Does a aux pump push fuel through return lines?

CRDMiller · **Posted:** Sun Feb 01, 2009 3:39 am

The subject is apt.

If the engine is not running, but your pump is on (aux pump) does the pressure from the aux pump push fuel all the way past the cp3 to the rail to the injectors and into the return line? (thus removing any and all air in the rail and head)

There is (obviously) no path for pressurized fuel to take out of the head, other than to the cp3.

Witch would mean that, if i had air in my lines, or in the head, the only way for that air to escape is into the cp3 and into the rail. (this is not good, right? we want no air, compressed air is NOT diesel)

This is the same as if I had no aux pump. So how does the aux pump fix the problems I had before I got the pump. And why do I never have air in my head?

Is it simply, the positive pressure helping the air into the rail every time I start the engine, compressing it, doing a nice cavitation jig, and then going through the return lines (or into a cylinder) ? Because i'm pretty sure my fuel system is still gaining air. Not as much as before I got a non leaking head, but still via the quick connects.

This is not a big deal, just something I've been curious about for a while.

ATXKJ · **Posted:** Sun Feb 01, 2009 10:01 am

no aux pump is going to push past the injector pump.
It can pressurize up to that point - but that's it.

the advantage of the lift pump is that air won't leak in -
fuel might leak out if it's a big enough hole - but air shouldn't come in.
(there are some backflow situations - but that's a lot less than a leak)

CATCRD · **Posted:** Sun Feb 01, 2009 11:28 am

The chief benefit is pressurizing the filter head. A fine filter used in suction, like on the stock CRD, will actually strip air bubbles out of the incoming fuel, and they'll accumulate in the head. This slug of air in the filter head can break the "siphon" of the CP3 and you end up with "limp mode" (wimp mode), no start, shudder etc. Plus it keeps air from being sucked in any connections to the filter head.

Joe Romas · **Posted:** Sun Feb 01, 2009 11:30 am

The CP3 has a programmed in air purge routine on startup. So the Aux pump would push the air to the CP3 that would then expel it :lol:

nursecosmo · **Posted:** Sun Feb 01, 2009 11:56 am

My lift pump does not push fuel through the CP3 when it is not running. One of the reasons that the lift pump eliminates air in the fuel filter is because it keeps a positive pressure across the actual filter medium at all times. In most CRDs with no lift pump, there is not an actual air leak, but if the filter is even a little bit plugged, the supply pump in the IP nose creates enough of a vacuum on the filter to suck air out of the fuel which builds up in the filter head until a large chug gets sucked into the IP. In those vehicles which do have an actual air leak, the LP does the obvious job of preventing any air from getting into the system while running.

If you are worried about any air being pushed into the IP and being compressed, causing cavitation in the rail and injectors, then going back out the return line, Don't be. there is no concern unless you get a very large bubble of air into the IP, such as one so big that that the supply pump can not supply the IP with the high supply pressure it needs. When normal sized bubbles get sucked into the IP (all fuel systems have some bubbles in the line), The IP compresses it down to between 333 to 1600 times it's original size. When that happens the air goes into chemical solution almost instantaneously, thus you should always have nothing but liquid fuel in the rail.

ATXKJ · **Posted:** Sun Feb 01, 2009 4:35 pm

nursecosmo wrote:

In most CRDs with no lift pump, there is not an actual air leak, but if the filter is even a little bit plugged, the supply pump in the IP nose creates enough of a vacuum on the filter to suck air out of the fuel which builds up in the filter head until a large chug gets sucked into the IP.

This theory had popped up a couple of times and the issue I have with it - is that it would also be equally true of the newly redesigned fuel heads and the Racors ect.

The limited data base we have says that once you get rid of the original design fuel head - the problem's gone.

So - it's a minor point - in that either fuel head or lift pump will fix a leak problem.

- but entrapped air imply's that changing the filter - to 'unplug it' would also correct air in fuel problems -
I have not seen that occur.

