Engine Shut Off Rattle

Alright guys, I think I'm having some paranoia but I just want to make sure. Had an event with a lot of my family and got to show off my new to me 2002 996 Targa, I've absolutely love this thing. Had my brother in law in the car and gave it some juice like I had several times that day already, but this time when it came down to idle it sputtered a little but went right back to idle. Got back to the house no problem, no noises, but when getting in the driveway to a stop it started throwing a good amount of white smoke out. I know oil is a known thing so I just said it'll burn off and be fine. Which was mostly true after a bit of highway driving. Getting back to my house which was 30 min away, car drove fine, felt like it always does. Temps, oil pressure, etc all look/feel normal. Get home, smoke has subsided, open the door to listen, all normal, turn off the car and right at the end there's a rattle, very quick, but not something I'd heard before. So now between the slight rattle that after 4-6 starts/stops happened the same as above maybe 2 more times and the amount of smoke that was coming out earlier I'm a little paranoid. The rattle does not happen on startup or persistent during engine running, only when you shut it off. Any thoughts? Thinking of just taking it in next week. Definitely having IMS bearing fears but also think that's a bit dramatic. Thanks in advance!

Welcome to the 996 club. Awesome car. With what you describe, it sounds like your AOS is failing. Very common. The Oem AOS is a weak part. Get yourself a UAOS which is a superior product to the OEM AOS which you won't have to worry about again. You can find an entire thread dedicated to it here in the 996 forum. As a side note, if your water pump and IMSB hasn't been replaced, it would be advisable to replace them as well.

Out of an abundance if caution i dont think i would start it without a scaner hooked to it. It almost sounds like a timing issue. Either a variocam, tensioner, chain, sliped cam gear or pad issue. Many times an off idle a motor can compensate for bad timing but at idle it will stumble, also it can allow oil through the valves and cause white smoke. Good advice above too, check aos performance with a manometer..

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I've read a few other things about the AOS and have a feeling that may be the culprit with the smoke. I'll find that thread you mention and look to get it replaced as well as look into the other two items. Thank you!

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Can you expand on the off idle comment? Now that you're mentioning something slipping that seems to me like a high chance to be the culprit of the sputter after the pull because of the amount of smoke and time it took to subside.

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Most important thing is to get yourself a good Porsche Indy if your not mechanically skilled. You didn't mention any check engine lights going off, but play it safe and get to the Indy asap. Tough to say with what you shared with us whether you can drive it there or tow it. But De Jeeper's caution is right "just-in-case". A Durametric OBD scanner goes a long way in telling you what it may be. Despite all the fears that the 996 forum can create, the reality is these cars are very durable and the problems are preventable with a rotation of parts replacements over time. Because it's a 20 year old car, you should check the service records to see if any of these parts have been replaced recently. If not, you should consider replacing them. Engine Failure Parts - If these OE parts fail, you run the risk of grenading your engine. 1. OE AOS - replace w/ UAOS 2. OE Water Pump - replace w/ OE water pump 3. OE IMSB - replace w/ either the IMS Solution or Retrofit Kit. While you are in there replace your RMS, and check the clutch and flywheel. Check Engine Light Parts - if these parts fail, CELs will appear and will mess with the smooth running of your engine. 1. MAF 2. Ignition Coils & Spark Plugs 3. Fuel Injectors 4. Oxy sensors on cats Lastly, change your engine oil at least 2x a year. Clean fresh oil is the easiest way to prevent bore scoring and engine longevity. Plenty of other things you can do, but resist the temptation to upgrade exhaust, suspension and pretty shiny things on the car until you take care of the engine with the items above. Keep us posted! The 996 is an incredible car. Just like anything in life, all you gotta do is take care of it and it will take care of you.

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This is really great, thank you for all this. I definitely plan on taking great care of this car, it's my first Porsche and I'd plan on having it for a long time. You're correct no check engine lights and all gauges normal. I haven't started it up this morning but as said earlier it ran fine during my trip home last night. I've read some other items about pushing the clutch in during shut off would help the rattle that I've described. It makes me think it'd be fine to drive to the Indy from this posts plus other things over read. But that doesn't necessarily fit the abundance of caution. Ill give my Indy a call and talk through it and go from there. Thanks again, I'll keep updates coming.

