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Old February 9th, 2009, 15:40   #1
whitedog
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Default Injection pump case pressure

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.
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Last edited by whitedog; February 9th, 2009 at 16:12.
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Old February 9th, 2009, 15:50   #2
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Sweet. Learn something here every day!
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Old February 9th, 2009, 19:46   #3
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Quote:
Originally Posted by whitedog
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. Iíll try to share as much as I know about this.
So when this thing starts acting up, what are the symptoms?

It seems like we learn more and more about these pumps all the time .
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Old February 9th, 2009, 21:08   #4
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When what starts acting up? The T-pump relief? The only thing there that Da Man mentioned was the possibility of flakey timing at low idle.

If you mean the N108, then you will see low power, lots of white smoke, rattle, hard starting, inability to maintain timing adjustment, probably others, but those came off the top of my head.
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Old February 11th, 2009, 09:42   #5
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Very interesting information.

I do have a few q's re the assumptions on the thread for example if the plunger stays in the bore, they why the loss of pressure at low rpm just because the circlip is out a bit? If the plunger isn't in the bore, then pressure rise won't push it in, the spring will be more than enough to hold it out the way while diesel flows above it and into the valve body/ports.

Has Kyle actually measured case pressure on a pump with a disintergrated relief valve & one half disintergraded to test the theory? The theory elsewhere and that I have (so far) gone on is that when the circlip goes down enough, the top of the piston goes lower than the outlet hole, as the piston moves down further it's no longer in a tight bore, diesel can flow round it and straight out the uncovered port thus leading to lack of pressure, as you say the advance piston is spring loaded retarded so low rpm function is okay but max advance/high rpm function is not.

If I get a chance to run a vehicle back to back with the case pressure valve dismantled/normal and monitor timing and N108 duty cycle I'd get a bit more info. Case pressure would be good too, only I don't know where I'd tap into that! Maybe he as already done this/benched it?

Just theory talk here - I like to be convinced by trying the theory rather than just accepting it

Greg.
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Old February 11th, 2009, 09:50   #6
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You are right Greg and I have been thinking about about doing some testing on the Beetle. Unless the bracket is in the way, I can test case pressures with the retainer out. There is a plug on the engine side, kinda towards the rear of the pump that will show case pressure, I'm just not sure if that bracket is in the way, though I think I'll be OK.

I was thinking of doing this and reading duty cycle while cold, and revving it a bit, then duty cycle when warm and revving it a bit.

I would also do the pressure tests while cold and warm.
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Old February 11th, 2009, 11:54   #7
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There is a bolt on the engine side of the pump, just below the quantity adjuster plate. I think this is intended as a pressure tap.
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Old February 11th, 2009, 12:21   #8
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I have a fitting coming for that pressure tap.

And Greg, you Jinxed me. Now my Beetle smokes out the neighborhood at >2700 RPMs regardless of if the T-pump relief is assembled or not. I did use an extra T-pump relief so I know that the assembled one is correctly set.

I'm going to look at my data, but I don't think that there is much, if any difference between assembled or unassembled.
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Old February 11th, 2009, 13:02   #9
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Well, I'm not going to bother reporting what I found until I check my case pressure because I gots me some problems.
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Old February 11th, 2009, 13:04   #10
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oh come on, spill or start a new thread and link it

this is a good thread BTW. keep up the good work!
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Old February 11th, 2009, 13:15   #11
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After looking at the data, it looks like when warm, it makes no difference. But then, I'm working with a pump that may have troubles, so nothing is conclusive yet.

I do have an 11MM pump sitting on the shelf. Toss that in, run a Diesel Purge and see what it does?
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Old February 11th, 2009, 13:21   #12
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you have the ability and tools, why not

only takes a little time to swap it over
you get more done when you aren't on the keyboard tho
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Old February 11th, 2009, 14:43   #13
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I'm going to wait until I get some readings on the current pump case pressure. I have fittings and test hose coming.
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Old February 11th, 2009, 15:05   #14
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My experience with this is that if the guts fall out or the valve is set to low, you get no timing advance at load....less than 10 adv.

If the valve is set too high, you get un-controllable timing advance at load or even while cruising steady.
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Old February 11th, 2009, 15:11   #15
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I was up to 15 or 16 with the guts out.
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