Hello everyone, this is my first post on this forum and I have done a lot of searching up until now.

I recently purchased an auto 2008 H3 3.7L with 170xxx kms. It had a P0301 Cyl 1 misfire code which I cleared with a cheap scanner. It hasn't thrown the engine code again but still has a slight misfire.

I have tried the following so far:
Switching coils between cyilnder 1 and 4. Misfire stayed with cyl 1 and didn't move.
Installed all new plugs.
Sprayed power cleaner (similar to brake cleaner) around the intake manifold while running to check for a leak. Nothing found.

We have since used a high quality scanning tool to watch the misfires which are still occurring and it appears to be misfiring mostly at idle and it seems to go away when under load. It also seemed to get worse as the vehicle warmed up. It is getting fuel and the O2 sensors aren't picking up any irregularities with the mixture. My mechanic friend is suggesting our next step is a leakdown test and compression test. He is leaning toward the idea that a valve may be staying slightly open at idle but when under load it is seating properly.

I am wondering if any of you have had a similar issue and found a solution or could offer anymore tips before I get in to pulling the head.

thanks,
Michael