2002 C4S - HPDE Day Resulting in White Smoke and Cyl 6 Misfire

Hey all � looking for some sanity checks and a good diagnostic plan before I dig deeper or tow the car to a shop. 2002 911 C4S, manual, 62k miles, car is on a fresh oil change, less than 1.5k miles on it. Recent oil analysis (Blackstone): Sample taken ~1,500 miles ago Wear metals (iron, aluminum, copper, etc.) all at or below universal averages No coolant in oil, no fuel dilution, viscosity normal Comment from Blackstone was essentially �engine looks healthy, no abnormal wear� So as of 1,500 miles ago, there was no sign of long‑term, high wear consistent with chronic bore scoring. --- What happened � Did a HPDE day, first run was fine, second run I got some white smoke from both tail pipes at start, it went away. I added 400ml of oil, noticing the level went down after this from burning oil. I drove the car lightly to lunch, at start I had some smoke but no issues after. Did one more run after lunch, did two laps fine, then cut the next short with heavy white/blue‑white smoke from both tailpipes, and flashing check engine light - P0306. � During this run engine started running rough with check engine light and misfire on cylinder 6 (P0306), heavy white smoke. � To attempt drive home, I noticed poor throttle response, lugging engine and constant white smoke. I towed the care home, have not started it since. Now I am thinking, is this bore score in cyl 6, or a failed AOS. Prior owner had replaced AOS a couple years ago. --- Working theories Right now I see a few possibilities: 1. AOS / crankcase ventilation failure AOS lets a lot of oil into the intake, oil goes to both banks, heavy smoke out both tailpipes. Saw some oil weeping on the driver side bank, so I dont think the oil issue is localized to cyl 6. Oil‑fouled plugs cause misfires, with 6 being the first one to trigger a code in this instance. 2. New, localized mechanical failure on cylinder 6 Fresh bore scoring, broken ring land, or other cylinder‑specific damage that happened after the last oil sample. Would also explain the fixed P0306 and poor running, and the smoke if 6 is burning a lot of oil. But the smoke came from both tail pipes. 3. Combination of both AOS or other issue floods the intake with oil, which either triggers or accelerates failure on an already marginal cylinder 6. The clean Blackstone report makes long‑standing, severe scoring less likely, but obviously doesn�t rule out something that happened in the last 1,500 miles (fairly normal driving, two autocross days). --- What I haven�t done yet Because I don�t want to risk more damage, I have not restarted the car since the incident. I also haven�t yet: � Pulled plugs � Checked compression � Done leak‑down � Scoped the cylinders � Inspected the AOS or intake tract for oil --- Planned at‑home diagnostic order (no restart) My initial plan is: 1. Key‑on / engine‑off scan: Confirm codes (currently P0306), see if there are any other misfires or mixture codes. There are no codes currently when I checked before writing 2. Static checks: Oil level/condition, coolant level/condition, visual check of both tailpipes, check the underside of my car for leaks over the next couple days 3. Pull bank‑2 plugs (4, 5, 6): Compare plug condition: All three on bank 2 similarly wet/oily/sooty → points toward intake oil ingestion (AOS/breather). Plug 6 much worse than 4 and 5 → points toward localized issue (scoring/rings/valve on 6, or at least 6 getting hammered). Inspect coils, especially 6, for cracks or tracking. 4. Compression test on all six (fuel/ignition disabled): If 6 is in the same ballpark as the others → scoring/catastrophic mechanical damage less likely; AOS/ignition/fuel stay high on the list. If 6 is significantly down vs the rest → strong indication of new mechanical damage on that cylinder, and I�d then move to� 5. Leak‑down + borescope (probably at a shop): Localize leakage (valves vs rings/cylinder). Borescope 4�6 for visible scoring, especially on cylinder 6. I intentionally skipped any extra running so I don�t risk washing down the cylinder walls further or trashing the cats. --- Questions for the group 1. Does this diagnostic order make sense given the symptoms and oil history? 2. Anything you�d bump earlier or later in the process, especially given that I don�t want to restart it? 3. For those who�ve seen AOS failures and bore‑scored 996s in the wild: How often have you seen �both tailpipes smoking + single P0306� turn out to be mostly AOS vs mostly cylinder 6 damage? 4. If compression on 6 comes back significantly low: Any specific leak‑down/borescope tips on these engines that you wish you knew earlier? At that point, would you keep cranking for more tests, or stop and plan a rebuild/engine swap? Any feedback on my logic or suggested tweaks to the test sequence would be really appreciated. I�d love to make the most informed decision possible before I either start unbolting things or send it straight to a shop. If I do send to a shop, what would a AOS replacement run me. And do I replace it with the UAOS? Do folks often replace all the lines as well?

