I was changing a bad spark plug on my F150 4.6L V8 and despite cleaning the spark plug well as thoroughly as I could muster (rearmost cylinder and under the dash pretty well so very little clearance) I can't guarantee a bit of the residual dirt from the well didn't make it into the cylinder. For a bit more context, getting the new plug lined up took a couple attempts and each time the anti-seize had to be cleaned off and re-applied due to sediment sticking to it, not a whole lot but more than I really wanted to risk with the soft aluminium heads on these engines.

What are the risks of starting the engine at this point if some sediment did make its way into the cylinder. Will it just get blown out pretty well right off or will I be risking damage to the sleeve or valves (or anything else further down the line). Would I be further ahead to remove the spark plug and try to sneak a line down into the cylinder and try to blow anything out (keeping in mind that may introduce new contaminants)?

  • 1
    Always unscrew spark plug half way before bowing out debris from the well with high pressure air, 150 psi or above. It is best to use a blow gun with an extended tip to get down in the well. Also use brake cleaner prior to blowing it out.
    – Moab
    Jul 29, 2016 at 21:50
  • You will loose a tiny amount of power which won't be noticeable on your massive engine. But otherwise, yea this is why we have air filters. Before it goes through an exhaust valve it still will do it's job by leaving scores on cylinder walls and dots on valves and seats... As Moab says, clean it before removing... Jul 29, 2016 at 22:22
  • Good to know, I'll be sure to try to find some better way of cleaning the other wells before doing the rest of the plugs. Unfortunately I don't have access to compressed air outside of 'canned' air which is pretty low pressure (or feels like it anyway) and this plug was the one that needed doing the most (misfiring and causing no end of issues). Jul 29, 2016 at 22:41
  • If you're going to keep doing your own work (a good thing, you should :-) then look into a small compressor. Harbor Freight often has passable compressors for less than $100 (based on the F150 I'm assuming you're in the US).
    – dlu
    Jul 30, 2016 at 4:13
  • In addition to taking @Moab's advice, I'd encourage you to get an inspection mirror so that you can really tell how well you did.
    – dlu
    Jul 30, 2016 at 4:14

5 Answers 5


tl dr; I'm sure minimal damage (if any) would occur from this, but there really is no telling.

A small amount of dirt would usually not cause an issue. It also depends on the size of the dirt. Most "stuff" which would be sitting where you're talking about is probably fairly small. The biggest worry is if it gets stuck at the side of the piston and is forced up and down the side of the cylinder wall. This will cause a wear track in the cylinder and will just continue to damage it until it is dislodged.

More than likely what will happen is, on first start up the debris will be blown right out of the exhaust port. Mind you, it can get caught up in the catalytic converter downstream, but it really shouldn't cause much of an issue there, either. This is mainly due again to the size of the debris. While the honeycomb of the cat is fairly tight, there is some room for passage of very fine debris. More than likely it will just clog a single passage (or maybe two, depending on the size of the debris), but really shouldn't affect efficiency of it much at all.

Your better bet the next time you go to change out spark plugs is to use compressed air and a shop vac before you remove the plug. Use the compressed air to dislodge any foreign matter, and the shop vac with a tight nosed attachment to suck it up as it flies out. This way you won't have to deal with it another time.

  • Not sure if this will help clarify but I did blow out the well and even got a small scraper tool in there to free things up, I just didn't use something with high enough pressure to blow out the stuff that was really stuck to the walls of the well as I don't have access to an air compressor. Jul 29, 2016 at 21:24

I would be more concerned about debris damaging the internal plug threads on an aluminium head.

Aluminium thread is surprisingly easy to damage. A little grit or sand trapped between the plug and threads will lead to a quick realization of why some heads are compared to cheese.


most mechanics in India just don't bother about the debris entering the spark plug well . This is a very important point to be taken care of.

  • I don't believe this answers the question.
    – SteveRacer
    Mar 16, 2018 at 23:14

A good bright flashlight helps a lot along with a straight snug socket to manipulate the plug in with your fngers, my biggest concern after blowing dirt out of the well with carb spray (smaller size cans) is on a old steel head how should you get carbon and rust out of the threads in the heads without getting particles from the threads into the engine

  • Use a tap to re-furbish the threads... Make sure it is not cross-threaded before you start...
    – Solar Mike
    Jul 17, 2019 at 8:40

I would think rust and most top soil like dirt wouldn't pose much of a problem... it might go through a few cycles in the cylinder before it got broken down into small enough pieces to fall down into the crankcase in suspension with all the rest of the gunk in the oil, but it's the sand and rocks that pose problems. The bigger and harder the particle the longer it takes to break down into something small enough to become harmless. It's possible for things to get ejected through the valves but kind of hard until they get light enough to float with air. I'd think they'd get between the gaps in the piston rings before that happened. There's a much clearer path just by taking out the spark plug and fuse to the fuel pump and turning the engine over, although if the sand didn't happen to land on the piston head and clung to the cylinder wall, still might be stuck in the film of oil that coats the wall. In that case I'd try spraying a good amount of fogging oil in the cylinder and then vacuuming it out with a shop vac. Oil should pick up the sand and hopefully get sucked into the vacuum. I'm sure the more times you do it as well the more assured you would be of success. If you don't have any fogging oil just stick the vacuum tube down into the cylinder and then find a narrow air compressor tip that will fit next to the vacuum tube, and blow into the cylinder a few times with the vacuum on. The turbulence from the air should knock anything big enough off the cylinder wall which will then get sucked up into the vacuum.

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