You said white smoke. Are you sure it is white, and not gray? Think steam. If gray smoke, all of the above advice sounds like the way to go. If it is white smoke you have another issue entirely. I don't know enough about these engines yet to give you any ideas as to exactly what, but white smoke indicates water, not oil. You can also tell by smell; water 'smoke' is going to have a sweetish smell from the antifreeze, and oil smoke smells like, well, oil.

Raider, you can open your coolant cap to see if you have a mix of coolant and oil, as well as do the same with your oil cap and oil dip stick. Odds are you don't because it's a 996.2 versus a 996.1. The 996.1 suffers from D-Chunking which is the cracking of cylinders due to how Porsche held then with clamps in the manufacturing process. 996.2 doesn't typically suffer from it. But it's always good to check what your coolant looks like, pull the dipstick and check the oil, etc. Add/Edit: Assuming your engine is safe to run, you can test if your AOS is failing by removing your oil cap when the engine is running. If it is difficult to get off due to lots of suction, and once you do get it off, the engine starts to stall, you most likely have a failing aos.

No need to panic over any of it. The 3.6 engine is pretty damn tough. The key is you want to replace the parts I mentioned previously, and take care of your bores with frequent oil changes of at least 2x a year. If you want to sleep at night, get your bores scoped to see what you got and test your oil every so often to see what's floating around in it. Other minor bolt-on precautions will need to be taken if you want to track your car that we can talk about when ready if applicable. Funny things can always happen to anything mechanical, but since the 996 is 20 years old, we pretty much know the what, when, how and why of the car. Enjoy it!

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I was reffering to the engine being able to overcome timing issues while at higher rpms. The inertia of the motor makes it harder to detect misfires. Its a simple check of a few numbers with a scanner to verify that the variocams and timing is in good shape. I also agree that lazy o2 sensors and a failing maf sensor can show up as a bad idle. Again a scanner can help determine the sensors functionality.

The rattle is probably the trans, they will do that in neutral at idle and shut down. Nothing to worry about. The white smoke was probably from the AOS getting some oil into the intake after spirited driving.

Itusler brings up a good point, you may want to consider replacing your engine mounts and transmission mount. I know, I know, it all costs money, but the car is 20 years old. Just set up a rotation over the next few years to replace parts that are necessary and can be grouped together to save you some money. As an example, next week my car is going in to the shop to replace my motorsports aos with the UAOS with drainback, replacing my IMS Retro with a new one, putting in a new RMS, looking over the clutch and flywheel, replacing the oe oil cooler, seals and rubber with a new one, and going thru the water cooling system. While I replaced my cooling system including the water pump 4 years ago, I am still having my shop go thru it with a fine tooth comb. ​​​​​ After that the next visit will be more oil cooling and the UIDS. Next year is a suspension refresh, followed by an exhaust refresh. Then an engine rebuild. Do what you gotta do to keep it running safely and then after, do what you wanna do. No timeframe on the 996 if you are a long term owner.

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Happens on my car; depress the clutch when shutting down...no rattle, at least for me.

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Man I can't believe all this advice, this truly is a great community. Your way of thinking of this over time is exactly what my plan is, take care of things that need to be taken care. Out of curiosity what are you expecting that visit to the mechanic to cost next week? Seems like a list I need to may need to make happen sooner than later.

Update: The car continues to run fine once started up, I had to move it in and out of the garage today as we're doing renovations. On both startups, this morning after overnight obviously, and then about 6 hours later. Both times I got white smoke on start and then almost immediately subsides. Morning the rattle on shutdown was very slight, evening it was a bit more but still not crazy. I don't hear the rattle on startup. I think my biggest concern is the smoke on startup. It seems to happen every time now vs it only happened before if the car sat for a few days, which I've read is normal. I think I'm on the AOS line of replacement. The coolant and oil are clean. I haven't done the vacuum test with a lot of other stuff going on today. Thanks again for everyone's comments here, I've learned a ton and while I'm no longer panicking a little I'm still concerned and will get it to an Indy soon. This is my daily (50/50 work from home so not a ton of miles), I can make other arrangements for sure, but I'm thinking I shouldn't commute with it before the Indy. Thanks again to everyone!

