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I own a 2004 Volvo XC90 T6 2.9L AWD with 6 cyl. (Twin turbo, with catalytic converter.)

While changing the spark plugs, one broke-off (all the ceramic below the threads and possibly the metal finger of the plug - I'm not sure) and fell through the valve, after bending it and causing the valve to burn (possibly). The cost to replace the engine or valve is huge! So I intend to drive the SUV back home (about 20 miles) and then replace the plugs with iridium NGK and drive it. I am hoping the spark plug piece that broke off will melt in the piston and not destroy the piston.

What do you think?

Can I drive the car or will it destroy the engine?

The spark plugs were less than 2 years old.

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    Do not start the engine. Bore-scope first.
    – Arturs Bolsunovskis
    Commented Jul 19, 2016 at 16:43
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    If your spark plug broke, what makes you think the cylinder will get hot enough for anything to melt? Did you replace the plug?
    – TMN
    Commented Jul 19, 2016 at 18:26
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    I am curious how the plug broke and how it fell through the valve, bent it, and why the OP thinks the valve is burnt. The T6 petrol engine sits 'upright' w/ cams at the top, not sure how it would have defied gravity, unless it was running when the plug fractured. If everything described is true, it ingested the plug, a rebuild is a certainty.
    – RomaH
    Commented Jul 19, 2016 at 19:10
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    NO! At the very least tow the car home!
    – GreatAndPowerfulOz
    Commented Jul 19, 2016 at 22:10
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    Just one "engineering" comment on the "DON'T DO THIS" answers: the acceleration of the pistons when the engine is running is 1000 (or more) times the acceleration due to gravity. There is ZERO chance that debris in the cylinder will "sit on top of the piston". The piston will try to "hit it out of the ball park" a few thousand times a minute, until it gets trapped somewhere out of range. If it gets into an inlet or outlet port, the valve will probably break when it hits it. If it gets trapped between the piston and the cylinder bore, you may be looking at a new block, not just a new head.
    – alephzero
    Commented Jul 21, 2016 at 23:45

11 Answers 11

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That sounds very risky to me. The portions of the plugs that reside in the combustion chamber are designed to tolerate the heat and pressure there. I don't think they will melt.

So what will happen is that those parts are likely to remain in the cylinder and may get caught between the top of the piston and the head and/or valves. That is likely to do more damage. If the engine is turbocharged any pieces that do manage to escape the cylinder will pass through the turbocharger turbine and may do significant damage there as well.

I think a better move would be to have the car towed home and then use either compressed air or a vacuum to pull out the spark plug pieces. Then do a compression check to see if there is any evidence of damage. If you can get access to one, I'd be inclined to use a fiber optic inspection camera to verify that everything was out.

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    I don't think it's risky, I think it's absolutely dead certain it will cause damage somewhere. Those bits won't melt, and there is not enough clearance for those bits to pass out safely, they WILL damage some or all of the engine, turbos, and cats on their way.
    – John U
    Commented Jul 21, 2016 at 17:07
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Do NOT drive a car with a piece of sparkplug inside the engine. It can (and likely will) lead to serious damage (up to a catastrophic engine failure). Get your car towed to a service and have the pieces of the broken plug removed. Additionally, get the oil changed (probably with a flush too, to remove all the debris). You will have to spend some money on that, but that'll be definitely way cheaper than replacing the whole engine.

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    My wife just tested this - is was raining too hard to stop & wait for assistance, so that's just been a whole new engine - the plug remnants went through the cylinder head & the debris wrecked the bore.. I doubt it would have helped to have stopped though, probably too late when the lights came on.
    – Mark Williams
    Commented Jul 19, 2016 at 9:19
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That's a nice car - don't be a consumer and break it because you're too lazy to fix it properly.

Sadly the "proper" fix is to tow it to your mechanic, who will take the head off and fish out the broken pieces. He'll also check the head and valves for damage while its off, and make a recommendation.

An old engine idling at low speed might happily burp out some fragments into the exhaust, but your car has a catalytic converter and possibly a turbo, and at least one muffler. There's no clear hole out of the system.

The pieces will not melt - ceramic and spark tip are in the combustion chamber for every explosion, so they're built for these conditions.

Also - you should change all the spark plugs even if the others look OK.

Spark plugs can break because they're in too tight, manufacturing fault, or the lead is too long and flapping around which stresses the plug. Accidents with tool also damage the ceramics, starting a crack that turns to a full fracture later.