CRDMiller · **Posted:** Sun Feb 01, 2009 9:08 pm

Interesting comments, I did not know that on startup, the cp3 has a routine for clearing air, nice to know. I was hoping that the IP was obstruction free while non-op.

I though that the highly compressed air would be bad for the rail. But if it's not, great! I'm not going to cry if i have to replace the cp3 and injectors and the rail. (in 10 years :P) I'm going to smile because it gives me an excuse to turn wrenches.

Also, please note that, i don't have any fuel problems, i was simply asking a question :P

I replaced the head as soon as the new one came out, and i rigged up the aux pump shortly after (i kept getting fuel to hot CEL's i suspect because there was air in there, somehow) And it runs better than ever.

Anyway, I was thinking of actually rigging up a return line off of the bleeder nipple, that would ensure no air was in the head, and thus the cp3 could never suck air. I like to tinker with things is all.

Thanks for your comments

ATXKJ · **Posted:** Sun Feb 01, 2009 10:51 pm

I posted this a while back - it's the only top - air bleed - fuel return I've seen (as noted not mine)
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?t=25299&highlight=lift+pump

I think BlacklibertyCRD ran a return line - using the side bleeder valve.

CRDMiller · **Posted:** Sun Feb 01, 2009 10:56 pm

atx, Excellent, thanks!!

dtempler · **Posted:** Mon Feb 02, 2009 1:59 pm

I have also been wondering this. I replaced my leaky filter head last week and saw dramatic improvements. However I have tried bleeding the new head after a couple of times driving and found air in the new head every time! I would like to see if I can stop this by eliminating any possible air leaks, such as the plastic clips, before I add a lift pump.

Will I ever eliminate all leaks, or will I always end up with air in the head until I get a lift pump? Do those with lift pumps ever have air in their filter heads?

BlackLibertyCRD · **Posted:** Mon Feb 02, 2009 2:18 pm

ATXKJ wrote:

I posted this a while back - it's the only top - air bleed - fuel return I've seen (as noted not mine)
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?t=25299&highlight=lift+pump

I think BlacklibertyCRD ran a return line - using the side bleeder valve.

That bleeder back to the return is great for bleeding new filters. Just cycle the key a few times and your done. I shut it off after bleeding for the air or lack of pressure don't return. Keeps the driveway clean!

ATXKJ · **Posted:** Mon Feb 02, 2009 2:22 pm

I have the tank mounted lift pump - and I never have air if I try to bleed it.

While the fuel head is the single biggest problem - the connectors at the tank are an issue

and the air entrapped in the fuel separating out - has been mentioned by several people that have a lot more diesel experience than I have - so they may have a point - I just don't think it's the main point.

but the lift pump - seems like a easy fix - even if it's overkill
I think the advantages outweigh the disadvantages (cost and installation)

nursecosmo · **Posted:** Tue Feb 03, 2009 1:19 am

dtempler wrote:

I have also been wondering this. I replaced my leaky filter head last week and saw dramatic improvements. However I have tried bleeding the new head after a couple of times driving and found air in the new head every time! I would like to see if I can stop this by eliminating any possible air leaks, such as the plastic clips, before I add a lift pump.

Will I ever eliminate all leaks, or will I always end up with air in the head until I get a lift pump? Do those with lift pumps ever have air in their filter heads?

I don't know of anyone who has installed a lift pump to have ever reported air in the line afterwards. The push on clips which join the flexible hose to the metal lines at the tank have been suspected by some for air leaks but I don't really think that anyone has ever had an actual example of a leaking fuel line coupler. On the other hand, if you are concerned by the possibility of it happening, it is an easy fix.

