P0306 code turned into almost dead engine

Okay so I need some help here. Let me start off my acknowledging that yes I was a moron and drove on the car with the P0306 code and random misfire and I understand that is why I am writing this today. On March 17th 2018, my 2003 civilian crown vic started shuddering when I accelerated heavily. Didn’t think anything of it until about 5 days later when is threw a CEL. again, the problem was still intermittent when I first got the CEL. On Thursday March 22, I had autozone read the code and was informed that I had a misfire in cylinder 6. They recommended replacing spark plugs and coils. The misfire started getting worse on Saturday the 24th. It became for frequent and more regular. I only drive the car on the weekends and usually about 10 miles so I figured it would be fine until this weekend when I had a chance to replace the coils and spark plugs. Well, last night I heard a hissing sound that stopped and about 5 miles later I started hearing a metallic pinging sound that is 100% consistent. It does not speed up when I accelerate. The car is lagging horribly and it’s shaking so bad it feels like the motor wants to come out of the car. Now back on the 2nd of March I had an oil change done and the serpentine belt and fuel filter was also changed by Ford. What the hell did I do to my car?? If I understand the other postings correctly, a misfire in cylinder 6 doesn’t digress I to complete engine failure in under three weeks averaging 40 MPH over 10 miles? Any help is appreciated.

That hiss followed by a metallic noise and a really rough engine sounds a lot like a spark plug that has backed out of, or blown out of, the cylinder head. That is a known weak spot on these engines. Ford put out TSB 07-21-2 for stripped or missing spark plug threads on 1997 to 2008 Crown Victoria and Grand Marquis with the aluminum-head 4.6L 2V.

To answer your question: no, a plain coil or plug misfire usually does not destroy an engine in three weeks of short weekend drives. The usual cause of a P0306 is just a bad coil or plug on that cylinder, which is what AutoZone told you. But a plug whose threads are letting go will also misfire first, often on and off, before it finally lets go with a hiss. So the misfire may have been the plug working loose all along, not something you caused by driving on it.

How to check it:
- Cylinder 6 is on the driver side, second from the front. The front one, nearest the radiator, is 5.
- Pull the #6 coil. If the coil was sitting high, the plug is loose, or the plug is gone and you can see the hole, that is your answer. A blown-out plug normally ticks faster as the engine speeds up, and you said yours doesn't, so look rather than guess. But this is still the first place to look.
- If the plug is fine, swap the #6 coil with a neighbor and see whether the code moves to that cylinder.

If the threads are gone, Ford's authorized fix is a LOCK-N-STITCH aluminum thread insert. Ford says the insert does not hurt heat transfer from the plug. If the car were still under warranty, Ford's recommendation is a new head.

The March work by Ford is very unlikely to be involved. A fuel filter or belt problem affects the whole engine, not one cylinder. Don't drive it until it's fixed. A dead cylinder sends raw fuel into the exhaust, and the catalytic converters don't like that.

Dana