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Discussion Starter · #1 ·
Need some help. Be warned: this is an engine swap. Inlet to fuel pump is stock 5/16. Outlet from regulator is 1/4". Fuel pump is Walbro 255 external fuel pump. Regulator is stock. I have a pressure gauge on the supply side, just before the rail. Pressure when cranked initially hits 55 lbs, but then the needle sweeps to max pressure (100 lbs). Here's what I've done:

removed return line and ran into can- no change. fuel flowing very well into the can.
replaced FPR with stock new- no change
replaced FPR w/ Accufab FPR- no change, but car starts to stumble badly when screw is backed way out like it's not getting fuel. Gauge doesn't budge.

replaced gauge- no change.

This is a somewhat new problem. I had a plugged return line and the pressure maxed out, but I replaced the line and pressure was around 65 lbs. Replacing the stock FPR dropped the pressure to 55 lbs (for a second). Pressure has always been steady. I'm really scratching my head over this one. Car seems to run and idle fine w/ no black smoke. I've never had it on the road and would like to see the pressure come down before I do.
 

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The fuel pump always operates at max PSI, rail pressure is controlled by the FPR opening and closing allowing excess fuel back to the tank through the fuel return lines.
If pressure is high the issue is with the FPR or the return lines.
You can check all of this by manually actuating the ASD Relay without starting the engine.
1)Remove the fuel return line from the FPR, connect another piece of fuel hose to the FPR and place the other end in an approved gasoline container.
2)Manually actuate the ASD Relay.
3)If pressure now remains normal the issue is a restriction/collapsed hose in the return lines.
4)If pressure is still not normal the FPR is faulty or there is a restriction/debris in the fuel rail.
 

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Discussion Starter · #4 ·
FPR is new, but I’ve installed three different ones. I ran a 1/4 inch line directly into a gas can and cranked the car: no change, but flow was very good. I’ve read that it is possible for the fuel pump to overpower the FPR, but I don’t recall seeing this problem before. I’ll try the APR trick later
 

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I have been using a Walbro 255 on my stock VNT for over 12 years without issue with a stock FPR.
Rail pressure is controlled 100% by the FPR and return system back to the tank.
There is still the possibility that there is debris in the rail which is intermittently restricting flow out of the FPR.
You may also want to connect a different fuel pressure gauge to the service valve to be sure your other gauge is accurate.
When you manually actuate the ASD relay you will read Static Rail Pressure (max regulated pressure 53-57 PSI), if you connect a handheld vacuum pump to the FPR and apply vacuum you should see rail pressure decrease (1psi for every 2"HG applied to the FPR), as vacuum increases, and rail pressure will increase as vacuum increases.
If you have access to a handheld vacuum/pressure pump and you apply pressure to the FPR you will see rail pressure increase 1 psi for every 1 psi of pressure applied to the FPR.
 

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Discussion Starter · #7 ·
Strange symptoms. Bypass ASD and the gauge creeps to max pressure. Turn the key off and pressure gauge reads around 50. Put a vacuum on the FPR and it drops to 40, but immediately returns to 50 when the vacuum is removed. I would think once the pressure was reduced to 40 it would stay there until the pump turns back on.
 

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As long as the pump is running total pressure across the injector tip will = max regulated pressure.
As you apply vacuum rail pressure drops, when you release vacuum rail pressure will go back to max regulated pressure as long as the pump is running.
 

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Discussion Starter · #9 ·
This was done without the pump running. The pressure should have been relieved when I put a vacuum on the FPR. My return line is good
 

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Discussion Starter · #10 ·
I think I’ve made progress. I first ran the pump straight into a gas can to check flow. Good flow. I completely removed the fuel rail from the car and inspected it. Didn’t see anything. I bypassed the ASD relay and ran the return line into a can. Good pressure. I hooked everything up normally (fuel rail still outside of car) and good pressure. I realized that in my setup there is no check valve, so pressure should return to zero when car is off. Absolutely no idea how pressure remained in the system, even with the return line in a can of gas. I looked at the fuel dampener on the end of the rail as it looked like it had a spot of fuel on it, but after cleaning it and running the pump I did not see any more fuel. I doubt it had anything to do with it. No idea what happened.
 

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Discussion Starter · #11 ·
So much for progress. Reassembled and cranked; all looked good. Shut car down and noticed it still had 2 psi of pressure. Sure enough, needle pegged on restart. I’m beginning to think it’s my pump or prefilter.
 

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Discussion Starter · #15 ·
I believe I may be onto the cause. As long as I'm running the pump by shorting the ASD relay, pressure stays somewhat steady. Crank the car and it pegs the needle. Here's the theory: At a lower voltage the fuel pump runs slightly slower; just enough for the stock FPR with 5/16 input and 1/4 output to do its job. Crank the car and the voltage jumps up and the pump runs much stronger, overpowering the FPR and lines. I've double and triple checked all lines and ran thru 3 different FPR's. My Walbro is an external 255. Not sure if that makes a difference. Also not sure why some people can run this pump with no problem but others can't. I'll keep plugging along. NAJ, thanks for the help so far. Keep the ideas coming!
 

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Is your alternator putting out correct voltage? I had a GLH years ago that would randomly run like 16 volts and the car would bog due to too much fuel.
 

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Again, the fuel pump always runs at max capacity.
With the ASD relay manually actuated dead head the pump (momentarily pinch off the return line), you should see 100 PSI + immediately and it should go back to static pressure (53-57 PSI) as soon as you release the return line.

Does the problem occur if you allow the engine to run with the FPR vacuum line disconnected and plugged?
 

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Discussion Starter · #18 ·
I apologize for taking so long to bring this thread to a close. I found the problem and thought I’d share with the class. I didn’t know before, but the stock fuel rail is actually a tube within a tube, both coming together at the fuel “buffer” which is the round device at the end of the rail opposite the FPR. It has a membrane inside and is used to smooth out pressure spikes in the rail. It seems the diaphragm had ruptured, causing fuel to seep behind the diaphragm and close off the fuel supply. This caused the input line pressure gauge to peg and reducing fuel to the injectors. The FPR never had an opportunity to do its job.
275786
275787
 

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Good catch!!

A lot of guys have removed the inner tube for high boost use.

Thanks
Randy
 
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