Hard Start
I have had slow start problems since I bought this truck. I thought I had it fixed a couple of times. The first time was adding fuel additive, which led me to think I just had dirty injectors. The second I changed the glow plugs and harness, but that only lasted a short time, and I think i might have moved an airbubble and just got lucky on the few good starts it had. Am I on the right track? Would air in the fuel lines cause it to hard start?
Yes. Air in the fuel lines will make it difficult to start and may even cause the engine to die.
Fuel Delivery Problems
To start and run properly, injector timing has to be accurate. A quick visual inspection will tell you if the timing marks are lined up. Refer to the vehicle manufacturer’s timing procedure if you suspect timing is off or the pump has been replaced recently. On newer diesels with electronic injection pumps or direct injection, you’ll need a scan tool to make any changes.
Any air in the fuel can also be a cause of hard starting or a no start condition. Air can make the engine die after it starts, and make restarting difficult. Air can enter the system through any break in the fuel line or via a bleedback condition.
Determine if air is the problem
To determine if air is the problem, install a clear return hose on the return side of the injection pump. Crank the engine and observe the line. Air bubbles in the fuel would tell you air is entering the inlet side of the pump. The injection pump itself is usually not the source of the air leak, so check the fuel lines and pump.
A worn or clogged pump can also make an engine hard to start. If the condition has been getting steadily worse accompanied by a loss of power, and the engine has a lot of miles on it (more than 75,000), the underlying cause may be a pump that needs to be replaced.
Before condemning the pump, though, check the fuel filters. Clogged filters can cause fuel restrictions that prevent the pump from doing its job properly. The primary water separator/fuel filter usually needs to be changed about every 30,000 to 40,000 miles, and the secondary filter about every 20,000 to 30,000 miles. Newer fuel systems with a single filter usually require service about once a year. If the filter has been neglected, chances are it may be restricted or plugged.
Source: http://www.autotap.com/techlibrary/diagnosing_light_duty_diesels.asp