MrMopar64 · **Posted:** Tue Feb 03, 2009 9:03 am

If you were to run the aux pump with the ignition key in the off position such that the ECU was off and, consequently, the metering unit (on the CP3) and the PCV (located at the back of the rail) were de-energized, then the fuel would flow through the head, pump, rail, and return back to the tank. No specific air purging routine upon startup since the common rail is air purging by design.

gmctd · **Posted:** Tue Feb 03, 2009 1:02 pm

I haven't actually tested this, but aren't the FQS valve on the CP3 and PCS valve on the rail normally-closed until ECM is controlling fuel injection in a starting-to-running engine? The COV in the CP3, being spring-loaded on the output of the CP3 lift pump, bypasses air but not fuel back to the tank at aux lift pump on, engine not running - at supply pressures over 73psi, engine cranking, COV will bypass fuel back around to lift pump input, sending any excess fuel back to the tank - the aux electric lift pump will never reach that pressure level

MrMopar64 · **Posted:** Tue Feb 03, 2009 1:22 pm

No, they have to be fully open in the de-energized state. In the event of an emergency with the vehicle, the rail pressure has to deflate as soon as possible. At least with the configuration of the KJ, this happens most quickly when both actuators are wide open (no power) as the fuel would (if still supplied by the tank pump) build no pressure.

BlackLibertyCRD · **Posted:** Tue Feb 03, 2009 1:33 pm

I know that when I turn the key off and the pump not running, the filter head and primer holds pressure. If the return was open thru the cp3 pump then we would never need a bleeder. I have to look it up but I think that the pump only bleeds during cranking.

GOT IT!

OPERATION
Cascade Overflow Valve
Instead of using an electric supply pump, this fuel system uses a gear supply pump located inside the rear of the
high pressure pump. The pump is driven by an eccentric on the end of the high pressure pump shaft. The gear
pump draws fuel from the fuel tank through the fuel filter.
The pressurized outlet side of the gear pump provides pressurized fuel to a branched circuit internal to the high
pressure pump flange, which supples both the fuel quantity solenoid and the cascade overflow valve. Because the
gear pump increases fuel flow and pressure as the engine rpm increases, the pressure is regulated by the cascade
overflow valve. The cascade overflow valve and gear supply pump are not serviced independently of the high pressure
pump.
The cascade overflow valve has two functions:
² Regulation of lubrication fuel to the internal moving parts of the high pressure pump
² Regulation of the fuel pressure being supplied to the fuel quantity solenoid
The cascade valve has a machined center piece that has three drillings. One for overflow, one for lubrication and
one for supply. The valve works in three stages based on the pressure entering the inlet of the valve.
Stage 1
When the fuel pressure entering the tip of the cascade valve is between 0 and 3 bar (44 psi), the spring force is not
overcome and fuel only flows through the center drilling. This drilling always allows fuel flow through to the pump
center ring and lubricates the pump bushings and internal moving parts. This circuit also allows air to bleed during
initial cranking and returns the air to the fuel tank. The cascade valve is only in stage one during cranking.
Stage 2
When the fuel entering the cascade valve exceeds 3 bar (44 psi), but is less than 5 bar (73 psi), the center piece
of the valve moves against the spring force aligning another passage for lubrication purposes. Stage 2 can be
reached during cranking and initial start up.
14 - 54 FUEL DELIVERY - 2.8L DIESEL KJ
Stage 3
When fuel pressure exceeds 5 bar (73 psi), the center of the valve aligns with the overflow passage. This stage
relieves the pressure into an overflow circuit that sends the fuel back to the inlet side of the gear pump which limits
maximum fuel pressure to 5 bar (73 psi). Lubrication fuel also continues to flow though the other ports during this
stage. Excess is sent back to the fuel tank through the return circuit.

Joe Romas · **Posted:** Wed Feb 04, 2009 8:32 am

This circuit also allows air to bleed during
initial cranking and returns the air to the fuel tank. The cascade valve is only in stage one during cranking.

BlackLiberty.

Thanks, I knew I had read that somewhere before when I typed it above :lol:

Does a aux pump push fuel through return lines?

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