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  1. #1
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    Help me diagnose really confusing intake backfires

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    2013 A4 allroad, 2.0 TFSI

    See videos:

    Cold start: https://www.youtube.com/watch?v=MCWngzwMGlM

    Warm: https://www.youtube.com/watch?v=HM_RXmoIgGY

    Edit: TLDR: The pops are intake backfires. Sometimes they're strong enough to blow off a loose charge pipe. If I disconnect the #2 coil, the intake backfires go away, and it's just normal dead cylinder symptoms. Tried swapping coils and plugs, compression and leakdown all good, timing chain marks line up. I'm stumped.

    Every few times one of those ticks/jerks happens, it feels like a donkey kicked the car when you're sitting inside. You can see the whole car shaking in the second video. Not every tick is a strong jolt. Overall this is not at all like a normal misfire I'm used to. It almost feels like a cylinder is firing backwards, before it hits TDC. It's not on every revolution. It's only once every second, sometimes no jerks for a few seconds. Car has absolutely no power, can barely move. Putting the car into drive or reverse form park or neutral makes the shaking somewhat worse. At first I thought the tranny was out of oil and something binding inside, but it's very unlikely given that disabling cylinder #2 makes all the jerks go away.

    If I disconnect the coil from the #2 cylinder, the jerks go away. It's then like a normal cylinder being out, like a dead coil. Much smoother and no noise. I'm quite familiar with these normal misfires.

    Shuffling coils and plugs randomly doesn't change that #2 is the bad one. VCDS shows much higher misfire counts for #2, though it shows some for #1 as well. But I think that's just cross-bleed from #2 being so horrible.

    Disabling any other cylinder makes the idle much worse and the jerks remain. So I'm quite sure it's #2.

    Compression test 150 across all 4. Leakdown test 7% across all 4. Bore scoped the cylinders, all fine. Cranking with the plugs out is smooth, no noise.

    So it's not coils or plugs, it's not pistons or valves.

    Injector seems like the only possible culprit. I just replaced all 4 injectors a few weeks ago, because one was stuck open. No indication of a problem since, until my wife completely broke down yesterday going straight, slow, in a residential street. Car could not drive up an easy hill. Injectors are some no-name from O'Reilly, but they just looked like Bosch manufactured injectors, same markings as the factory ones, same manufacturing techniques, Bosch logo, everything.

    I did the VCDS injector test, and I can't tell a material difference in the ticking sound between #2 and the others. Sounds the same.

    I tried disconnecting the helper injector that's in the bottom-middle of the manifold. No change.

    No codes in VCDS in any module other than random misfire and #2 misfire.

    Cranking the engine with the plugs out spews the same amount of gas out the spark plug holes from every cylinder, and in the correct rhythm. Definitely not like the masses of fuel that was coming out when an injector was stuck open. Also no strange smell, like the masses of unburnt fuel when the injector was stuck open. But I can't think of what else it could be other than injector #2. I also don't want to rip everything apart and have it not be the injector, because it's more work than I have time for these days.

    The only other thing is that I forgot to put back the intake butterflies when I replaced the injectors. But I didn't notice any effect for weeks, car drove the same as before, until it broke down yesterday. So I have a hard time believing it's the missing butterflies.

    But the ticks/taps/pops do sound a bit like intake backfires. I don't hear much from the air filter area, but a bit around the intake manifold, and a bit around the turbo. Really hard to tell. The turbo piping muffles everything. I don't hear anything in the exhaust. I do feel the pops though when I squeeze the charge pipe, so I'm quite sure it's an intake backfire.

    Could it be the timing belt skipped? Strange that it seems to only affect #2. But what else could cause intake backfires. My valves seal correctly according to compression and leakdown tests.

    I even disabled the helper injector. I guess unless it's stuck open?? But I don't smell excess fuel, and disabling #2 stops the backfires.

    Disconnecting the cam adjuster connectors makes no difference.

    If I raise RPMs, the kicks get a bit stronger around 2000rpm, and I start getting exhaust backfires, I think. But around 4000rpm the kicks and pops go away, engine feels like a normal misfire, with minor exhaust backfires.

    Sorry for the long description. But I tried everything I could think of, and nothing makes much sense. Hoping some of you have experienced something like this and can point me towards a fix.

    Thanks a lot!
    Last edited by steel_3d; 12-21-2024 at 06:20 PM.

  2. #2
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    A misfire in one cylinder can cross-bleed into other cylinders.

