I know there's an idling issues post just a few threads down, but mine's different.

Backstory - end of September, I hydrolocked my engine. Replacement finally arrived and was installed a week ago. All sensors from the old engine were transferred over, new spark plugs were installed. Shortly after picking the car up, there was a clanking noise from the bellhousing, which stopped shortly after, and was assumed to be due to the dual-mass flywheel having been disturbed in the swap. It only occurred for about 20min total, and has not recurred since.

Since I got the car back, there has been random "puff" sounds that come from the exhaust. RPMs are not really affected noticeably, it hasn't stalled and only threw a code (P0304 - cylinder #4 misfire) once, and after restarting the code never came back. I did swap out coil #4 with one from my wife's car with no change in symptoms. I chalked the puffing to stale gasoline, and I ran it until the idiot light came on and filled up. The problem remains.

Since I have a pair of g35s, I swapped out the MAF sensor as well, since that was the most likely to have been damaged from the ingestion of water, but it didn't change the puffing.

The oil put in at the change still looks amazingly clean after about 400 miles of driving, and when the car is cruising, I don't think I feel any roughness. Engine temperature has no affect on the idle either.

 

What's the power like? Same as the old motor?

 

It was a new engine, so I was still breaking it in, so no idea on total power, just that it didn't feel particularly weak.

Anyway, I found the culprit. That off-center plug? The ceramic insulator was broken, and if I had bothered to look at it while it was in the angle it was in the car, I would have seen it sliding around. Swapped it out and everything's good now.