Gray green substance under oil cap - Cummins ISX15

[Waterloo]

The fuel system adjusted? This ain't an old CAT... Like Rawze said, you are being taken to the cleaners. And do not be offended, hell, many of us have been there, I know I have, and the reason I am here. You are going to have to invest in the tools and learn if you are going to own one of these EPA trucks, there is no way around it if you want to stay in business, or have very deep pockets and like taking it up the rear end at every turn. Don't be that guy.

And that engine knock, you said they put a new compressor on, I wonder if they lined up the timing marks, or like most shops, totally skipped that step and threw the compressor on?

The ISX is notorious for its compressor knock, when it is building air. When it reaches full pressure, the knock goes away. You said the knock is at or near the compressor, I wonder if it was installed correctly? And it seems a bit odd that a compressor would fail with less than 400,000 miles on it... They normally make it to around the 800,000 mark, at least that is when mine bit the dust, of course it was the middle of winter and I froze my rear end off installing it in the driveway.

You need some help here, I would get the truck down to Jerry (Mr Hag), let him explain what is going on, fill in the blanks, give you some honest answers. And you still need the tools and the Inline 6 Reader with the software, I carry mine in the truck at all times. That Inline Reader will pay for itself the first time you need to use it.