    At face value it sounds like an issue with ignition timing. Have you checked your timing, phaser (vane cell camshaft adjustor), etc?

  3. #3
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    Thanks for your input!
    How do you mean to check the timing?
    Ignition timing?
    Or cam timing marks?
    If it's ignition or cam timing, why only cyl #2?
    How do I check phasers? Car behaves the same with them connected or disconnected. I remember similar behavior when it was running right - disconnected phasers don't affect much. But I guess a broken phaser could cause intake backfires. No idea how to diagnose that, haven't had them in any other cars.
    I wish I had known to check the cam adaptation for timing chain stretch when the engine was running right. Too late now...

  4. #4
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    Sorry I was thinking about the variable lift solenoids. But those are on the exhaust side only, so probably can't cause intake backfires... Found this video that shows how they work. Pretty neat. https://www.youtube.com/watch?v=gWrrS4FoAQI

  5. #5
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    I meant check your phase position at idle with VCDS. Given everything else you stated maybe there's a problem with the valves like the springs broke or the tension isn't right but I am not sure why it would be OK at higher RPMs.

    When you replaced plugs, coil packs, and injectors did you use all parts from Audi?

  6. #6
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    "I meant check your phase position at idle with VCDS" Which value do you mean and what's the expected value? Do you mean "Camshaft adaptation intake"? Doesn't the engine have to run properly, warmed up, to get a valid value on that?

    "maybe there's a problem with the valves like the springs broke or the tension isn't right " If a spring was broken, the valve would've dropped and it would be very obvious. Tension - not sure how, but maybe. The fact that compression numbers are the same on all cylinders seems to rule out anything being broken in the valvetrain on #2.

    "I am not sure why it would be OK at higher RPMs." it's not really OK, there's still the misfire, and exhaust backfire, but the intake backfire seems to clear up.

    "When you replaced plugs, coil packs, and injectors did you use all parts from Audi?" No. But given that I've swapped plugs and coils all around with no change on #2 seems to say it's not plugs or coils. The injector is a possibility, but not sure how that could cause intake backfires. It felt very different when an injector was stuck open. Now I have the same amount of gas spewing out from each cylinder when cranking with the plugs out, and no smell of unburnt fuel.

    I'm sort of suspecting timing chain skipped. Can that cause such symptoms? Can I just take the upper timing cover off, get the crank to TDC and check the timing marks on the cam gears? Like 62mm from the benchmark to the intake gear mark, and 125mm between the marks on the intake and exhaust gears?

  7. #7
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    Yes if the chain skipped it's possible that's the cause of the problem. If you suspect that could be the case I would check it to verify.

  8. #8
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    I'll check timing marks. Can you elaborate on this? "I meant check your phase position at idle with VCDS"

    Really appreciate all your input!
    I'm surprised no one else has opinions on this :) Is this the wrong forum?

  9. #9
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    I am talking about the values that everyone is posting in this thread:
    https://www.audizine.com/forum/showt...ser-adaptation


    I am assuming you are in the correct forum but it doesn't look like you posted your year and engine model...

  10. #10
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    Good point. I updated the original post. 2013 A4 allroad, 2.0 TFSI

    I assume the adaptation can't settle on anything when the engine is running this bad, but I'll check. I wish I had known to check before things went south.

  11. #11
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    I think you should still be able to check even with the engine running poorly but I have never tried it to be sure.

    For that engine and model, yes you are in the correct forum.

  12. #12
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    So my cam adaptation is 4.5.

    Idle: https://drive.google.com/file/d/18h7...ew?usp=sharing
    2000rpm: https://drive.google.com/file/d/11Pw...ew?usp=sharing

    Does anything look out of the ordinary? Yeah my chain is not the freshest, but nothing there seems like the end of the world.

    My timing marks line up correctly at TDC: 62mm to the intake mark, and 125mm between the intake and exhaust. So it's not timing chain.

    The chain was a little loose, though, as it sat from when the engine was shut off: https://youtu.be/3I4x4cZivCY
    But as soon as I rotated the crank, it got pretty tight, and I could never get it into such a loose state again. The chain is pretty tight everywhere. So I'm assuming I don't have a failed tensioner.

    So I'm stumped again.

    It's 100% confirmed that it's intake backfire. I've had it blow off loose charge pipe. It even blew out the actuator linkage for the intake butterflies. But I was able to push it back into place and got rid of the vacuum leak. The blowout happened only today, and fixing the vacuum leak (confirmed with smoke tester) didn't fix the misfire.

