In the last 24 hours, I've had a strange problem with periods of misfire cropping up randomly during driving. It doesn't seem to be associated with any particular load level or RPM - I'll be cruising fine one minute, and then all of a sudden SES flashes, DCS comes on, and the car sounds like it's missing on 1 cylinder. This will persist until the ignition is cycled, and then it's fine again - I can even do it on the fly without stopping the car. As long as the engine goes to 0 rpm before restarting, it seems to fix the problem.

Before I go any further, I will state that yes, I have searched this issue. It seems like it's a fairly common thing with several causes. I'm trying to narrow down what it could be so I don't end up going in a million different directions this weekend trying to troubleshoot it... Now, I've ran only 93 octane (not always "top tier" though) through the engine since I bought the car about 3 weeks ago. It's been 100% fine for about 1k miles up until yesterday evening. The misfire happened once yesterday, and 4 or 5 times this morning (ambient has been down in the upper 30's). I'm thinking that since it always seems to reset itself after cycling the ignition, maybe it's a engine software issue? I would think that if it was a fouled plug, or god forbid a burnt valve, it would be a fairly persistant problem.

The car is an '05 MCS with milltek cat-back, JCW cai, RMW-tune, JCW 380cc injectors, JCW plugs, etc. I'm going to run a code-scanner on it this weekend, and then pull plugs and check compression as needed to see if I can find anything that way. But if it were a software issue, those tests may not tell me anything, right?

 

check the coil. pull the #3 wire and see if the nipple is corroded. also check the others to see if they're dark in color. if they are that signs of the coil intrl shorting. i normally see #1 and #3 nipples dark in color which is not good.also , pull each wire off the plugs and make sure the ends arent white. if they are you know your not connected to well down there.

 

The ECU will deploy countermeasures by shutting down the fuel to the cylinder if it detects misfire, this may explain why it goes away when the key is cycled. The coil is used to fire two plugs at the same time (DIS series circuit) if there is excessive resistance in the coil or connections the companion plug will also be affected. A misfire is defined as the absence of combustion in the cylinder; causes can be mechanical (lack of compression); electrical (lack of spark) of bad air/fuel delivery or ratio. Use a vacuum gauge (17 to 22 in/Hg at idle) to get an initial assessment of the mechanical integrity including timing.
Also check for misfire codes to see if it is cylinder specific or random misfire; makes a difference in determining next steps.