would welcome any dissertation anyone out there may have as to why this may be so, although I have my theory and told Porsche that in 2004.

Have at it.

Great job! Did you find it a difficult repair? For $90 this must be a DIY correct?

Yes, DIY. Took about 3 hours to remove the top manifold sections and throttle and clean all the oil out, but otherwise it was not bad. Anything in the Boxster engine compartment is tight, but the AOS was just a little tedious, that is all. The AOS cost anout $78 and the bottom hose was $12, and that is at the local dealer. I posted a fix and some photos just before this post which goes into a little more detail, but it is mostly about the AOS having blown. If it was just replacing the AOS and not all the clleaning it would be considered almost easy in my mind.

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I just replaced my Boxster's AOS on a 2003 with a 2.7L engine, 65,000 miles, 5 speed manual transmission. An unexpected byproduct is that an engine vibration at about 3200 rpm's that Porsche said was normal is completely gone. I also cleaned the MAF but surely that was not the fix. If you have a vibration that occurs after warm up when accelerating thru this rpm, driving on the highway at this rpm, and more pronounced decelerating in gear thru this rpm, and you have not replaced your AOS, you might just want to try this. I would welcome any dissertation anyone out there may have as to why this may be so, although I have my theory and told Porsche that in 2004.

Have at it.

I would like to know your theory how the AOS would cause a vibration. I have a 996 and sometimes notice a vibration at 2000 rpm.

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That is the first time I've heard that as a solution to the vibration while decelerating

below 3k rpms. Could you elaborate on why you think changing the AOS would impact engine vibration?

I recently found that this vibration went away on my car after replacing a worn engine mount, and think it did it by allowing less engine movement.

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That is the first time I've heard that as a solution to the vibration while decelerating

below 3k rpms. Could you elaborate on why you think changing the AOS would impact engine vibration?

I recently found that this vibration went away on my car after replacing a worn engine mount, and think it did it by allowing less engine movement.

I will tell you first that I am an architect and understand that many things are inter-related, but I do not know how they are inter-related.

The theory is has not included the AOS until now. I have believed all along that the VarioCam was the culprit. The valve overlap is controlled by VarioCam somewhere between 1,250rpm to 5,120rpm according to Porache literature but the service manager 5 years ago, who I trust but who is no longer there, told me that the VarioCam is the only thing that "happens" in this 3,000-3,200rpm range, where I have had the vibration. I asked him what was different at this rpm and that was his reply.

The vibration started when I took off in the car just to see/feel the power. The car had less than 10,000 miles on it. Got to 80mph in 3rd gear and that was good enough, and short shifted to 5th and let the clutch out. You would have thought I pulled 2 opposing plug wires out. I just knew I had broken something. Put clutch in, slowed a little, then went on. All was fine.

Some time later but before the 1 year service I started getting a slight vibration decelerating thru about 3,200rpm to 3,000 rpm. It had gotten worse as time went on, including every time I accelerated thru this rpm, to where driving at 3,200rpm on the highway caused constant vibration. This is at 75 to 80 mph, the exact speed I had to drive for 2 hours going to Kitty Hawk regularly. Made me crazy. To get out of the vibration I either had to slow down or get up to about 85mph, somewhere above 3,500rpm. You can check the graphs, the speed/rpm's match so performance was never affected.

Yes, I had a motor mount replaced, had no affect. Transmission replaced early on since they said the pinion gear was chewed up, but I had no symptoms whatever, no affect. They were sure that would fix the vibration.

So 5 years of this with no relief.

So my AOS blew and I replaced it. While in there I cleaned the MAF just for fun. Now if someone out there tells me the MAF cleaning solved the problem, I'll accept that but even Porsche Service would have tried that. Don't know if it has ever been cleaned but looked pretty good. Have a photo of it before cleaning if you are interested.

All I know is trhe vibration is completely gone and I have duplicated all the scenarios that caused the vibration, cannot get even a little shutter.

The VarioCam was redesigned for MY2003 to include an oil filled stadia rather than electronic operation. The oil pump was made 5% larger. I got this from "Porsche: Excellence was Expected" and other Porsche literature.

Now here is the theory: If I had a breech inside the AOS effectively reducing the oil pressure slightly (5% or more, the pump size increase made to overcome the 2003 VarioCam design?) which affects the valve timing during this rpm transition, would that cause a too long or too short overlap in the valve timing, causing the out of balance vibration? And if the breech got progressively worse would the symptoms get worse? Something happened to get progressively worse, that is a fact.

The alternative is that the MAF was dirty. Those are the only 2 things I did, besides cleaning the manifold and throttle. The vibration is gone. Completely.

I will admit that one of my greatest faults is to learn a little about something and think I came up with something ingenious. This may be one of these times. If you have the same vibration, clean the MAF. If that works, groovy. If it does not, change the AOS. That is the simple way to look at it.