    Not sure if I got it wrong last time, or it changed, but the backfire doesn't go away at high rpm. In fact it intensifies. That's how I blew out my butterfly actuator.

    My timing advance is -10 at idle, and +30 if I raise the rpms. But it misfires either way, so it doesn't seem to be the advance/retard.

    So what could it be?

    Anything else I can check before I replace the #2 injector? I just don't see how an injector could be causing intake backfires, especially since it's not stuck open and seems to be behaving like the others.

  13. #13
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    So your timing chain is stretched but not the cause of your problems. If you take the engine apart to fix the issue I would do the timing chain.

    Strange problem, I don't really know. If it was my car I would be sure the plugs, coils, and injectors were OE Audi parts to rule out any issues. The plugs don't have to be per se but you would need to ensure they are compatible and gapped properly, using OE Audi plugs are cheap and would rule out any issues there.

    Sounds like you fixed the vacuum leak but a bad leak will make the engine run crappy. After my rebuild I had a vacuum leak and the engine was running terribly, freaked me out, but the gasket on the intake manifold wasn't seated correctly. After fixing that it ran perfect.

    This is a tough one...

  14. #14
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    Thanks, man. I've never had a car problem I couldn't diagnose in my life, in 20+ years. And I've dealt with plenty :D This one is weird.

    The plugs are bosch iridium, the correct ones, correct gaps. A couple months old, replaced at the same time as injectors. All look good. Coils are NGK, replaced at the same time. The fact that it doesn't matter which coil or plug I shuffle to it, #2 is always backfiring, and if I disconnect the coil the backfire stops, I can't imagine how it could be coils or plugs. Or how those could even cause intake backfires.

    For now I'll replace the injector, since that's the only other replaceable thing. After that, it must be something in the valvetrain. But very strange that valvetrain issues wouldn't show anything on compression or leakdown tests.

  15. #15
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    So weird. It is almost like the intake valve is sticking open but you say compression is fine, really bizarre.

  16. #16
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    I agree. I don't see how intake backfires are possible if the intake valve is not open at the wrong time, but then compression should be horrible.

  17. #17
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    So I took the intake off and found the #2 butterfly valve stuck in the #1 intake port.


    Note how much more soot there is on #2, and hot it's much more dry than the other cylinders, from so much combustion happening in the intake port...



    Seems like this did matter, after all:

    "The only other thing is that I forgot to put back the intake butterflies when I replaced the injectors. But I didn't notice any effect for weeks, car drove the same as before, until it broke down yesterday. So I have a hard time believing it's the missing butterflies."

    By butterflies I meant the fixed vanes that go in the intake ports in the head:



    Maybe not having the vanes in there left the butterflies more exposed and prone to breakage?

    Now, I'm not sure how the broken butterfly got from #2 to #1. Either a backfire blew it into the intake and #1 sucked it in, or it just fell in there as I was taking the intake off. But the angles make the latter pretty unlikely.

    The other mystery is whether the broken butterfly caused the backfires, or the backfires broke the butterfly. I will verify this by putting it together with a fresh intake, and see if the backfires are gone, which would confirm the broken butterfly caused the backfires.

    But I can't quite see how a missing butterfly could cause intake backfires.

    Given that the butterfly shaft was broken and the angle sensor at the end was not turning (along with the #1 butterfly), I would've expected some trouble codes if the butterfly broke a long time ago. So it must have broken right before or after the breakdown.

  18. #18
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    Those intake runner static dividers are meant to optimize and direct airflow. I think you have found your problem for sure. Any problem with air delivery and AFR will cause the engine to run like complete crap. Good work...

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    I replaced the intake with a brand new one, I swapped the #3 injector to #2 and put a brand new injector into #3.
    No change. #2 still gives intake backfires unless I disconnect the coil.
    Compression is still exactly the same in all cylinders.
    This makes no sense.
    I have no idea what I can even take apart anymore or try.
    Maybe take off the valve cover? But I can't imagine I'd be able to see anything interesting given that compression and leakdown are perfect.
    Any ideas?

