Engine RepairDear Ole Mechanic;

When I replaced the air cleaner in my truck, I dropped and couldn’t find a wing-nut.  I had another one that fit, so I didn’t worry about it.  At least I didn’t worry about it until I started the engine after replacing the air cleaner.  At first the engine ran fine, then it started missing, popping in the air cleaner and running rough.  I revved it up and and at first it ran smooth, then it started knocking badly.  Why would changing the air-cleaner cause these problems?  Did losing the wing-nut have something to do with it?

Wondering what Happened
 

Dear What Happened;

It wasn’t the air cleaner that caused the problem--it was the wing-nut!  When you dropped the wing-nut, it went into the worst possible place--the engine air-intake. 

After you started the engine, it took a little while for the nut to vibrate down past the throttle plate and get to the intake-valve.  When the valve opened to let air into a cylinder combustion chamber, the wing-nut got part-way past the valve, and it held the valve open.  With the valve held open, the spark-plug could ignite the fuel and air in the intake manifold.  That caused the engine to miss, because of the lack of compression in that cylinder due to the open valve. The popping noise was caused by the fuel burning in the intake manifold.  Up to that point, very little damage had been done to your engine.  Unfortunately,the wing-nut got past the intake-valve and got between the piston and the head or some other expensive part of the engine.  The knocking indicates that the piston is trying to pound the wing-nut into a place that is smaller than the wing-nut’s original size.  Also, the ongoing knock probably indicates that the steel wing-nut has partially embedded itself into a softer piece of metal such as the aluminum piston and is not going to make its way out of the combustion chamber past the exhaust valve. 

If you are very lucky, the wing-nut will not have hammered the top of the piston near the piston rings,causing them to break or be stuck.  That would lead to severe oil consumption and the piston and rings would need to be replaced.  You will also be very lucky if pounding the wing-nut has not cracked the cylinder head, as that would cause a constant coolant leak, overheating and possible engine-bearing  failure.  If you are really luck, the wing-nut may not have been caught between the piston and a valve, as that can lead to a bent valve that will need to be replaced even if it doesn’t crack the head.

In short, if the engine can be disassembled enough to remove the wing nut and be put back in service with only new gaskets and seals--go buy a lottery ticket.  You will be one very lucky person.

The way my luck usually runs, the valve would be bent, the head cracked and leaking, the piston broken, the cylinder wall scored from the broken piston, the connecting rod bent and the crankshaft broken.  In other words a complete engine-replacement would be needed.

It is time to dig out your tools or visit your favorite mechanic.  You have or are going to learn an expensive lesson.  When anything is dropped anywhere near the intake system, FIND IT before you go any further, even if it means removing additional parts.  Nearly all mechanics have a magnet or pick-up tool to make their lives easier when this happens.

Herr Professor Nuzanbolts