Changed out fuel filter and now tractor won't start

[Jerry/MT]

Ok..now that OP has tractor running and I won’t hijack thread…

Mechanical system: I understand air will compress, but if the pump is pumping fuel behind it, so what? Won’t the air eventually compress to the fuel pressure the pump is supplying and push air through the injector?
Or would this just take too much time cranking given the relative volume of the fuel lines to the amount of fuel injected?
Or does the fuel line “decompress” back into the pump between injection cycles, thus pressure never “builds” to pop injector.

I know lines from pump to injectors have to be bled of air, I just don’t know why.

Here's my theory:

A liquid, like water is virtually incompressible while air is very compressible. (The bulk modulus of water, dp/dV, is 15,000 times greater that air)
I presume diesel fuel compressibility is on the same order of magnitude as water. So as the pump attempts to pressurize a fuel line, it has virtually no resistance, so it builds no pressure in the line. The air acts like a soft spring and as the pump attempts to push the fuel into the line the air compresses to increase the available volume in the line and the pressure does not increase. Since the injectors need a certain pressure (pop off pressure) to deliver fuel, fuel release doesn't happen. The internal volume of an injector is very small so it can't hold much air so we get the air out of all the fuel line by cracking the injector fuel connection and releasing the air. Now the pump can build pressure to a very high level and when the pop pressure is reached, the last little bit of air is released from the injector body with the fuel and the engine can begin to run.
If the injectors are a bit leaky than you'd expect that you might eventually force enough fuel into the lines to push the air out. I have never started a tractor with air in the lines by pulling it but I would think that you can get higher rpms with the pump and that combined with leaky injectors would also help prime the system.
Does that make sense?

 

[fruitcakesa]

I’d give it some time, still might have air in the system.

I believe that will be the case since I went out to start it after parking it a few hours earlier and it cranked a number of times before catching and stutteringly [sp?] come up to full song.
Tomorrow will be the day to get it sorted before the supposed big storm hits us.

 

[gstrom99]

I believe that will be the case since I went out to start it after parking it a few hours earlier and it cranked a number of times before catching and stutteringly [sp?] come up to full song.
Tomorrow will be the day to get it sorted before the supposed big storm hits us.

Yippee!

 

[CobyRupert]

Here's my theory:

A liquid, like water is virtually incompressible while air is very compressible. (The bulk modulus of water, dp/dV, is 15,000 times greater that air)
I presume diesel fuel compressibility is on the same order of magnitude as water. So as the pump attempts to pressurize a fuel line, it has virtually no resistance, so it builds no pressure in the line. The air acts like a soft spring and as the pump attempts to push the fuel into the line the air compresses to increase the available volume in the line and the pressure does not increase. Since the injectors need a certain pressure (pop off pressure) to deliver fuel, fuel release doesn't happen….

“So as the pump attempts to pressurize a fuel line, it has virtually no resistance, so it builds no pressure in the line.” - I think it would only have no resistance if the end of the fuel line was open. If there was no blockage / injector.
Think of an empty garden hose attached to a 60psi faucet and with a closed spray nozzle at the other end. When you turn the faucet on, all the garden hose will be at 60psi, even if very little water entered it. The amount of water to enter was the same volume that the compressed air lost when going from 0-60psi. The 60psi water will push the air through the spray nozzle if you open it (or if spray nozzle automatically pops off at 58psi). See what I’m getting at?
Now let’s say you only open the faucet for 1/2 a second at a time. It may take a few of these cycles for enough water to enter and build pressure to 58psi before nozzle starts passing the air then water.
However, only if the line decompresses between these open cycles (using the compressed air to push back ) would you get a situation where the pressure never builds over time to 58 psi.

Thus I believe you have to crack at injectors because:
Otherwise it takes too long cranking to bleed air.
Or compressed air in fuel line decompresses itself by pushing back fuel into the pump when that cylinder is “off cycle”.
Or, like I think Lou suggested, a combination of the two.

 

[Jerry/MT]

“So as the pump attempts to pressurize a fuel line, it has virtually no resistance, so it builds no pressure in the line.” - I think it would only have no resistance if the end of the fuel line was open. If there was no blockage / injector.
Think of an empty garden hose attached to a 60psi faucet and with a closed spray nozzle at the other end. When you turn the faucet on, all the garden hose will be at 60psi, even if very little water entered it. The amount of water to enter was the same volume that the compressed air lost when going from 0-60psi. The 60psi water will push the air through the spray nozzle if you open it (or if spray nozzle automatically pops off at 58psi). See what I’m getting at?
Now let’s say you only open the faucet for 1/2 a second at a time. It may take a few of these cycles for enough water to enter and build pressure to 58psi before nozzle starts passing the air then water.
However, only if the line decompresses between these open cycles (using the compressed air to push back ) would you get a situation where the pressure never builds over time to 58 psi.

Thus I believe you have to crack at injectors because:
Otherwise it takes too long cranking to bleed air.
Or compressed air in fuel line decompresses itself by pushing back fuel into the pump when that cylinder is “off cycle”.
Or, like I think Lou suggested, a combination of the

Your analogy of a water hose and opening and closing a valve makes sense but having a constant pressure source is not what we have. We have a pump that puts out flow depending on the back pressure. You come to the same conclusion, i.e. after a certain amount of time, the pressures build.

First of all the amount of fuel being pumped by the IP is very small and the delivery pressure with virtually no back pressure is very low. Only when the line is near full will the pressures rise toward pop off pressure. The reason I say "virtually no back pressure' is because of the compressibility ratio between fuel and air. the bulk modulus ratio dp/dVair/dp/dVfuel is on the order of 15,000. That means that the rise in air pressure due to a change in volume is 1/15,000 change in volume from additional increment of added fuel in the line.