Given its an AWD you're going to either have to tow it in neutral (if its a manual) or put the whole thing on a car trailer or a flat deck tow truck. It'll be okay to winch onto a trailer or truck, but only short distances.

Remember a repair is cheaper than a replacement car - its not a disposable cellphone.

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    Respectfully disagree with the emotion. When a vehicle fails in a way that surpasses the repair capabilities of an amateur mechanic, bringing a professional into the equation is apt, not sad.
    – Mathieu K.
    Commented Jul 19, 2016 at 22:17
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    By the time the mechanic has taken the head off, you're probably looking at a bill that rivals the cost of a new engine anyway. So I don't think this is very good advice. It would be much better to find a mechanic who knows how to use an endoscope to examine the situation with at most an hour or so labor in order to determine whether you actually need to spend thousands of dollars.
    – R.. GitHub STOP HELPING ICE
    Commented Jul 19, 2016 at 23:17
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    @r. A new engine will be $several-thousands, plus the costs of getting it fitted. A mechanic who charges that much is over-charging. You're right in that investigation is worth trying - a vehicle is not disposable.
    – Criggie
    Commented Jul 19, 2016 at 23:52
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    @Criggie: I've seen head gasket jobs cited as starting around $1200-1500, and I can't imagine this would be any less expensive since it involves the same disassembly, and since you'd have to replace the gaskets and head bolts and all just like if you were doing a hg job for its own sake. So there may still be over a $1000 difference between this and replacing the engine, but it's still on the same order of magnitude and crazy to spend that much before you even know if it's needed.
    – R.. GitHub STOP HELPING ICE
    Commented Jul 19, 2016 at 23:59
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    @Criggie "A mechanic who charges that much is over-charging" - a bit redundant wouldn't you say? ;)
    – Darren Ringer
    Commented Jul 21, 2016 at 13:10
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If this happened during a plug change

If you have roadside assistance/towing, I would highly recommend getting it towed home, then purchase a cheap USB inspection camera or a inspection unit like this one from Harbor Freight and look inside the spark plug hole to see if there is

  1. any remnants of the plug and
  2. any trauma to the exhaust value.

If it happened while driving

It has mostly likely

  1. done all the damage it will ever do, or
  2. simply went out the exhaust if the wastegate wasn't directing exhaust to the turbocharger.
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No question - running the engine will break it.

If you're "unlucky", there's enough clearance between the piston and the valves to fit the spark plug. In that case you'll have the spark plug rattling around inside the cylinder. In approximate order, this will first destroy the valves which will not seal properly when their edges and seat faces get damaged, then the cylinder walls, then the piston rings as they're damaged by the gouged-out cylinder walls. The cylinder head and piston will also be damaged beyond repair, but they probably will not immediately cause the engine to fail. After that, the crushed-up ceramic will run through the oil and scratch up everything that gets oil and needs to be kept smooth and clean. The engine may run for a short period, but it will be permanently damaged. After you've turned the engine on, your next step is to book your garage to fit a complete new engine. Total cost in the UK is probably £2000 for a recon engine and the time.

If you're "lucky", the rising piston and descending valves will hit the spark plug. This will immediately destroy the valves and seize the engine. The reason you're lucky here is that this is repairable because it happens immediately. There's some risk to the piston and cylinder head, but you stand a fair chance of getting away without damaging them. Total cost is probably around £500-1000 for taking the head off and fitting new valves.

I've no idea what you're talking about when you say it "fell through the valve". The spark plug doesn't go anywhere that it can fall through a valve, because that's not how an engine works. The valves cover one set of holes into the cylinder (carrying air and fuel), and the spark plugs go in completely separate holes (which are sealed by the plugs). I don't mean this to sound insulting, but I'm not sure you know enough about engines to make an informed assessment of what's gone wrong and how to fix it.

If you have breakdown/recovery cover, call them and get them to trailer the car to a garage. If you don't, call round the local garages to find one who'll pick it up. You might not need a new engine, but you certainly do need them to take the cylinder head off to retrieve the broken spark plug. I'd expect to pay around £500 for that.

