It could be a number of things or a combination of them causing your problem. It could be the lifters are low (out of adjustment) for the injection pumps inside the housing or the pumps themselves are getting worn & also the spray nozzles (injectors in the head) are worn & not giving a good spray at low turning.Also the valves should be set on the diesel & it could be low compression due to worn rings & liners but you said it doesn,t smoke while running so maybe they are not worn.If you have low fuel pressure esp. while turning it over to start maybe the filters need changing or the transfer pump is getting worn & not building up sufficient fuel supply for starting.So these are a few things to check out to find the problem.
DeWets, I never saw any specs on compression ratios & not sure if Cat really listed it but the D4400 did have a much different sound when firing than the D315. Cat never give compression in lbs. but when testing was done they had a method of pumping air in on top the cylinders & then block the supply going to the cylinders & then check for leak down to determine cylinder condition & really could tell you a lot of where low compression was from or if a head gasket was letting compression get into the cooling system or low liner or leaking prechamber & whatever.I made up some of my own testing parts but Cat did have a couple items for doing it.
Get yourself a can of starting fluid & when you,ve turned it over for short time with the compression on give it a short sniff of the spray into the air cleaner inlet & it should pop right off then.Always put the compression release in the run position as soon as possible if you don,t have real low temp. & then heat will build up faster in the cylinders which is necessary for starting.
Yes AJ I forgot to make myself clear & yes leave the throttle off till some heat builds up inside the cylinders before trying to start it.Then is the time if she doesn,t crack off soon then give it that sniff of starting fluid in the air cleaner inlet & she should start then.