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Is there any reason to keep the rear fuel pump with EFI?

As i sort through the low fuel pressure issues on my ride, a recent thread on fuel pumps got me wondering: Why do you guys with EFI installs even keep the second (rear) fuel pump. If the pumps are in series, and only needed for CIS, why keep it?If the second pump fails, will there be pressure issues because the rear is not pumping? I have replaced my front pump with a Bosch 44, only to discover that my fuel pressure still remains low (37). I am replacing my fuel filter tommorrow, then checking the FPR, and the rear pump seems fine, but can i just get rid of it and be ok?
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Old 05-29-2008, 02:05 PM
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Assuming your new EFI system runs with reasonable fuel pressures (35 to 55 psi) Yes, you should be able to get rid of the second pump, but do the same tests and procedure you have proposed.

Change the filter, check the FPR. Also, do a volume test.

Do you have an adjustable FPR?
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Old 05-29-2008, 04:39 PM
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Yes the rear pump is not needed on EFI cars. But a large single high volume, high pressure pump (like an 044) up front is. The two pumps used in series is one pump to make sure the other pump is feed a constant supply of fuel. The only reason on a EFI car to have multiple pumps is to have them in paralell with each other to make suer there is enough volume of fuel based on the amount of HP. I use Aeromotive pumps as well as Bosch. The Walbro 255 pumps are also a reliable pump.

Now the real question is...... although your fuel pressure is still 37psi..... When you drive the car, does the fuel pressure slightly raise during instantaneous revs, and start to increase under boost? Or does it still drop when brought up to 3500RPM? If it drops then it could be the rear pump and or a clogged fuel filter. Or it could also be the FPR but until the rear pump is removed and a new filter is installed the FPR is not a candidate yet.
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Old 05-29-2008, 04:46 PM
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GJ, The fuel pressure actually drops a little with trottle to 35, so i pretty sure its either the filter and/or rear pump. We have been working on the car as time permits, and by tomorrow hope to have at least the pressure sorted. The FPR looks like the stock (non- adjustable) unit, but for some reason the vacume line was not connected. Connecting it makes no diff in the pressure change.

We decided to remove the rear pump and do a volume/ flow test in the am as it seems the rear pump is superfluous, and work our way back to the FPR. Have decided on the Aeromotive FPR if the stock is not working. How often do the stock FPRs fail?
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Old 05-29-2008, 05:59 PM
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You have 934 cams and they have alot of overlap. So I would suspect the vacuum at idle to be all most non-existant. So that would explain the no change in pressure when instantaneous throttle response. But with the vacuum line not connected it would not allow the fuel pressure to rise during boost conditions (bad). Does your boost gauge have a vacuum portion to read vacuum or if you look at your ECU during operation look at the current cell at idle and see what the ECU registers at idle, you can compare this to the amount of change your fuel pressure will see off idle. So if your car has 1" or 2" of vacuum at idle with the vacuum line connected the the pressure should drop the 1-2 psi But rise when you rev to "0" vacuum. I would remove the pump, replace the filter and get a "Mighty Vac" and connect it to the FPR and apply 5-10 psi to the regulator and your fuel pressure should rise as long as it is plumbed properly by the amount applied. If it doesn't change then the regulator is bad. If you want take the IC off and check the routing of the fuel line in and the regulators return to make sure they are plumbed correctly. If you are not sure take a picture and I can tell you if the plumbing is correct.
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Old 05-29-2008, 09:24 PM
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The regulator/s MUST be connected to vacuum from the intake plenum! It being unplugged makes them usless and unbelievable that the car has made it this far.
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Old 05-29-2008, 09:27 PM
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FPR usually never go bad. the stock Bosch units are very reliable. I use the Aeromotive due to my HP output which required larger fuel line than stock. If your FPR/s are working then there is no need to change them. You just need to attach the vacuum line to them and make sure you tie wrap the lines to them good as boost can pop them off.
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Old 05-29-2008, 09:32 PM
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GJ, thanks for the invaluable info. My shop guy is grateful as well, as his 'older' porsche experience is limited. Is this a stock 930 or Carrera FPR? Would you suggest going whole hog to the 'Going super fast' fuel rail/ Aeromotive FPR combo whe we really start into tuning , as you seem to imply that the stock fuel system may be a rate limiting factor?
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Old 05-30-2008, 08:58 PM
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Can't answer on what FPR you have. As GJF said the vacuum/boost line should always be connected in order for FPR to work properly especially during boost. If the FPR is not faulty and the vac/boost line was disconnected during the dyno run then that may explain the lean condition.
I do not understand how you could have a "stock" fuel system at the rails and injectors. Is there a way to attach a fuel pressure gauge in your current setup? I know that there's an 1/8 NPT port in the Aeromotive FPR for the FP gauge. Also, there's no need to change the fuel rail to use Aero FPR. Just need to find adaptors fittings to make it all work. One more suggestion. I would tee off the vac/boost line for the boost gauge so as to see if there's vac/boost at the FPR. Of course the vac/boost should be fed directly from the intake manifold after the throttle body (not from other lines that may reduce its vac/boost readings).
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Old 05-30-2008, 09:25 PM
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That is a stock Carrera FPR the 930's didn't have these they were all CIS which meters the fuel differently. The stock FPR works in many cars out there with 500+HP. I'd say if there is a problem with any of your lines or components then the GSF stuff would fix the problem. It would take care of the non adjustable issue too. Most of the aftermarket and stock fuel injectors we use are rated @43.5 psi. So when you set your static fuel pressure should be set to 43.5 (vacuum not connected when setting this).
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Old 05-30-2008, 09:52 PM
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Here is one of my intake setups using modified stock rails and AN lines.
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Old 05-30-2008, 10:05 PM
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Replacing stock Carrera FPR/fuel lines - can someone ID part?