A garage should easily do it in a day. It's generally a whole-weekend job for a very competent amateur mechanic with a good set of tools and plenty of experience. (That's how long it took me when I had to change a cylinder head, anyway.) Actually taking the head off is really the easy bit - what takes most of the time is disconnecting everything else around it. This does need skill and care though, otherwise you can permanently wreck the engine. Under the circumstances, I would not recommend you attempt this, unless you intend to scrap the car anyway and you fancy having a try to develop your mechanic skills.

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I've seen this before on a Zetec Ford Focus Estate. The noise of the engine running with a missing plug made it sound like a lawn mower. Whilst the car could theoretically have been driven a very short distance (i.e. to move it out of danger, not 20+ miles) we decided that it wasn't worth the risk to try and move it under it's own power and towed it with another vehicle.

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Many people are not reading the question. This plug didn't fail on the compression stroke, it failed when the user broke it while trying to remove it. I'm not sure what "fell through the valve, after bending it and causing the valve to burn (possibly)" means. The engine wasn't running. How would anything bend or burn?

STOP. Do not run the engine. The cost of a tow will be a lot less than that of the possible damage. So far, all you have to do is fish the broken remains of a spark plug out. Keep it that way.

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It's very likely that the engine is already severly damaged. A twenty mile trip would most possible kill it. The best you can do is to get your car towed to a service

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A small piece of ceramic from a brand new iridium plug broke off as I was trying to get plug socket off. I didn't realize it but had to take plug out to retrieve socket & that's when pieces fell into valve. I took my dirt devil with hose attatchment & began making smaller & smaller attatchments to eventually get small enough to fit through hole. Last tube was clear with a ball point pen piece at the end so I watched the pieces go through into vacuum. I then emptied vacuum bag with the pieces & matched their size with what had broken off.

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I had 1987 Toyota Celica which at about 5 years old (just out of warranty), suffered a nasty engine malfunction.

A screw holding an intake butterfly onto its shaft (one per intake port) came loose and fell into the intake, along with butterfly itself. This, of course, happened while I was driving on a highway. All I knew is that the engine suddenly started making very loud unhappy noises. I exited the highway as quickly as I could, found a location with a phone (I didn't have a cellphone) and shut the car off.

Once I got it to the dealership, the mechanic knew the exact problem without even looking at the car (it was a known problem with that engine), but I asked him to open it up anyway. He recovered bits of brass from inside the affected cylinder which were the remnants of the butterfly, and showed me the bent valve (occurred as it chewed on the butterfly), and the affected piston and cylinder head (both had numerous dents due to the piston crunching the bits against the head).

The engine was repaired (very expensive - about the same price as replacing with a rebuilt), and was never quite the same again.

Bottom line: if any debris falls into the cylinder, it needs to be properly cleaned out to avoid expensive engine damage.

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At the very least drain the oil, but I suggest that you take the oil pan off, clean it thoroughly, fill it up again, run for a day or so, and repeat.

The piece that broke my engine probably went through the oil channels 30 to 100 times before it hit the right spot.

I speak based on experience. I made the mistake of running an engine after this happened to me. Ford V8, 428cid. The result of this mistake was a broken crankshaft about a month later.

My best guess is that a piece of the ceramic spark plug was caught by the spinning crankshaft in the oil passage in the crankshaft bearing. The cranked stopped, and the mass snapped the crank. I was in second gear (automatic), going about 25mph, lightly on the gas.

I can not prove that the spark plug broke my crank, but I am pretty sure that is what happened.

BTW - the heat in the combustion chamber will not melt or in any way damage the broken bits of ceramic insulator. That is the whole point of using ceramic for the spark plug insulators.

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    I can not imagine anything going from the combustion chamber to the crankcase; unless it's a two-stroke engine.
    – Lenne
    Commented Jul 19, 2016 at 5:36
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    The only way from the cylinder to the sump would be down the sides of the piston, around the rings. Any broken pieces going down there are going to scratch the bore, allowing a path for compression loss. If any did make it down this way, would more likely be sandy powder, which could have totally done a number on your bearings but should also have been caught by the oil filter.
    – Criggie
    Commented Jul 19, 2016 at 8:35
  • The piston is not 100% sealed and small particles can pass through the oil film on the wall. It's a 12 years old engine, some ammount of wall is already worn out. I would perform general repair of the engine - the combustion chamber needs cleaning, so do the piston rings and whole oil system. Call it overkill, but better safe, than sorry.
    – Crowley
    Commented Jul 19, 2016 at 17:20

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