fuel pressure variation

G

GT240sx

New member
Does anyone here notice a 2-3psi fuel pressure drop between fuel pressure at idle and fuel pressure at full throttle?

fuel pressure is the main variable I change when tuning the car and I have been trying to get fuel pressure to stay as constant as possible. Is it reasonable to expect fuel pressure to stay completely constant under all engine conditions?
 
I know nothing about your Toy Auto but when we had the same problem the biggest help was installing a larger fuel line from the pump in the rear to the regulator at the firewall
 
Don't know which car you're asking about, but if it's the 240SX the fuel pressure should actually increase at full throttle. The FPR is vacuum controlled - when there is a vacuum in the manifold the pressure is decreased, as the vacuum decreases the pressure increases. Because of that the pressure will fluctuate depending on the amount of vacuum, but it should always be highest at full throttle.
 
You should check that your voltage to the pump is constant across the rpm range. If you are running vacumn to the fuel pressure regulator, the opposite should happen as mentioned. There is a relationship between volume and psi as well. If all things are as they should be, then you are no longer running the engine off the pressure regulator, but the injectors at high rpm. Rather than the regulator bleeding fuel back to the return, the injectors are bleeding the fuel pressure into the engine. You may need to increase the volume your pump is putting out. We use the Bosch CIS pumps on all our cars. These are high volume rotary pumps suited to a regulated 80 psi. They are cheaper than aftermarket performance pumps, readily available, and reliable. A good quality regulator is also a must. We use ones made by aero. Not sure how much of this is IT legal though. :shrug:
 
Hey Russell. Check the voltage at the fuel pump. My factory wiring (in its 20-year-old condition) was drooping the voltage under load. I fixed it by running a 10 gauge +12 wire back to a relay that is actuated by the stock FP +12 wire.

WRT Earl's comments, the reason for the vacuum controlled regulator is so the injectors see a constant pressure differential between the fuel rail and the manifold (I just learned that last week). Use it or not depending on what it does to your AFR. When I retune my engine next spring I'll attach the vacuum line.

Still planning to do the ARRC?

[Edit: Mark beat me to it. It should all be legal - FP & FPR are open in IT]
 
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Thanks for the replies everyone. To give a little more information.
The car is an 85 Toyota supra with fuel injection and a fuel return to the tank for the bypass flow. I am not using the vacuum port, so the pressure at the regulator is not being altered by the engine vacuum.

I have upgraded to an in tank bosch 040 fuel pump. I have a mallory fuel pressure regulator that is supposed to be good for regulating fuel pressure between 30-100psi.

The pump should have way more flow capability than the engine can consume. I have checked the fuel lines, they are all clear. I did check pump voltage at idle and part throttle (12.5V), but not under load. I will give that a shot tonight

The only thing I have not replaced is the fuel pressure regulator. I am not confident it is working properly. When I shut the car down the fuel pressure drops about 17psi instantly and then bleeds down to zero in a matter of minutes. If I recall correctly, when the fpr was new, the pressure would remain very close to operating pressure (37psi) for many hours after shutting the car down. Should the pressure remain close to operating pressure when you shut the car off?

I am still working towards going to the ARRC, but we will see how everything works out.:)
 
I watched a pressure reading this week , No vac ,the pressure drops off under WOT. With vac, the # comes up, and than back down, under throttle. Hook up the vac line, make it work, and you may fix the problem.
MM
 
It is also possible your return line is too small for the volume of fuel the pump is now producing. At idle, a much larger percentage of the fuel is being returned than at full throttle. Pressure would rise because of the restriction back to the tank. If this is the case, its not that big a deal, as the problem with pressure is not at full throttle, its at idle. If you can run the return fuel outlet of the regulator to a catch can and test pressure then, and it stays stable, you have found your problem. Please don't spray gasoline all over yourself and the surrounding area, creating a fireball that can be seen from both coasts. :026: That can cut into race budget, which is always a bad thing.
 
I did some more testing this weekend with an extra pressure gauge. I put the gauge in the fuel system at the return port of the pressure regulator.

The following measurements were taken with the car at idle.
When the original return line to the tank was attached the static pressure was 3.2psi
When sending the return fuel into a bucket just below the regulator the pressure was not registering on the gauge...somewhere between 0 and 1 psi
With 20ft of brand new fuel line going from the regulator to the tank the static pressure was 2 psi

The above readings seem to be reasonable, 15ft of fuel line is going to have some pressure drop. Based on that assumption I started working with the pressure regulator.

The regulator has two springs and the stated pressure adjustment range is from 30-100psi. It seems that devices like these are not as accurate near the limits of their operation. So, I removed the inner spring and re-assembled the pressure regulator.

I have not been able to test the new configuration under load yet, but the pressure does not move as much as it did previously while revving the engine. Hopefully I will be able to get the car out this afternoon and make a few runs. Thanks for the help guys and I will post any findings.
 
Yeah, this is not unusual. There's a good book on tuning Bosch EFI that discusses this, and how it goes leanish around 4000rpm too, and what to do about it, but honestly, this ain't no street car - what do you care what idle pressure is? tune at max load WOT and check EGT/o2. that will tell you what you need to do eh? I just put a adjustable regulator in place of the stock one and tune accordingly. (I have a 85 Corolla GT-S)
 
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