Recently purchased a D4 7U  (7U40968). It seems cylinder 3 isn't firing. When running if I loosen injector line there is no change when running. I swapped injectors 1 & 3 but it stayed the same. I'm wanting to test the compression. But I don't have an adapter to fit a D315. And I can't find anyplace that has a listing for one. If someone could tell me where to find one, or even a part number for one, it would be greatly appreciated. Was thinking about swapping positions with the injector pumps. I'd like to see what the compression looks like first. Someone at some point changed the pump for cylinder 3. It looks a bit different than the others. I'm assuming its an equivalent replacement from different manufacturer or a design change. If anyone has any information that may be of help would be appreciated also. I have extensive experience working on cars not so much with diesels.  

Welcome to the forum: So far you have done what I would have; take the rectangular inspection cover off the side of the pump and check to see if #3 plunger is free and moving up and down while running. It could be stuck and the culprit, rather than swapping things around. Good luck and keep us posted. JM

Juiceman is spot on where to look first before tearing into anything.

JM correct. Also, When you loosened the #3 line did fuel pump out? I assume you bled all four lines.
Do you have an infrared thermometer? Check each cylinder at the exhaust ports for even temperature.
You're thinking about compression test; Do you know if this engine took in any water? Exhaust seats badly rusted. #2 & #3 get it first.

Yes, fuel came out of all four lines. I bled them after removing the injectors. Couldn't really verify anything with the infrared. Hasn't been any water in it that I know of. Guy I got it from had it about a year  He flattened a spot for his swimming pool. Said he got it from an excavation company. Supposedly had engine gone thru not long ago. Does appear heads been off. The paint is different. But it's not real recent. Not that sure about the story either. It's not done much the way it is  Someone tried being a fabricator and failed. The arms connecting the blade to the track frames have been replaced for whatever reason. There are some decent welds and a couple not so good ones too. Main problem was material used and how it was used main piece may work, I hope. The links connecting the pivoting triangular piece (connected to cylinder) to the blade arm wasn't used. Instead they welded a flat piece of thin steel to the arm. Then pin on triangle thru hole in plate. It gets better. But we'll save all that great info  for another day. It doesn't seem to run that bad. From little bit I've run it, it seems to have power. Sometimes it sounds a bit off. When good and warmed up seems pretty good. Doesn't smoke. But when you open

Not sure what happened there. Adding pictures deleted the end. Improvement from the first time when it deleted everything. 🥺
Anyway, was appears to be about the same amount of fuel coming from all four lines.Need fuel filters. Can't believe the price for them. 60 dollars each? So $240.00 plus tax, shipping, and two gaskets. That's crazy. But from what I gather the gauge on the filter housing is pressure after the filter. It doesn't move from zero. I removed it and put pressure to it. Can't say about accuracy, but it does function. Would think that would effect all four cylinders . Also when I drained the oil from the injection pump, it looked a bit milky and smelled of fuel. I removed the valve cover to see all the valves are moving. And the oil running down the block showed me little bubbles coming from head gasket when it's running. Hoping retorquing head bolts will fix that. And maybe some sealer on threads of bolt behind the thermostat housing. It holds the radiator brace to the head. It has coolant seeping from it. Has had head off at some point. The paint doesn't match. Still can't find an adapter for compression gauge that I can verify will work. 

Milky pump housing may be from the hour meter glass cracked or leaking? Could be from years of sitting and not serviced in quite some time. I would keep a lookout for fuel in the pump though after you change it. Not that big a job to replace the seal between fuel filter tower and pump if seal went bad. If you end up removing the head if re torquing doesn't fix the seep; I would definitely do a valve job. May only take regrinding seats and valves. Valve stem guide clearance can be as much as .012" New would be .005"-.007" New aftermarket valves and guides are pretty cheap too.

Cat does not publish compression specs. A leakdown test will tell you what you need to know.

A leak down test would work. I could tell close enough with a regular compression test also. Published specs would be nice. However, if I don't have an adapter to put in the precombustion chamber, I can't do either. I've tried everywhere I could come up with. They all say they have nothing. I did find a cat part number that was supposed to be what I need. But don't have it now and can't find it again. Looked at so many websites I just don't know. If I had a metal lathe ....... 

In theory Cat never discussed compression pressures, but they did at least once, and only because they had some customers ask for the info, and I quote "at cranking speeds (200-350rpm) on a cold engine, expect to see
D339 and D342 445-485 PSI                                    At PSI below 365 PSI expect hard starting
D318 530-580 PSI                                                     At PSI below 430 PSI as above
D311 550-600 PSI                                                     At PSI below 445 PSI as above

Unquote