GJ, that set up is sweet! Well, it looks like the stock Carrera FPR is not cutting it. To recap, replaced the stock 930 front pump with Bosch 44, removed rear pump, and replaced fuel filter... still not getting above 37.5 at idle and drops with throttle, even with vacume connected Would like to replace entire Carrera manifold line system with the Going super fast set-up, but cant see spending $$$$ when everyone is telling me the stock Carrera fuel rails and injectors are good for up to 600+hp (and as GJ you demonstrates). Anyway, can anyone tell me what part #10 is on the diagram of the Carrera manifold fuel line diagram?

I am splicing in the Aeromotive FPR in the spot where the stock FPR (#15) was, and am running the return line to where #11 (return line) fitting attaches. Am scratching my head as to what part #10 is... it looks like a second pressure reg? Should i delete part #10 and simply run a line from the fitting on fuel rail #1 to the second port on the Aeromotive FPR? Thanks for any advice guys.
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Old 06-07-2008, 08:51 PM
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GJ, what Aeromotive pump are you using? AN-10 or -6 fittings? Do you have any other pix of your fueling set-up? Looks similar to the GSF rig.

ps - what are the lines for going directly into the throttle body?
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'79 930/934 replica
80 RSR-look(Now in Sicily)
914/6 2.7 (Projekt 908/3)
1965 Karman Ghia-Class winner 2007 Carrera Panamericana/Ducati 900ss/GhezziBrian STW
D-Zug Produkte/D-Zug.com
Old 06-07-2008, 10:32 PM
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cant see spending $$$$ when everyone is telling me the stock Carrera fuel rails and injectors are good for up to 600+hp

no the stock injectors cannot support 600HP but.... everything else can.

#10 is a damper.... It is not needed.

37.5 is fine at idle. But what happens when you get on boost does the fuel pressure rise? The dropping fuel pressure is a concern though even with vacuum connected. Are all your lines good and plyable? BTW where are you taking the signal to the FPR from?

I use an Aeromotive A1000 pump with -10 to the pump and -8 to the fuel rail. From the fuel rail to the reulator -6 and -6 to the tank for the return.

The two lines to the throttlebody are just overkill lines to a remote AIC valve.
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87 911 Carrera platform
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Old 06-07-2008, 11:12 PM
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I'll check Monday, but vacum lines were running to stock location in the as in this pix (disregard arrows)
I agree, does sound suspicious for vacuum line issue, but we looked for cracks pretty closely, but didnt see any. GJ, according to the Pelican parts diagram (which i found after posting), part number '15' is the damper, and '10' is the FPR. Is this wrong?
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'79 930/934 replica
80 RSR-look(Now in Sicily)
914/6 2.7 (Projekt 908/3)
1965 Karman Ghia-Class winner 2007 Carrera Panamericana/Ducati 900ss/GhezziBrian STW
D-Zug Produkte/D-Zug.com
Old 06-08-2008, 12:57 AM
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Sorry Juice, I can't makeout the numbers in the diagram. Looking at the diagram the canister on the left is the FPR and the canister to the right is the damper. The FPR has 3 ports as does the damper but the damper has a flange on the underside instead of a fitting.

You still haven't answered this......What is the vacuum reading at idle?

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87 911 Carrera platform
Twin Garrett's, Big Valve, Xtreme Cylinder Heads, JB Racing cylinders and machine work, custom JE pistons, GT2 EVO cams, GT3 oil pump, modified 3.2L intake, 80mm TB, custom Intercooler, equal length headers, DFI, KEP Stage 2 custom twin disc, G50 custom gear set, LSD, Tilton trans pump, Fluidyne cooler
Old 06-08-2008, 08:51 AM
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Will obtain actual vacuum reading at shop tomorrow. Did a 'finger test' at idle, and seemed ok, but will obtain actual numbers. What should it be at idle?
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80 RSR-look(Now in Sicily)
914/6 2.7 (Projekt 908/3)
1965 Karman Ghia-Class winner 2007 Carrera Panamericana/Ducati 900ss/GhezziBrian STW
D-Zug Produkte/D-Zug.com
Old 06-08-2008, 12:32 PM
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GJ, never mind, saw your previous post about 1-2 at idle. Will correlate.
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80 RSR-look(Now in Sicily)
914/6 2.7 (Projekt 908/3)
1965 Karman Ghia-Class winner 2007 Carrera Panamericana/Ducati 900ss/GhezziBrian STW
D-Zug Produkte/D-Zug.com
Old 06-08-2008, 12:37 PM
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I agree with juicersr, nice setup GJF!

Justin,
You said "still not getting above 37.5 at idle and drops with throttle, even with vacume connected"

Just letting you know that the pressure does drop under vacuum (i.e. not under boost). Also, you need to disconnect the vacuum/boost line when setting base fuel pressure. Once set then reconnect the vacuum/boost line to AFPR. It will read lower than base pressure reading. Main thing is to make sure that the fuel pressure rises when under boost. Run lower boost to make sure everything is working correctly.
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Old 06-08-2008, 04:17 PM
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How can the damper be tested?

TIA
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Old 06-10-2008, 11:03 AM
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