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~KnuckleDuster~

High fuel pressure after fuel pump swap..

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The motor is an RB25 non turbo.

 

So I was having some performance issues, motor seemed down on power a bit and I was getting some starting problems. I would start the car and it would die, had to blip the throttle to get her going. I suspected the old in line fuel pump needed replacing so I put a new Walbro 255 in as shown here with a pre filter to catch any gas tank debris. Also changed the fuel filter while I was at it.

 

gallery_2013_1393_262778.jpg

 

After I did this the fuel pressure was maxed out on the gague. I searched around and heard the rb25's fuel pressure regulators don't do well with a high pressure fuel pump like the Walbro 255 and an adjustable regulator should be fitted. So I bought an afpr and the issue still persists. Turning the fp down does nothing. When I key on the fuel pressure does it's normal thing and it will hold a psi within the normal range but once the car is started it maxes out. The car runs and can be driven this way. I've heard high fp can be a kink or obstruction in the return line but no kinks and what are the odds an obstruction happened right when I changed the pump, I never even touched the return line during this whole process. Any ideas? I'm at a loss here..

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Check your return line. I am not sure on your car, but my 240z had a tiny return line. I used the vent line that usually runs down the driver's frame rail as many do on the 240z for fuel injection. I have never had any issue with the stock RB fuel pressure regulator on the walboro pump. This is my third car running that combo. Pull the rubber return hose off the hard line and put it into a gas can and see if the problem persists.

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Yeah it's running back through the tiny little return line. Interesting I never heard of running the return through the vent line, the cars efi setup was already installed when I bought the car. With the old pump my FP was always around 43psi which should be 36 iirc. I'll try the gas can test and see what happens.

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If you get your in and out backward on the FPR it will max out the pump pressure.  Pushing on the wrong side of the FPR valve.

Pretty sure I've got that part right. The return line to the gas can solved the high pressure. There's definitely some good flow returning the fuel back to the tank, it's no wonder that tiny return line isn't up to the task. With the gas returning to the can my vac pressure dropped 5 psi or so..I'm assuming that's normal considering there was zero back pressure on the fpr at that point?

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The stock 240z return line is definitely too small. Your FPR can only do it's job if it's the most restrictive part of the flow path. I grabbed a roll of -6 cunifer line on eBay that I'll be running this weekend for both my feed and return. Since I'm going to start with a high pressure pump down-regulated to 3psi for SUs I'm wishing I would have gone with -8 since I'm concerned the -6 is going to be too restrictive. 

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The vent line does go back behind the head and joins in with the rest of the fuel lines. I cut it off near where they all join up and connected it to a rubber hose. I installed the regulator directly above it in the firewall. Looked clean. The hard line joins in with a hose in the back that goes to the top of the vapor tank. If you don't rearrange it, it will still work. It will just sound like someone pissing in your gas tank. I put the return line in one of the vent lines in the top of the fuel tank right above the original lines. I then ran the line that came off that tank vent to the original return with some size reducing. Kinda switching the two hoses.

 

Not really sure why your vacuum changes. Maybe when the fuel pressure equalizes, the engine runs the way it should, changing the idle load.

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Seems like a person could set up a parallel system, with a second FPR back by the tank, set to a few pounds higher pressure.  The fuel rail line would take only the flow it needed, as it needed it.

 

On the other hand, a lower rate pump might work too.  The reason the small return doesn't work is because the flow rate of the Walbro 255 is so high.  I think my numbers below are right, the Walbro 255 will support 650 horsepower.  Don't think that RB25 is making that.  Just another view...

 

255 liters/hour = 4,250 cc/minute

 

Punch in one injector and 650 horsepower target - https://www.rceng.com/technical.aspx

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I cut the vent hard line at the passenger side and still used the old small return nipple on the tank. It got rid of the crazy high pressure but I suppose I should still run it to a larger vent port up top, just not easy to get my hands up in there. I've always used Walbro 255's in my turbo cars. I just know it's a reliable brand, I didn't realize it would cause this domino effect in my fuel system hah.

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Ok last question hopefully. I ran the new fuel return to the 3/8 vent nipple above the fuel sender. I've got the 1/4" rubber line that was for the original vent hard line running to the unused little fuel return line. My question is what do I do with the 3/8 vent tube I removed from the top of the tank? Does that just need to vent to atmosphere somewhere now?

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