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Discussion Starter #1 (Edited)
This is a continuation of "Lift pump pressure"

Should probably call this "high pressure at the filter head"

After going after and fixing the sucking air problem twice and the spike of lift pump pressure after shut off now have a constant high lift pump pressure at speed. I'm seeing over 12 psi at the filter head with a factory diaphragm lift pump.

Disconnected the return line after the "T" junction of the injector and IP return. Just a little dribble coming out at idle and hardly a stream at 2K. No restriction in the return line to the fuel tank.

New AutoMeter fuel pressure gauge was reading 12 psi when I shut it down.
I checked the over flow restriction, nothing inside the banjo bolt. Blew it out with air, no change.
Left the banjo bolt of the over flow restriction loose to see what it would take to stop the over pressure on the lift pump. Had to leave it open, fuel pored out, still saw 10 psi @ 3K rpm, no load.

Since the lift pump pressure is controlled by a spring I doubt that the lift pump is at fault.
So the IP must be feeding back into the return line?
The pressure control valve is malfunctioning?
Any other suggestions for testing?
 

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Your pressure reading is very unusual for a diaphragm lift pump. Don't know that I've ever heard of one hitting 12 PSI and holding it. Usual readings are probably more in the 6-8 PSI range. Your numbers sound more like what one would expect on the VE piston lift pump. Somewhere in the 12-15 PSI range prior to the filter is normal for that one. It would be interesting to know what your pressure is after the filter. You normally expect several PSI loss when going through the filter.
 

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Discussion Starter #3
I did a manual test, used the priming lever pumped it like H**L and most I could get was 7psi. Fuel was dribbleing out the disconnected fuel return line.

The problem is not the lift pump.
 

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.... You normally expect several PSI loss when going through the filter.
Correct me if I am wrong, the pressure loss across a good filter should be very low in a lightly modified 4bt (the flow rate is not huge). Several PSI is the sign of a plugged filter.
 

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Discussion Starter #5 (Edited)
Let me backtrack and mention how this came about.

The air leak came back making the Scout no start after sitting for a week. Decided to add a Fram G12 in line filter just before the lift pump to check for air and crud. At that point I found that the flex hose on the lift pump had hardened over the barbed end on the lift pump. Fixed that then added the filter. Found that driving with that filter on the pressure went over 12 psi. And could not get the air out of the filter so scrapped that idea. Next day went to start it and the MaxTow gauge died. Could not verify that the fuel pressure was normal so the Scout sat until the AutoMeter fuel pressure gauge arrived. New gauge in and the good news it started after sitting for a week, bad news still had the high pressure (12psi+) at speed.

Just pulled the fuel pressure regulator out of the VE pump. Looks good, no crud, spring and plunger not stuck, it moves.
 

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Could just try the banjo bolt in the pump with the fuel line off and compare the flow to the flow you are seeing at the Tee. That is a fairly small line and maybe kinked or restricted. Cannot compare mine for you as I have a large return line right from the pump and the inject return is on it's own line.
 

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Your diaphragm pump must be on steroids. Specs from Cummins show average of 3-5 PSI and replace if it drops below 3 PSI. Those numbers are after the filter. Russ, maximum drop across the filter is listed at 5 PSI before needing filter replacement. Pressure at the injection pump has a range of 3-10 PSI. I wouldn't think 12 PSI would be any issue.
 

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Discussion Starter #8
Could just try the banjo bolt in the pump with the fuel line off and compare the flow to the flow you are seeing at the Tee. That is a fairly small line and maybe kinked or restricted. Cannot compare mine for you as I have a large return line right from the pump and the inject return is on it's own line.
I have yet to test the overflow restriction in the banjo bolt to prove that that is or part of the problem. My wife has graciously allowed me to use her ultrasonic cleaner for cleaning out the orifice of the overflow restriction.
 