  20. #20
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    Quote Originally Posted by steel_3d View Post
    I replaced the intake with a brand new one, I swapped the #3 injector to #2 and put a brand new injector into #3.
    No change. #2 still gives intake backfires unless I disconnect the coil.
    Compression is still exactly the same in all cylinders.
    This makes no sense.
    I have no idea what I can even take apart anymore or try.
    Maybe take off the valve cover? But I can't imagine I'd be able to see anything interesting given that compression and leakdown are perfect.
    Any ideas?
    Gotta be your ecm i think. We take those butterfly valve things out when installing ie manifold, they don't matter. Or something is wrong with your intake camshaft or intake valves at #2 however i feel it would show up on a compression test which makes me lean towards fluky ecm or wiring harness that is causing it to ignite the coil at the wrong time. What you could do is pull all coils and either put plugs in or an ignition tester and watch while cranking car, the spark should be consistently 1/3/4/2 with every revolution. If number 2 is sometimes sparking the same time as another cylinder then we got a problem

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    All good. All the plugs fire in turn, #2 fires on beat when it's supposed to, as well. The same amount of fuel comes out of the spark plug holes on beat. All cylinders act the same.
    I'm totally stumped.

  22. #22
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    Quote Originally Posted by steel_3d View Post
    All good. All the plugs fire in turn, #2 fires on beat when it's supposed to, as well. The same amount of fuel comes out of the spark plug holes on beat. All cylinders act the same.
    I'm totally stumped.
    Then id look at the intake cam, followers and valves on number 2, seems it must be occasionally not closing all the way

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  23. #23
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    Yeah, I can't imagine how it could only occasionally not close, but I'm suspecting something like that as well. So I took off the intake for like the third time and watched the valves while cranking. Nothing out of the ordinary on #2. See video:

    https://youtu.be/G_zIrkHjju8

    I did a leakdown test with the intake off, the valves are really perfectly sealed. They open and close at the right time, like the other cylinders, and behave the same under compressed air.

    The only thing I noticed, is that the left side #2 intake valve area is more wet than the right. The right is covered in soot, like the rest of the #2 intake runner and ports. The soot is from backfires, but no idea where the wetness is coming from. But it seems to indicate that the left side port is behaving differently from the right. But no idea how, or which one is correct. See video, in case it gives you a clue:

    https://youtu.be/cgUJfrTxu1g

    If you guys can't think oif anything, I guess I gotta take off the valve cover and hope I notice something visually. Then throw this car in the garbage.

    You guys don't think the PCV could cause backfires, right?

  24. #24
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    Could blocked oil passages or over/under extended lifters cause backfires? How could I diagnose such a thing? Visually the valves behave correctly while cranking.

    Also, should the intake valves be open at BDC? I need to rotate the crank past BDC against the compressed air pressure to close the valves when doing a leakdown test (I know it should be done at TDC, and that's all good, I just wanted to play with BDC as well to get a sense of how the valves behave). And if I let go of the wrench, the pressure pushes the cylinder back towards BDC and opens the valves. Is this normally what you guys see?

    I guess if the valves close too late after BDC, fuel laden air can be pushed back into the intake on the compression stroke. If the intake cam/adjuster is way too retarded, this might happen? Can the vvt fail in such a way? It doesn't feel like the valves are open THAT late after BDC, and I wouldn't expect them to be closed right at BDC, but I thought I'd ask to be sure.

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    This is what chatgpt thinks. Nothing really sticks out as easy to test. Other than just looking under the valve cover.

    ------

    Key Observations and Theories
    Wetness on the Left Intake Valve:

    The wetness could be unburned fuel pooling due to incomplete combustion caused by a stuck-open or poorly sealing intake valve.
    Alternatively, it could indicate fuel wash from an injector spraying fuel that doesn’t combust properly because of the valve’s behavior.
    Soot on the Right Intake Valve:

    The soot suggests combustion gases are escaping into the intake manifold through the right-side valve (or during a backfire), leaving residue.
    Random Backfires on Cylinder #2:

    If the left valve isn’t sealing properly, it could disrupt the intake airflow dynamics and lead to a backfire every few revolutions.
    Intake Valve Stuck Open During Operation:

    Even though the valves appear to move properly during cranking, dynamic operation at speed may reveal issues that aren't visible when turning the engine over slowly.
    Likely Causes
    Sticking or Malfunctioning Intake Valve on the Left Side of Cylinder #2

    A carbon deposit or mechanical obstruction could be preventing the valve from sealing properly during operation.
    Weak valve spring tension or a broken valve spring may allow the valve to float or stick open dynamically.
    A valve seat issue, such as pitting or damage, may cause intermittent sealing failure.
    Fuel Injector Spray Pattern Issue

    While you’ve swapped injectors, the wetness could be caused by a spray pattern problem or over-fueling for cylinder #2 specifically.
    Verify the injector signal and inspect the spray pattern (off the engine) to ensure even atomization.
    Camshaft Lobe or Hydraulic Lifter Issue

