whitedog
Veteran Member
I have been struggling with how the case pressure on our little TDI engine works and today I took the time to talk with Kyle (Da Man!) at Diesel fuel injection services and man, did I learn plenty. I’ll try to share as much as I know about this.
First off, we have our transfer pump (T-pump) that pulls fuel from the tank and provides fuel flow into the injection pump (IP) case. The fuel flows into the case and feeds cooling, lubricating fuel to the high pressure side of the pump. The fuel that doesn’t get sent to the injectors, flows out the top of the pump and out the return fitting. This return fitting is what creates the pressure in the case. Remember, a pump does not create pressure, it only creates flow. It is the restriction to flow that causes pressure.
In the VP-37 injection pump, the case pressure also serves to control the timing advance. The timing advance piston is spring loaded to the full retard position and pressure acting on the opposite end of the piston advances the timing. The N108 solenoid controls the amount of pressure pushing on the advance piston by acting as a variable orifice. The more pressure on the piston, the more injection advance you get.
Here is a drawing of where it is located. It's pointed to by the long blue arrow.
And here is the basics of what is going on with the timing:
Back to the T-pump.
On the outlet of the T-pump is the T-pump relief valve. This provides protection for the T-pump in case of blockage of the return. This relief valve has been commonly miscalled the case pressure regulator; it is NOT. It is simply a relief valve. It is very common for the guts of this valve to fall out. This has puzzled me why the pump still runs pretty good and that is what prompted the call to Da Man.
This is what things can look like:
It is located in the top of the pump right next to where the hose from the filter is. It takes a 10MM wrench to remove it.
That ring that holds the plunger in is not what sets the pressure. That fitting is hollow and it has a plug in the top end that sets the spring pressure to that specific pump when the pump is rebuilt. Due to variable leakage in the transfer pump, this can’t be set for all pumps, it must be set for each pump individually, therefore the T-pump relief should never be swapped between pumps.
I was also a bit lost on how that pump could still work with the guts all falling out. Well, as is typical with me, as I was about to ask Kyle the question, I had an epiphany and realized that even though the ring is no longer in the fitting, the plunger is still sitting in there and working just like it should.
You may be asking what all of this has to do with anything. Well not much really, LOL! The case pressure is vital to the timing, but that T-pump relief valve retainer ring falling out won’t have much effect on timing at road speed. At low idle, there is lower flow, therefore lower pressure, so there can be some flakey timing then, but once the RPMs come up, flow increases and the N108 can control the timing again.
That’s about it. I didn’t ask Da Man what the case pressure on our pumps should be, sorry. I also didn’t go into the electrical control of the timing, or how this could be modified for performance. Basically, I just wanted to clear up some misunderstandings.
I'll add some pictures after I post this.
First off, we have our transfer pump (T-pump) that pulls fuel from the tank and provides fuel flow into the injection pump (IP) case. The fuel flows into the case and feeds cooling, lubricating fuel to the high pressure side of the pump. The fuel that doesn’t get sent to the injectors, flows out the top of the pump and out the return fitting. This return fitting is what creates the pressure in the case. Remember, a pump does not create pressure, it only creates flow. It is the restriction to flow that causes pressure.
In the VP-37 injection pump, the case pressure also serves to control the timing advance. The timing advance piston is spring loaded to the full retard position and pressure acting on the opposite end of the piston advances the timing. The N108 solenoid controls the amount of pressure pushing on the advance piston by acting as a variable orifice. The more pressure on the piston, the more injection advance you get.
Here is a drawing of where it is located. It's pointed to by the long blue arrow.
And here is the basics of what is going on with the timing:
Back to the T-pump.
On the outlet of the T-pump is the T-pump relief valve. This provides protection for the T-pump in case of blockage of the return. This relief valve has been commonly miscalled the case pressure regulator; it is NOT. It is simply a relief valve. It is very common for the guts of this valve to fall out. This has puzzled me why the pump still runs pretty good and that is what prompted the call to Da Man.
This is what things can look like:
It is located in the top of the pump right next to where the hose from the filter is. It takes a 10MM wrench to remove it.
That ring that holds the plunger in is not what sets the pressure. That fitting is hollow and it has a plug in the top end that sets the spring pressure to that specific pump when the pump is rebuilt. Due to variable leakage in the transfer pump, this can’t be set for all pumps, it must be set for each pump individually, therefore the T-pump relief should never be swapped between pumps.
I was also a bit lost on how that pump could still work with the guts all falling out. Well, as is typical with me, as I was about to ask Kyle the question, I had an epiphany and realized that even though the ring is no longer in the fitting, the plunger is still sitting in there and working just like it should.
You may be asking what all of this has to do with anything. Well not much really, LOL! The case pressure is vital to the timing, but that T-pump relief valve retainer ring falling out won’t have much effect on timing at road speed. At low idle, there is lower flow, therefore lower pressure, so there can be some flakey timing then, but once the RPMs come up, flow increases and the N108 can control the timing again.
That’s about it. I didn’t ask Da Man what the case pressure on our pumps should be, sorry. I also didn’t go into the electrical control of the timing, or how this could be modified for performance. Basically, I just wanted to clear up some misunderstandings.
I'll add some pictures after I post this.
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