Compression combustion engines (diesels) do not rely on a fuel air ratio to operate like gasoline engines do. Although it's not the correct terminology, you could say that diesels run "lean" most of the time. Meaning that there is more than enough oxygen present to burn all the fuel that is injected. So to increase power, you simply increase fuel volume. You can do this until you get to the point where you are injecting more fuel than can be burned with the oxygen available. That's when you'd be looking at adding a turbo to force in more oxygen so you can combust a greater volume of fuel, but that's beside the point. Since a naturally aspirated engine like your 7.3 can only draw in a volume of air equal to its displacement, the amount of oxygen available for combustion is more or less fixed. There are some variable like atmospheric pressure, temperature, and compression leaks, but again beside the point. So with that in mind, when you step on the accelerator you are opening a metering valve in the IP and allowing more fuel to be injected. That creates more power and turns the engine faster until the engine speed levels out with the position of the metering valve (throttle) or you're bumping against the governor.
It's a fairly simple principle, but it gets a bit more complicated when you figure timing into the mix. The ip needs to create enough pressure to "pop" the injectors at a specific time (8.5* or 9.5* BTDC +-1.5*). Our idi's, with their precups, are fortunately forgiving with their timing. However, if you get too far out of spec in either direction you can expect similar results. Too far advanced can cause poor fuel vaporization resulting in incomplete combustion (smoke). Early combustion can also cause the engine to "work against itself" robbing power and economy. Too far retarded can cause poor fuel vaporization resulting in incomplete combustion (smoke). Very late injection can also result in there not being enough time left in the combustion stroke for all of the fuel to ignite resulting in a large quantity unburnt diesel being pushed out the exhaust.
Additionally, there is a mechanical "lag" of sorts from when fuel enters the IP to be pressurized, and when an injector's pintel actually gets unseated and allows fuel to be injected into the cylinder. As the volume of fuel being injected and engine speed increase this lag make a more and more significant difference. If this effect wan't compensated for, then as the fuel delivery and engine rpm increased, the timing would retard. To compensate for this lag and keep the engine in proper time as rpm increases, there is an internal mechanism in the IP that advances timing as the engine speed increases. This works as a function of ip housing pressure since the housing pressure rises when the increased engine speed causes the mechanical lift pump and the ip's internal vane pump to move faster and generate more pressure.
What's really important to note is that the injectors need to "pop" at the correct time to get complete and efficient combustion. Stepping on the accelerator causes the timing to become more retarded. The ip has a mechanism to counteract this, but it doesn't kick in until the engine speed actually catches up with accelerator position.
Knowing all of that can let you "read the smoke." Haze, or a small amount light blueish smoke means you are getting combustion, but it isn't as efficient as it should be. Maybe the fuel isn't being properly vaporized during injection, and it's robing some heat from the combustion stroke to fully vaporize the fuel. Dark blue or black smoke means the same thing, but that there is a greater volume of poorly combusted fuel. Either there is more fuel being injected than there is oxygen available to burn, or something with the combustion event (timing) is seriously off, or a combination of both. White or very light gray smoke, especially in larger volumes, means something with the combustion event is bad enough out of whack that all the fuel isn't being vaporized and you're pushing raw unburnt diesel out of the exhaust.
Applying this to your situation:
1. Blue smoke at cold idle (first start)
Pretty normal. These engines have a lot of slop in the rings. You probably aren't building good compression until the engine heats up and the rings expand to seal better. I reckon there is a small chance this could be caused by leak back or air intrusion, but I'd expect the smoke to be white in that case.
1. ... slight blue haze at warm idle intermittently. (Timing?)
This seems like more of an issue. Likely related to timing being too retarded. Over time the injector springs will compress and the ip will wear both causing the timing to become more retarded. I believe it is recommended to check and adjust timing every 30k.
2. Air intrusion/white smoke after long descent or after cresting a long hard pull when I let up on the accelerator.
This one is a little confusing. Seems like something that would normally happen with valve guide / seal issues. I'd just leave it be for now until you can get more info.
It’s when I engage OD and step on the accelerator that the white smoke appears.
This makes a lot more sense. From the above, probably overlong, explanation when you are at a low rpm and step on the accelerator hard your are increasing fueling and retarding timing. Normally it's not an issue but if you are already running too retarded, then this could push it far enough that she'll blow smoke until the rpms catch up with accelerator position. I would imagine there is more smoke if you do this going uphill?
There are simple diagnostic steps you can take with very little effort:
To check if drain back is causing the smoke on cold starts, put the transmission in park and parking brake on. Energize the cold idle advance plunger and make sure the fuel shutoff switch does NOT have power. Pop the hood and jump the started relay and turn the engine over for 15-20 seconds. Normally you would depress the Schroeder valve one the fuel filter head after this to see if it spits out air or diesel, but that won't work for you with the filter being where it is. So I'd just wait a minute and turn it over again. You can do this a couple of times, but be careful not to burn out your starter or drain your batteries. Then get back in the cab and start it like you normally would. If the smoke decreases and/or it runs better, then you have a leak back to tank issue. However, if it's more or less the same, then air likely isn't the problem.
A quick way to check for retarded timing. Once it warms up, i.e. the fast idle and timing advance switch off and the temp gauge is in the normal range (if it works factory gauges are pretty bad), take note of the smoke/haze coming from the exhaust. Then go back to the engine disconnect the fast idle plunger and apply 12v to the timing advance solenoid. If the smoke decreases and or it runs smoother then you likely just have a timing issue.