My short take on this: Oil ingested during hdpe and subsequently a crack in cil wall of #6 due to hydrolock (coolant seeping into cilinder causes the white smoke and misfires/CEL)

Understood - adding ‘cracked head/cylinder with coolant leak into 6’ to the possibilities. Though wouldn't that cracked cil wall lead to smoke only out of one tailpipe? I checked the dipstick, oil is not latte like, nor is my coolant level lower than when I started the day.

Usually burning oil is blue smoke and coolant / water is whiter and fuel is blacker, AFAIK. You may have had some sort of coolant leak initially, or some sort of failure that let coolant into the cylinders... You may have a larger problem related to the leak, but the code could be from something as simple as fouling the spark plug in that cylinder... You could have introduced some oil into the engine by adding the 400ml of oil after your AM session. You mention white smoke before lunch and blue smoke in the afternoon, right? I would start by ensuring the oil is not overfilled and the AOS is not allowing oil into the engine. I would check the spark plug in Cylinder 6. I would check the coolant and see if it is low or oily. I would check the oil to be sure it does not smell like anti freeze is in it or look like it. If you think the engine sucked oil into the cylinders by AOS being overwhelmed, pull all 6 plugs and turn over the engine to clear the cylinders (as long as you are sure you do not have some major mechanical failure that you be making worse by doing so). Hopefully, you have two issues: one the AOS dumped in some oil in your afternoon run - hopefully easily solvable by making sure not overfilled and hopefully not pumped into cylinders. Then, you need to figure out where the coolant leak is - could be a cylinder head - you will need to hunt it down. Just my 2 cents. I hope it is not a major breakdown.

You could initially pull the throttlebody out (easy & quick) to see if the intake runner is flooded with oil. If yes, there is good chance that the AOS has given up. Next would be plugs to verify the findings to be true or not.

Your planned steps look good. Personally I would get an endoscope, ideally one with articulating head (180 deg in one plane) and look into cil 6 first and then 5 (which should be inconspicuous in appearance), 4, 1, 2 and 3 before doing compression or leakdown test. If visual yields nothing definitive, then leakdown. As a next step, AOS health can be confirmed by measuring crankcase vacuum (should equal ~5" of water). If greater vacuum, then the membrane inside AOS probably torn. If much less vacuum, then a vacuum leak is probable or you have a lot of blow-by. There is a thread on here started by Porschetech3 that explains the procedure and has results added from fellow rennlisters. Good sign that coolant level is not lower than at beginning of the day but be aware, it takes only a little coolant to cause a lot of smoke. From personal experience I can say that AOS dumps oil into cilinders 1 and 4 first, as those are closest by the AOS hose connected to the throttle body. So you having codes for nr 6 could well point to a different cause. Coolant flows through AOS to prevent the PVC valve freezing (in winter/cold climate), so depending on how it has failed, coolant could join the oil being sucked into the intake. Re-reading your original post properly, my earlier reply is perhaps too much of a shot from the hip. Do the analysis yourself.

First of all, your factory AOS failed. I see it all the time at the track as an instructor and folks think that since the have a new factory AOS that everything will be fine. Nope. The factory AOS is a poor design and cannot process oil mist under heavy G. A very common issue and easily preventable with a better designed AOS like the UAOS. Second, when oil mist from a failed AOS floods the intake, you run the risk of hydrolock occuring which is when the pressure buildup from the oil mist in the cylinders cracks or even shatters the cylinders. Lets hope you dont have hydrolock which will require a rebuild. Whether you need a rebuild or not, the first thing I woukd do is get the factory AOS off the engine and replace it with the UAOS w/trackdrain so this doesnt happen again.

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"measuring crankcase vacuum" = Bad Idea. Engine running to do the testing. Agree with GC996, failed AOS. Even a "new" Porsche Motorsport AOS, their "zoomie" edition fails right out of the box. Porschetech3 and UAOS have a million page long thread on install, how-to, tips & tricks, and so on. Mentioned over and over is if you want to do HPDE track stuff, get a UAOS + Track Drain on your engine. Cost is about +/- the same as P-Motorsport parts. Some folks also add the UIDS, but there are different opinions on that part- just FWIW. Search on RL for comments. While it "may not" be failed AOS, it could be failed spark plug, cracked ignition coil pack, or a failed fuel injector, among other things. Also, a higher viscosity oil for "track stuff" is recommended along with more oil in the engine (which standard AOS doesn't like). My C4S "runs/ran fine". It is "under the knife" getting a new rebuilt engine which will have new UAOS+TD, all the ignition bits, WP + Coolant Rez, etc, etc. (The "ran" part is because this is a "pro-active" job, fix everything B4 it breaks, so obviously it is RFWP). Anyway, if DIY, start with the simple stuff, plugs, coil packs, etc, hope for the best. At 65k miles, the "life span" of the water pump is also "done - up", so something else to think about. Do some first simple steps and post results. RL folks can help. YMMV

