A pop up sound, spark plug lost, distributor wires off, while car is running
 

After driving for about 15 minutes in normal operation, I heard a sound as if I drove over a tree branch, and as if it was hitting the bottom of the car, then it slowed down continued to make sound as if the muffler broke off. The tires, and muffler were ok. I looked under the hood and saw that 2 out of four probes like things (wires from the distributor cap, that go into the engine top were popped out from the engine. I waited a while and re-pushed them in. Then started the car, and the same 2 popped out again, although the car started. Towed the car to a mechanic, and he told me that there were only 3 spark plugs- no sign of the forth plug. Havenot replaced the spark plugs, cables, distributor cap etc in 2 plus years. The car is used about 11k per year. Oil change and all repairs are done in a single shop. Car is parked in garage and in secured work site (inside the security gates). Strange that the car only had 3 plugs (not even a broken piece of the 4th plug was discovered). The spark plugs replaced last time were Bosch. What could be the cause of sudden behavior without any warning signs (no lights of any kind on the dashboard)? I am getting new plugs, new wires, distributor kit (genuine Toyota) to see if this helps. I do a religious oil change 3-3.5k miles, transmission check and drain, coolant fill and wash, tires, power steering check and fill. Car has 168k.

The spark plugs actually came out of the engine? Did the spark plug threads in the engine become damaged?

Any chance the porcelain blew out of the spark plug shell? The shell itself may still be in the head. Wouldn't be the first time it's happened.

RonJ

There were no signs of sparkplug remnants
 

The mechanic was able to install 4 new spark plugs without trouble, including the one in inlet missing the 4th spark-plug (No broken pieces found). no issues with threads according to him.
Now- is it a coincidence or what, that every time I buy gas, I have to put gas dryer to make car run in idle without stepping on gas. Mechanic says, it has nothing to do with the timing belt, since it stops after ading the gas dry. I have tried gas from 2 separate gas station in NJ. Could it simply be the cold temperatures (cooler than normal) we have been experiencing in the east his yr? Or, is there something I can do to flush off the engine system in the car?

How cold is it there?

This is not normal for Camry.

Did the mechanic state why the plugs came out? This has happened on rare occasion but not more then one at a time.

There might be water in the gas tank. An internet search will find a lot of hits on what to do about water in the gas.

Prior to the spark plugs popping out had you filled the tank or added a lot of gas dryer?

The torque setting for plugs is quite low to keep from stripping the head threads. It is not uncommon for a plug to work loose and cylinder pressure can blow it right out, sometimes with enough force to dent a hood. I use a torque wrench set to the proper torque and have often found a plug or two has loosened when I remove them to check or replace.

I have no idea why the extra alcohol in "dry gas" (above the 10 % already in pump gas) would make the car idle better.

I wonder how the spark plug got loose and went missing. That happened to me before when I checked on it the distributor wires were disconnected but the plugs were intact.

i have seen plugs with broken porcelin. and stripped out threads in the head. pretty typical failure. consider yourself lucky if the head was not damaged. could be a $$ repair. you pushed the plug wires back onto the plugs and they blew off again? don't quite follow that comment.