Lift Pumps: What are they doing, anyway?

Stealth TDI · November 12th, 2009, 19:03

Hello,

Some of you may have read my "Dyno Letdown" topic in which I questioned the operation of my Walbro FRB-11. My troubleshooting compelled me to install a fuel pressure gauge so I could see what the pump was actually doing. It is specified to flow 43-gph at 6-8 psi. After installing the fuel pressure gauge, I learned the pump was pressurizing the fuel line to 6 psi when I first turn on the key (N109 cycle). Once the car is started, pressure hangs out around 3 psi at idle. It's about 2 psi at 50 mph cruise and 1 psi at 80 mph. It drops to ZERO under anything more than casual acceleration. I thought it was safe to say the pump was not delivering enough fuel to keep the feed pressurized under all conditions.

Tonight, I installed a brand new FRB-5. It's specified to flow 45-gph at 8-11 psi. They seem to be a popular upgrade (add-on?) for the Duramax folks. I got everything installed and hooked up. When the key is turned, the pressure jumps to about 5-psi and slowly climbs to 7-psi. Pressure drops to 3-psi at idle. In short, the pump behaves quite similarly to the FRB-11. The good news is my FRB-11 is fine. The bad news is either these pumps don't do what they're supposed to or I don't understand the external influences on these pumps (likely the latter).

I've been told, "get a PD lift pump," "get a Carter," "your pump is bad..." I suspect the majority of us read the specs of a given pump and accept that is what the pump is doing. But I'd like to request something a bit more scientific:

I'd like the input of those who actually have a fuel pressure gauge installed. Please post what lift pump you use, where your pressure is sampled, and what your pressure is just prior to start-up, at idle, and under acceleration. Perhaps a format like my example here:

Pump: FRB-5
Pressure sampled between the fuel filter and the IP
Pre-start: 6-psi
Idle: 3-psi
Acceleration: 0-psi (unknown if this is "zero" or vacuum since there is a mechanical stop... but IP timing is fine)

I don't mind a few theories of how pumps work and why my pressure reads what it reads. But please don't jump to the conclusion that I have the "wrong pump" or another "bad one" off the shelf unless you have a lift pump which maintains its specified pressure under all conditions and you have a fuel pressure gauge installed to prove it (meaning you understand how lift pumps behave in our application with fuel returning to the tank).

While I understand my topic is diagnostic in nature, I opted to post it in the Power Enhancements section due to the fact lift pumps are a popular power enhancement and my goal is to learn what the lift pump is actually doing (is it really providing "pressure" to the IP, or just ensuring there's not vacuum?). Others stand to learn something as well since I'm sure the vast majority of drivers do not have a fuel pressure gauge installed. Of course, moderators are welcome to move this at their discretion.

Thanks,

Scott

Whitbread · November 12th, 2009, 19:25

Woops, looks like i'm a minute too late. I just posted in your other thread haha. But anyway,

Pump: Aeromotive A1000 and Mallory 4309 bypass regulator
Pressure sampled between the fuel filter and the IP
Pre-start: 24psi
Idle: 24psi
Acceleration: WOT@5.5K 12-13psi.

Fix_Until_Broke · November 12th, 2009, 19:39

I've got a PD in-tank lift pump, cat 2u filter and 11mm IP.

I can hook up my pressure sensors and take data similar to my EMP/IMP measurements while taking a VagLog of Channel 1 (RPM, IQ, Collar Position and Coolant Temp).

Then we can compare fuel pressue and delivery vs RPM or whatever else you want me to measure.

Actually on the quick and dirty - I've got a 30"Hg-15psi analog gauge that I could hook up and just visually see what happens when driving around. What's another gauge on the dash - I've already got 4 extra ones

If someone can point me to where I can measure the case pressure I can record that as well (runs at ~80 psi I think right?)

To dig into the theory a little bit (until I can take some data).

The pumps you're using flow 45 gallons/hour or ~0.75 gallons/minute. The vane pump in the injection pump is a fixed positive displacement pump so for an easy test, we can remove the return line from the IP and run it into a bucket and measure the volume that comes out at idle (~903 RPM) for a 20/30/60 second time period and calculate the flow rate.

I did this once when I first bought my car (don't even remember why) and I seem to recall that I got ~1/2 gallon/minute at idle but can't be sure of that.

Anyway, being that it's a fixed positive displacement pump, we should get double the flow at 1806 RPM, triple the flow at 2709 RPM, etc. With this information we should be able to then at least get in the ballpark selecting a lift pump on specifications (I suspect that 45 gallons/hour is not enough).

