So, I'm picking up dinner at the drive thru, when out of the blue, the car idle drops and the engine starts "loping", and the check engine light comes on. No mechanical knocks or anything. Torque Pro reports it's a P0303 (Misfire on cylinder 3) - between each step, I clear the codes in the ECU.

I swap spark plug wires around, no change - still misfire on cylinder 3.
Listen to the fuel injectors - all are clicking away. (use the screwdriver trick).
Check the compression on the cylinders - All are about 150 (except Cylinder 4, which is 135... well, that's within spec according to the manual, but still concerning).
Replaced all the spark plugs (they've never been replaced, I assume). Runs 100% better. But still lopes, and still reports cylinder 3 misfire.

At this point, I'm not really sure what's wrong. I can't seem to do a spark check, as when I pull all the plugs, not a one shows spark. But obviously cylinder 1,2,4 are firing.

Can a bad coil cause one cylinder to misfire?
Before I pull the fuel injectors, is it possible to test them some other way?
I'm really not sure where to go if it's not one of those two. Anyone else see something like this?

 

Same thing happened to me on the track at Watkins Glen last month. My R53 doesn't specify which cylinder was responsible, but I suspect it was the one that had to have a Helicoil installed after the #3 plug blew out on another track several years ago. Limp mode was not much fun on either track

Fortunately, after clearing the code, the car was back to normal. If you didn't change out the spark plug wires when you put in fresh plugs, that would be my suggestion...

 

Also check if the connections on the coil are still good.
number 3 has a tendency to corrode.

 

The interesting thing about the ignition system on these cars is that there's only 2 coils that fire all 4 plugs. So when cyl 3 is firing, cyl 2 is also firing, a so called "wasted spark."
So, you should be able to connect cyl 3 plug to cyl 2 coil output and connect cyl 2 plug to cyl 3 coil output without any effect on the way the engine runs.

What I'm suggesting is you do this- and if the misfire is now on cyl 2 you've found a fault in the coil.
If the misfire stays on cyl 3 then I think you have bigger problems.

 

Checked the connections - no corrosion, plug wires didn't make a difference - thanks.

I'll try swapping 2 & 3 - thanks @mrbean . if that's not it, I hope it's a clogged fuel injector. Otherwise, it's cam/valves/etc. :(

 

This is the first good news I've had in a while now. The misfire moved to cylinder 2. A new $39 coil arrives tomorrow! Thank you!

 

Nicely done, MRBEAN!!

 

I agree! MRBEAN is awesome. I put the new coil pack on, and it's purring like a kitten!