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I am having a strange problem with my fuel pressure and before I blame the FPR, I want to make sure I do not have other problems.

I set the pressure to 45# (vac off for a Thrasher)and with the vac on it holds steady at 38# at idle and cracking the throttle. After driving for a while, the pressure drops to 38# with the VAC OFF! Once it drops, it holds steady. If I readjust it back to 45#, looks good at idle, but after driving, it jumps to 52# with vac off. Definite pattern of + or - 7# of the desired pressure. I intend to try a stocker to see what happens.

My question to the board has to do with an RCI aluminum fuel cell I just installed. The cell has a -8 feed in the rear sump, a -6 return on top of the cell and a vent with a rollover valve on top. It has no foam. I can hear the fuel dripping from the return and can smell the fumes as it is currently vented to the atmosphere. Is there any possibility the pressure problem could be because of the cell? I have heard that aluminum tanks, because they flex, can act like a lung. However, if the vent is functional, I have a hard time seeing how it might be the culprit.

Any input or suggested tests would be appreciated.


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Scottie's GNZ
 

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Scottie, you said ANY input
These are longshots, but it seems you have tried most of the obvious stuff.
I'll ASSume your alternator is working correctly and you have good voltage at the pump.
Your fuel might be getting aerated (sp?). It would probably be a good idea to put a hose or something on the inside of the tank and allow the fuel to return to the bottom. This should also help on the fumes, too. Another issue (that I currently have with my car), is the fuel pump is sucking so much fuel out of the sump area that when the gas gets low, it creates a little vortex and starts pulling air (similar to draining a bathtub). This shows up as low (usually 10-15 PSI on my car) fuel pressure. When you drive the fuel will slosh towards the pickup and the problem may disappear.

These are just some possible things to consider. An easy way to test both of the aeration problems would be to run a FULL tank of fuel.

Also, I hope you are using a mechanical fuel pressure guage and not an electric one.

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Discussion Starter · #3 ·
Thanks for the response. I probably need to be more descriptive of my setup. The cell size is 20x18x10 with a rear sump. There is a -8 line from the sump that goes into a Fram HPG filter which feeds into an external pump. Both filter and pump are on the side of the cell about mid-height. The pump dumps into a -8 line to the rail with a mechanical guage on the rail. The pump is "hot-wired" in my own way. I ran a 10-guage lead from the battery to the pump through a relay and used the original lead to trigger the relay.

I am speculating on the problem but puzzled. It seems "mechanical" and by that I mean the pressure only seems to change when the car is driven and there is additional load that would cause the FPR to adjust one way or the other. However, it seems to always "remember"
that it should be at 45#, because if it drops to 38# and you adjust it up 7#, it seems to add 7 to 45 and registers 52#. Then if you adjust it down 7#, it seems to subtract 7 from 45 and registers 38#. I say registers, but since I have a DS, I know the mixture is real. That probably did not read right but I have no better way of describing it.

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Scottie's GNZ
 

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I also have a fuel cell. I would suggest getting a -4 or -6 vent tube leading out side the car. Also, get some foam. The foam in the fuel cell keeps the fuel from having bubbles when it falls from the return line.
Question: If you take the fuel cap off does this still happen? If so, then the rollover vent is the culprit. It probably is not venting properly.


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Jason White
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Originally posted by V6RACER:
I also have a fuel cell. I would suggest getting a -4 or -6 vent tube leading out side the car. Also, get some foam. The foam in the fuel cell keeps the fuel from having bubbles when it falls from the return line.
Question: If you take the fuel cap off does this still happen? If so, then the rollover vent is the culprit. It probably is not venting properly.

The foam will also eliminate the bathtub drain effect. (we use it in oval track cars to eliminate side to side sloshing of fuel from the pickup. It'll also slow fuel escape in case of a rupture.....god forbid.
 
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