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Discussion Starter #9
Your diaphragm pump must be on steroids. Specs from Cummins show average of 3-5 PSI and replace if it drops below 3 PSI. Those numbers are after the filter. Russ, maximum drop across the filter is listed at 5 PSI before needing filter replacement. Pressure at the injection pump has a range of 3-10 PSI. I wouldn't think 12 PSI would be any issue.
Char1355, the diaphragm pump cannot put out more psi than it was designed for. The pressure is generated by a compressed spring not from the lobe of the cam. The cam lobe does pull the diaphragm down for the suction/intake of the fuel, not for the pressure.

How a diapahragm fuel pump works Half way down is a animation of a working diaphragm pump.

When this overpressure happened the first time I caught it was going past 14psi. Since then I've stopped it at 12psi.
 

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14 PSI pre filter is about what the piston type lift pump for the VE produces and that's acceptable. I've just never heard of a diaphragm pump producing that much pressure.
 

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Discussion Starter #11 (Edited)
14 PSI pre filter is about what the piston type lift pump for the VE produces and that's acceptable. I've just never heard of a diaphragm pump producing that much pressure.
In this case it is NOT the lift pump that is producing the high pressure it has to be feed back from the IP. The lift pump for the 4BT is designed to produce a max of 10psi, it cannot produce more than that. Or it could be that the overflow restriction orifice is blocked.
 

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Discussion Starter #12
Next subject: The Pressure-control valve.
Took it out and looks good but it rattles or should I say there is play between the spring and the shuttle valve. And there is daylight showing through the orifice. Has the spring weakened? I believe this condition has something to do with this high pressure back feed.
127490

The valve is shown upside down.
127491

The bright spot in the oval is light showing though from the flashlight.
 

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Discussion Starter #13 (Edited)
Here is the pressure control valve disassembled. Per the instructions on page 16 I stretched the spring 1mm so that the orifice was closed by a distance of .5mm.
And the overflow restriction orifice was blocked with junk. After cleaning the return line there was a steady stream of fuel at idle, not at a dribble like it was.
127492
127494
127495
 

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Discussion Starter #14
The test drive:
Idle fuel pressure 7psi, @840rpm.
Highest fuel pressure observed 10 psi.
The fuel pressure is still higher than it used to be before all these problems but it's below 10 so I'm happy. Now to sell it before I have to fix it again.
 

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Sounds like you cured the problem. Junk in the injection pump is never good for anything. Someone probably needed to change the fuel filter more often in the past. I'd say your new numbers a super good and should give no problems.
 

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As I was reading through your updates, I thought it had to be a return restriction like BlackDuck was alluding to. Wondering if you had just cleaned the orifice and not stretched the screen, you would have been down at normal level. Based on the drawings you posted, that spring presses against fuel flow to regulate pressure, so stretching the spring would net higher running fuel pressure, the opposite of your desired effect.
 

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Discussion Starter #17 (Edited)
As I was reading through your updates, I thought it had to be a return restriction like BlackDuck was alluding to. Wondering if you had just cleaned the orifice and not stretched the screen, you would have been down at normal level. Based on the drawings you posted, that spring presses against fuel flow to regulate pressure, so stretching the spring would net higher running fuel pressure, the opposite of your desired effect.
In hindsight I would have preferred to have done the overflow and the pressure regulator "fix" seperately. But I had the pressure regulator apart as the overflow was in the ultrasonic cleaner. I wouldn't have repaired the regulator if I hadn't noticed the spool valve in the regulator was leaking and slid back and forth when shook. Did that "fix" add to the higher ovserved fuel pressure, I don't think so. From page 15, "If fuel pressure increases beyond a given value, the valve spool opens the return passage so that the fuel can flow back to the supply pump’s suction side" as shown in fig 7. But I am now seeing 10 psi at startup cold, and a slightly high pressure reading after the engine reaches operating temp, 8 psi vs 7 psi. And the pressure will go to 11 psi until the engine warms up. The "over pressure 12 to 14 psi" was definitely caused by the overflow orifice being plugged.
 
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