    A worn camshaft lobe for the left intake valve could reduce lift or cause erratic operation under dynamic conditions.
    A failing hydraulic lifter may not provide sufficient pressure to keep the valve train functioning correctly.
    Recommended Next Steps
    1. Inspect the Intake Valve Closely
    Use a borescope to closely inspect the left intake valve and seat for:
    Carbon deposits or debris preventing proper sealing.
    Damage or irregular wear on the valve face or seat.
    Manually push the valve (if possible) to confirm it isn’t sticking or catching.
    2. Perform a Dynamic Test on the Valve Train
    Check the valve spring tension and ensure the left-side spring for cylinder #2 is not broken or weakened.
    Inspect the camshaft lobe and lifter for cylinder #2, focusing on the left intake valve.
    3. Check Injector Spray Pattern and Signal
    Remove the injector for cylinder #2 and test its spray pattern (most shops can do this).
    Use an oscilloscope to ensure the injector is receiving proper signals and isn’t over-fueling.
    4. Check for Exhaust Valve Overlap or Pressure Issues
    If the exhaust valve isn’t fully evacuating combustion gases, it could disrupt airflow through the intake manifold.
    Inspect the exhaust valve and port for damage or obstructions.
    5. Test Cylinder #2 Under Dynamic Load
    Use a cylinder pressure transducer or oscilloscope to monitor real-time cylinder pressure during operation. This can reveal subtle timing or sealing issues that don’t show up in static tests.
    6. Recheck Cam Timing Under Tension
    While the timing marks are aligned, there could still be slight chain slack or tensioner failure, causing timing irregularities that only manifest at certain RPMs.
    Hypothesis Testing
    Based on the evidence, the most likely culprit is an issue with the left-side intake valve (sticking, sealing, or dynamic operation failure). The wetness and backfires point toward improper valve function as the root cause.

    If the valve, injector, and camshaft all check out, the issue may involve less common scenarios, such as:

    A cracked intake valve guide allowing unmetered air or disrupting the valve’s motion.
    A micro-crack in the cylinder head around the intake port for cylinder #2.
    Final Thoughts
    Focus on a dynamic inspection of the valve train and injector for cylinder #2. The wetness and intake backfires are strong indicators of a valve sealing or airflow issue that may not manifest in static tests.

  26. #26
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    didnt read the whole thing ,,,, so you can discard this if it has been discussed ..
    intake back fire on one cylinder ? did you check for a broken valve spring ?
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  27. #27
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    Didn't look at the valve springs directly yet.
    The valves move correctly while cranking, so not fully broken, but I guess that can change while running.
    Sounds like it's worth it for me to take the valve cover off at this point.

  28. #28
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    Also, I keep looking at the intake side, but maybe come blockage of the exhaust side, like exhaust valves not opening correctly, could also cause intake backfires? Has anyone seen that happen?

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    I can't figure it out, guys. Do you see anything wrong?



    So at least my last hunch was right. If the exhaust valves don't open, you will get intake backfires.
    I guess I just didn't think the compression test would look normal if the exhaust valves are stuck closed, but it makes sense. Not like you can build up any more pressure, since it all leaks out as soon as the intake valves open.

    So how the hell can rockers fall off?
    Well, if the tabs keeping them on the valve stem break off:



    But how could those break off? Both at the same time:



    I guess if the solenoid shifting the lobes is worn down, and it doesn't shift the lobes over fully, and the rollers fall off the edge of the cam lobe, or something else funky happens due to the lobes not being shifted over properly.

    I don't know why the solenoid stem got worn down like that. I don't see anything unusual with the track on the cam. Maybe it got nicked, and once it's nicked, it just gets worse and worse with each shift, over hundreds of shifts. Let me know if you guys have seen this before and know the reason.

    Out of curiosity, does anyone know why these engines default to the high lift lobes when off? I assume it should be at low lift most of the time, ie: at idle, ie: after startup and before shutdown. So why is it on high when off?

    Anyways, I think I can replace the old tensioner with a new style one without taking the bumper off. Take out the exhaust cam, replace the rockers, the solenoid, and put everything back together. If I had the new style tensioner already, I could just pin it and not have to take the lower timing belt cover off. As long as the timing marks line up at the end, it should be all good.

    Thoughts?

  30. #30
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    Glad to see you got it figured out! Don't think I've seen that happen before, definitely strange

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    Wow I don't think I've ever seen that before.

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