Let me also say that just because you have a plume of smoke at the track doesnt mean that you cracked your cylinders from hydrolock. As an example, it happened to me 2x back in the mid 2000s before i went to the Porsche Motorsports AOS and then 4 years ago to the UAOS w/track drain which is the best. You will first notice the smoking when you start the car after it sat for awhile following a track session. White/light grey/blue smoke. Thats a good indicator that oil mist is getting into the intake. But it can and will get worse on the track if you are under acceleration in a G turn and you will see grey/black/blue smoke that plumes out of your exhaust. Yes, the corner workers may even see flames coming out of the exhaust. when this happens, you are at extreme risk of hydrolock. You will know it because its not white smoke, its black smoke. When all this happens you blow a ton of oily smoke that messes with your cats, sensors, plugs, coils, etc. causing all kind of problems. I wouldn't over react either way. Just be smart and systematically go thru your list. Your worse case scenario is hydrolock. Best case is everything is fouled from the oil mist and oily smoke. But as i said before, do NOT use a factory AOS on your car on the street or track. Especially of you are going to track it. It is a poorly designed part that just cannot process the oil mist. The best solution is the UAOS w/track drain.

Super helpful information. If this misfire and smoke situation is a repair I can handle, ill certainly upgrade to the UAOS. I thought I was in the clear since this AOS is the car's second. What do you think about the general oil weeping on the car? Could be due to the high gs the car felt and then I topped off with a little of oil? was a couple drops coming from what appeared to be the coils / headers area. I am going to try and pull my dash cam footage to get better footage of the the smoke to determine color etc.

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The first thing I would want to know is if i hydrolocked the engine. So i would scope the bores thru the sparkplug holes to see their condition. He second thing i would want to know is whats in the sump and oil filter. If any of this is beyond your pay grade, I would tow it to your indy. The 3.6 is a very strong and durable engine. It can handle the track without a problem. But the key to its longevity is to upgrade 3 parts. (1) upgrade the factory IMSB to the LN Retro or Solution so you dont have IMSB failure. (2) upgrade the factory AOS to the UAOS so you can effectively process the oil mist and run higher quantities of oil to eliminate oil pressure instability/starvation (bearing/rod failure) and the risk of blowing smoke and causing hydrolock (cracked cylinders). (3) replace the factory water pump every 5-7 years to prevent the plastic fins from breaking off and lodging in the head and cracking the head. These three items are pretty easy to do. For sure, we can discuss water and oil cooling and deep sump/UIDS at a later time. But the 3.6 is very strong and durable if the 3 above upgrades are addressed.

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EVeryone always think they are in the clear until the **** hits the fan... I hope the damage is nothing major ....

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...or the fan (WP blades) hit the engine block,,, YMMV

I pulled dash cam video from my track day, sure enough, as I exited a corner, hard on the gas, built speed, a puff of light blue smoke, and then my flashing check engine light for cyl 6 misfire. I pulled in to pit and shut the car off. Then as I tried to leave the event, same light blue smoke came up during acceleration. Yesterday I pulled coils and plugs for every cylinder. They were all looking pretty tough, with dark residue on the tips. Cyl 6 had the most oil and second and third most oil on the plugs were the remaining cylinders on that bank. Pics of all the plugs below. As I scoped the cyl, the ugliest ones were 6 (light honing, and oil pooling). Cyl 3 had oil pooling in the same quantity as 6, with what I thought was a crack, but I don't think it is based on further research. Pics below. Cyl 1 and 2 were at top so I could only see call it one cm of the wall. No oil. Cyl 4 same at top, saw some oil. Cyl 5 slightly below top, no oil and no evident scoring with the little bit of wall I could see. I would be curious to hear folks' feedback on these pics. If I had to make a determination, I do not think my engine is blown. I think that I suffered some oil in a couple cylinders due to AOS failure. (if I had a blown engine, I feel like cyl 6 would have looked much worse, and my engine would have seized before I shut it down. It was just running bad. I also believe that if I didn't have oil in any of the other cylinders (1-5, the non misfiring ones), that would have been much worse than me having oil in at least 3 cylinders. Thanks a million once again. Hoping to get back on the road soon! slight honing, didnt seem overly alarming like scoring, just some wear oil pooled up Cyl 3, had this crack looking thing, I don't think its a crack but worth sending here.