I have no idea if the rpm of the 996 is in the same range or not, so the previous question may not be ansewerd even if it is for the 986.

Thanks for sticking with me this long, that is if you are still reading. Architects are imaginative, aren't they?

Alan.

Edited by octantman
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Alan,

That’s great the vibration is gone but you probably affected something else when changing the AOS. I don’t know the Boxster, did you have to remove and replace the exhaust? The variocam adjustment is independent of oil pressure.

Take a look at this thread http://www.renntech.org/forums/index.php?s...c=17104&hl=

Also, did you notice the vibration in all gears and stationary?

George

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Alan,

That's great the vibration is gone but you probably affected something else when changing the AOS. I don't know the Boxster, did you have to remove and replace the exhaust? The variocam adjustment is independent of oil pressure.

Take a look at this thread http://www.renntech.org/forums/index.php?s...c=17104&hl=

Also, did you notice the vibration in all gears and stationary?

George

All gears but never when stationary.

Also, I think VarioCam for Boxsters and 911 are different. Here's what Ludvigsen says on page 1370 of "Excellence" specifically for the 2003 MY and updated VarioCam on the Boxster:

"Assurance of enough hydraulic pressure to operate the VarioCam was given by a five-percent increase in the pressure pump's size."

Could be one of those times I am over my head, but there it is.

Alan

Edited by octantman
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Alan,

That's great the vibration is gone but you probably affected something else when changing the AOS. I don't know the Boxster, did you have to remove and replace the exhaust? The variocam adjustment is independent of oil pressure.

Take a look at this thread http://www.renntech.org/forums/index.php?s...c=17104&hl=

Also, did you notice the vibration in all gears and stationary?

George

Hey, George. I participated in the thread you reference. Had NO vibration when stationary or before warming up to 185 degrees. You may be right so here is what I did:

1. Read and cleared misfire faults cyl 2,4,5.

2. Removed and cleaned (just wiped off oil, used no cleaners) throttle/manifold section and forward manifold section taking care to see that connectors and vacuum tubes had been secure before removing.

3. Removed old AOS.

4. Cleaned MAF. It's connector was secure and clean when removed.

5. Replaced K&N air filter with OEM filter. Used since June 2006 and cleaned annually. Decided to not use based on posts I have seen about K&N not keeping particulate out. Note vibration predated 2006 use of this filter.

6. Cleaned a little oil out of intake air duct that had dripped into it.

7. Installed throttle and manifold section with precise care for alignment and reconnecting of everything.

8. Cranked and ran, cleared other codes for precat O2 sensor and CEL not being present.

9. Been to work and back 3 days, no codes, no vibration.

So what could it have been?

Alan

Edited by octantman
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I think your vibration problem was caused by a not 100% effective resonance flap(s) in the intake manifold vertical pipe(s), before cleaning.

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I will tell you first that I am an architect and understand that many things are inter-related, but I do not know how they are inter-related.

George Costanza is an Architect too! :) Does this "line" really work?

Good right-up, Alan. Maybe this will help other owners too.

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I think your vibration problem was caused by a not 100% effective resonance flap(s) in the intake manifold vertical pipe(s), before cleaning.

Now there is a possibility, I made sure that was clean and functioned fully, operating the linkage attached to ther vacuum canister. I assume you are talking about the (horizontal duct forward of the throttle body) this thing:

post-27343-1237723292_thumb.jpg

post-27343-1237723311_thumb.jpg

Silly for Porsche Service to not even try. My explanation is quite elaborate but it is because I spent hours making sure they understood all the conditions precedent to the vibration.

Thanks for the evaluation.

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The ducts showed in your pics are indeed the resonance flaps.

At one time I had the engine compartment open revving slowly up to about 4,000rpm and this flap did operate at about 3,000, although there was no vibration. Could definitely hear something click and indeet you can see the stud on top rotate. There was a lot of oil when cleaning it but did not notice grit. But since I operated it about 5 times plus cleaned the linkage outside with a toothbrush this could be it.

So there is something else relatively simple for folks to try.

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  • 3 weeks later...
The ducts showed in your pics are indeed the resonance flaps.

At one time I had the engine compartment open revving slowly up to about 4,000rpm and this flap did operate at about 3,000, although there was no vibration. Could definitely hear something click and indeet you can see the stud on top rotate. There was a lot of oil when cleaning it but did not notice grit. But since I operated it about 5 times plus cleaned the linkage outside with a toothbrush this could be it.

So there is something else relatively simple for folks to try.

Well, the vibration came back. If Renntech believes this is a misleading post and thread, please remove it in its entirety so people will not think this will solve their vibration problem. It is a fact that the vibration was gone for about 4 days and over 100 miles after this work. I have another question which I will post as a new item.

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