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For next week's visit, I purchased the parts which tally up to a little over 2k (UAOS, IMS Retro, RMS, Oil Cooler). Probably be a little over 15 hours of work at 150 an hour. Maybe less since we are grouping things together with a lot of "while you are in there" stuff. I was just in last month going thru the borescope and running thru the engine, suspension, brakes, exhaust, rubber and plastic bits, replacing differential and trans fluid, checking for leaks, replacing engine mounts, fuel injectors, sensors, etc. The car is in great shape but I want to get it on the track again, so it comes down to replacing parts that may be fine for the street for a few more years but on the track you don't want the possibility of a part failing. My shop is very seasoned just working on Porsches with a heavy club racing and DE clientele. We have done the same stuff a few times before over the past 20 years.

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So I don't plan on tracking this car unless I end up getting a different daily. I'd love to maybe one day but it's highly unlikely. Too many other hobbies to pick up that for right now. With that said is a UAOS worth the money if this is just a half daily/weekend car? I'm sure it's a far superior part but I'm seeing costs that are significantly higher vs an OE AOS. Unless I'm looking at the wrong things (UOAS I'm seeing is $700-$1000 vs OE AOS is less than $200)

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Is it absolutely necessary? No, but it is truly far superior and worth the money. You'll replace it once and never have to worry about it again. On the other hand you will find lots of stories of factory AOS failing within a year, and that means you will be shelling out the money for the part and labor again.

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Yes it's worth the money. The cost of the OE AOS is cheap, but so is its quality. I had 2 fail within the first 4 years of ownership in my car. It's expensive to replace them with service hours. So I then went to the motorsports aos which was a superior part to the OE AOS in durability. However, today it's more expensive ($900) than the UAOS ($710), as well as when the MS-AOS does fail, like mine is now, my shop will have to go thru the same 4 hour exercise to replace it. However, with the UAOS, once it's installed, you will never have to replace it again. Just the diaphragm in the engine bay, which takes 5 min and is a very cheap part. There are other benefits to the UAOS as well, such as the ability to run an extra quart of oil if you get the track/drain. But in your case, you don't need that for the street.

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Totally get it and makes sense, seems like it's worth it regardless of tracking or not. Thanks!

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Ok perfect, just wanted to make sure I wasn't shelling out money like that for a part that only benefits track type environments. Do you mind linking the one you're looking at, just want to cross reference what I'm seeing to make sure I'm seeing the right thing. Thanks!

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​​​​​​You want the UAOS-AL. You can get it at ultimateaos.com or thru LNEngineering.com. You do not need the track drainback which is an extra $175 since you won't be tracking your car. You can always get it later if you decide to track it. Porschetech3 (Skip) is the inventor. Hopefully he sees this and helps to give more direction. You will find him on his UAOS thread as well. He likes questions so feel free to ask.

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Question for the assembled AOS expertise: does a failed AOS cause the car to emit smoke on every startup, even if the car is only driven lightly afterwards? Or just backed out of the garage and then driven back in?

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Yes, with the 2 OE AOS failures I had in 4 years, they smoked every time I started the car. The smoking eventually stopped if just driven on the street without any heavy braking or turning. However, heavy braking and turning will cause a huge continuous smokescreen and flames from the exhaust. This is what you wil see on the track.

Two follow ups: How risky is it to drive a little in a conservative fashion to get around town? Does anyone recommend an Indy in Dallas area? We have our mechanic who did the 3rd party inspection on purchase who works on Porsches but I wouldn't say he's a specialized Porsche mechanic.

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At minimum, the risk is fouling your plugs and ruining your cats. At worst, you can hydrolock and destroy your engine.

The UAOS is better than the Porsche OEM AOS in EVERY way, even economically, unless you plan on flipping the car for a quick buck soon. Happy to answer any questions. Also BTW, lots of good advice above. and BTW :: the rattle sound on engine shut-down can also be cause by a dual mass flywheel that is out of factor specs. I have seen this scenario several times although not as prevalent as other failure modes of the DMF..