The other thing is that the timing piston can consume/expel siginificant ammounts of fuel in very short times if there's very quick timing changes. This could cause momentary dips/rises in fuel pressure as the piston extends/retracts to adjust the timing. Not too big of a deal if there's sufficient supply pressure there to feed it, but something to think about (secondary)

Fix_Until_Broke · November 12th, 2009, 20:33

Had a moment of motivation and went and spliced in my -30"Hg to +30 PSI gauge in between my fuel filter and injection pump and went for a trip around my "off road" course.

Key on engine off: 7 psi
Key on engine running at idle: 7 psi
Neutral rev to 5000 RPM: 6 psi
Normal driving around 25-40 MPH at all kinds of throttle positions and RPM's: 5-7 PSI
Full throttle 5500 RPM 3rd gear pull: 3-4 psi minimum at high RPM: ~5 psi typical
3000 RPM overrun (foot off of throttle) turn key off - goes from 7 psi to ~10inHg

I made a Channel 4 Timing log last week (RC5 ALH) and it holds pretty well
[IMG]

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Apparently the PD lift pump keeps up pretty well?

Let me know if you still want Vag logs vs case or inlet pressure.

Hope this helps you out.

oldpoopie · November 12th, 2009, 21:14

SERIOUSLY THE MOST INTERESTING THREAD IN MONTHS!!!!!!!!!!!!!!
BRAVO!!!!!!!!!!!

If I have some time this weekend I'll hook up a gauge as well.

D_Bat · November 13th, 2009, 00:13

I haven't installed my FRB-11 yet but it did say that it had to be oriented a certain way and that it couldn't be turned a certain way. I forget but it was on the install instructions that came with it. Just something I'd like to quickly mention.

Jasengine · November 13th, 2009, 05:57

Yes your correct on that, it has to be installed vertically.

I have the FRB-11, which I installed as I have fuel stavation when pulling hard in 4th or 5th gear. Once I installed this, the 'surging' had almost disapeared. I took logs before and after lift pump install so I will post them up later.

silverbox · November 13th, 2009, 06:10

Left Coast Resident did some testing on the oe lift pump as well.
http://forums.tdiclub.com/showthread...ight=lift+pump
He got similar results.

Gearhead51 · November 13th, 2009, 09:56

I know there was debate on some IDI forums to a lift pump's abilities to skew timing. The timing actuator in those is pressure based. I guess this is not an issue on the TDI pump, though. I know the fuel return line is a busy place to be at high RPMs, so you'd have to have a heck of a pump to keep pressure on the inlet in these circumstances.

I measured case pressure on my IDI once by putting a gauge in a standoff between the return port in the pump and the banjo fitting. The return port on IDIs had a restriction orifice in the bolt.

MethylEster · November 13th, 2009, 10:20

I'm still waiting for dyno numbers with and without the lift pump to see what, if any, additional performance you get. Unless I've missed something recently nobody has yet to do that. Again I don't want subjective seat of the pants feelings. I am looking for hard facts.

Stealth TDI · November 13th, 2009, 10:47

My recent dyno testing showed just 2-hp difference with the FRB-11. It appears that's because there's no pressure at higher RPMs. But a lift pump that can actually pressurize the feed at all RPMs... ???

The Shootist · November 13th, 2009, 11:00

This is the pressure case pressure tap according to my research...

Jasengine · November 13th, 2009, 13:32

Here are my results for the timing before and after lift pump install:

dvst8r · November 13th, 2009, 13:40

Old setup.

Small Holley Red 67gph
KOEO: Never really paid attention
Idle: 7psi
WOT: under load 4-5psi.

New setup:
Bosch044: -6an feed on an areomotive bypass after CAT filter, -8an after the bypass (helps dampen pump pulses)
Idle: 25psi
WOT: Under load 24-25psi

I am quite certain that I could push this setup to 60psi+, but I know what happens at that point, and I don't feel like getting this pump machined for the retaining ring yet, as it is just a daily driver, nothing fancy.

shizzler · November 13th, 2009, 14:41

Good thread Scott.

I too would like to see a before/after dyno. But there are several documented instances like the one above where injection timing better matches requested after installation of a lift pump. I wonder if actual timing slipping from requested as an indicator of fuel starvation also implies that actual vs. requested injection quantity would suffer too? Has anyone logged that data?

This thread is quite timely as I was thinking about purchasing a walbro or carter pump, but then went with a PD in-tank pump when Aaron ran his killer October special. Hopefully it'll keep my "light haze" from disappearing over 3500 rpm....