My old eyes see no bad problems in your pics my 2� 1) first thing, remove the big vacuum line from aos to intake. Put the line in an old water bottle to the side. 2) Spin over engine with plugs out by the starter for 10 seconds or so to try and clear any pooled oil out of cylinders. Do this a few times. 3 )If engine sounds ok while spinning it over in step 2, no abnormal rattles bangs knocks vibs, do a compression check and note all cylinders. 4) If compression checks out ok, then install some good plugs. 5) Start it up with AOS line still in water bottle and aos line plugged off at intake. Can get a vacuum line plug/cap at corner auto store. or make something up with hose/bolts/clamps. AOS Intake port must be plugged off. Bottom line, we do not want a possibly failed aos connected to intake ingesting more oil. Leaving the line in a old water bottle with a rag wrapped around the neck of bottle will be fine and let the crankcase breath fine. 6) see how it runs at idle, check/listen for any abnormal rattles bangs knocks vibs, might still be some residual oil/smoke. Will take some run time to clear it out. 7) If ok so far then its time for a easy drive easy up to operating temp. Make sure aos line water bottle is secured out of harms way. A little oil in bottom of bottle is fine/normal. If heads were flooded might be more oil temporarily, if so empty bottle and should be ok now. Thats your 'catch can', a setup that many race cars use. 8) This is just a troubleshooting test, if things all work out ok, you'll eventually need a new AOS/UAOS preferably, let us know how things work out. But many different type race cars use the catch can system indefinitely. JMHO

I also noticed a hose that was dangling around coil pack 6. I could not determine if this is one of the hoses that is supposed to be dangle, or it came loose during strain of the engine. it was coming from the top of the enging downward with the opening facing the floor

The cly r fine and that "crack" is not a crack and oll of our engines have that line. The hose by 6 is pribably the coolant resi overflow drain. Get a uaos installed and enjoy your car.

From your pictures, it looks like you escaped cylinder damage. Good news! @allcool and @De Jeeper made some good suggestions. What you will come to appreciate is how damn tough the 3.6 really is if you (1) upgrade your AOS, (2) upgrade your IMSB, and change your water pump every 5 years or so. I am amazed at how good the 3.6 is with those 3 simple solutions. For sure, a 2qt deep sump or the UIDS certainly helps as well on the track.

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This is helpful. Where do I access that big vacuum line? Is it under my car? I pulled the airbox and I have have much access / don�t know which wire to get.

So by now you�ve figured out that adding oil probably wasn�t a good idea� In any case, it looks like your cylinders are good - cyl 6 misfire probably coil pack or spark plug. First thing I�d do is change out that AOS for a UAOS before driving it hard again�

Turned the engine over manually a couple times, no oil hit the catch can device however the engine turned smoothly. Compression test went well. Passenger bank was a little lower but nothing too far out of the 10% bands cyl 6 - 7 cranks 183 Cyl 5 - 7 cranks 178 cyl 4 - 7 cranks 190 Cyl 3 - 7 cranks 190 cyl 2 - 7 cranks 190 cyl 1 - 7 cranks 200 Cyls that were up top under previous bore scope showed fine. A little oil pooled and some minor wear on the cylinders. One of the cyls had this debris in them, I think it�s carbon. Next steps appears to be UAOS and new spark plugs. Excited to get the old girl back on the road. Let me know if there�s any other tests I should do ahead of this.

UAOS is here and the bitch bolt is almost off. Skip has been very helpful on tips and tricks getting this last one bolt off. Can begin to see the light at the end of the tunnel.

Installed and have driven around 25 miles. I will add some more color tomorrow but Skip was super helpful with some questions I had during the install. When I finished driving today I heard what sounds like a slight whine from the engine. I can upload a video later, but I wonder what can be causing this. I dont think its a vacuum thing as my idle and engine response is great. I will snug up everything tomorrow.

100 miles and things are fairly smooth, no more smoke and smooth running engine. The whine was from the hose coming from the AOS to the UAOS coming loose. It has come loose again and I am wondering if anyone else has had this problem. I may go to Home Depot or harbor freight and get a screw down hose clamp vs the one that comes with the UAOS which is the plier style. Let me know what you guys have experienced. I notice that the hose takes some struggling to sit flush on the back housing.

Cylinder looks fine. That line you are seeing is a casting line. Not a crack. But yeah. Get the UAOS.