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Stock FPR with walbro 255


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#1 MerloZ

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Posted 3 weeks ago

Ive read another post on here that the walbro 255lph worked for them on their l28et 280zx. I have recently megasquirted my car and its running rich due to fuel pressure. My car is an l28et swapped late model 260z. The engine has stock injectors and stock fuel rail. Also the Stock FPR that might be damaged. I have a fuel pressure gauge in line before the injectors and the FPR after the injectors. I have the walbro 255lph pump in line and fuel pressure is reading about 44 without any vacuum disconnected. And around 40 with it disconnected. Anyone have any ideas. Is the pump too much for the fpr or is the fpr bad? . Also i belive all my lines are 5/16. Thankyou

#2 NewZed

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Posted 3 weeks ago

You have fuel lines for carbs so you might have the common problem of a too-small return line behind a high volume pump.  

 

You seem to have your pressure readings backward, you show pressure higher without vacuum, but that's not how it would work.

 

You can test if the return line is are too small by disconnecting the return line hose after the FPR and running it in to a gas can.  See what pressure reads then.  If it goes down, then your return lines are causing a pressure increase.  The stock FPR should give 36-37 psi.



#3 MerloZ

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Posted 3 weeks ago

Ok ill try that. But no i have pressure higher with vacuum

#4 cgsheen

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Posted 3 weeks ago

The stock FPR should maintain an approximately 36 PSI pressure differential between fuel pressure and manifold pressure.  So, higher pressure with vacuum is wrong - fuel pressure at the rail should drop corresponding to the amount of vacuum (10 PSI vacuum (-10 PSI) should equal ~26 PSI fuel).  Higher fuel pressure with boost is correct - add the boost amount to 36.


1974 "Early" 260Z: '83 L28ET - F31 ECU MAF Coil TPS - T04E - FMIC - 400cc Injectors - MTX-L wideband - NIStune enabled! - '83 5 speed - coilovers - Big Brakes up front - Sakura Garage, Tempe, AZ


#5 MerloZ

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Posted 3 weeks ago

The stock FPR should maintain an approximately 36 PSI pressure differential between fuel pressure and manifold pressure.  So, higher pressure with vacuum is wrong - fuel pressure at the rail should drop corresponding to the amount of vacuum (10 PSI vacuum (-10 PSI) should equal ~26 PSI fuel).  Higher fuel pressure with boost is correct - add the boost amount to 36.

Whoops sorry you are both right i have 40 psi with vacuum and 44 with vacuum disconnected and crimped. All my fuel lines are 5/16 OD. I dissconected the outlet of the stock FPR and ran the pump (not the engine) with the fuel going into a gas can. So this deletes the return line from the equation and i still get the exact same reading 44 psi. now when i remove the stock FPR and run fuel into the gas can fuel pressure reads at 22 psi. I am now going to order an aeromotive FPR and gauge however do or will i need bigger fuel lines. with the walbro 255 and a 5/16 line from the tank to the fuel rail at full throttle will my stock injectors use more fuel than the 5/16 lines can handle. if thats the case i assume i will also need a larger fuel rail and probably return line correct? thankyou.



#6 NewZed

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Posted 3 weeks ago

You've shown that the return lines don't affect pressure at battery voltage.  The only way to get a restriction would be at higher voltage, when the alternator is working, making the pump spin faster.  You can test that by reconnecting everything properly, then revving the engine enough to max out your voltage readings.  If fuel pressure doesn't increase, there's no problem.

 

As far as the 5/16" (8mm) supply lines go, they're the same size as the ZX Turbo supply lines.  Overall, it looks like you don't really have a problem with fuel supply, and with Megasquirt you can tune around whatever fuel pressure you set.  So you're probably wasting your money on the Aeromotive parts.

 

If you hadn't asked the direct question about your fuel lines somebody probably would have pointed that out.  You should be focusing on tuning your Megasquirt.

 

Edit - actually the site's activity levels are way down, and there was an outage a day ago.  So your thread isn't really getting much attention.  Plus your title isn't very informative.  No offense.   But an interesting title gets more views.


Edited by NewZed, 3 weeks ago.


#7 MerloZ

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Posted 3 weeks ago

You've shown that the return lines don't affect pressure at battery voltage.  The only way to get a restriction would be at higher voltage, when the alternator is working, making the pump spin faster.  You can test that by reconnecting everything properly, then revving the engine enough to max out your voltage readings.  If fuel pressure doesn't increase, there's no problem.

 

As far as the 5/16" (8mm) supply lines go, they're the same size as the ZX Turbo supply lines.  Overall, it looks like you don't really have a problem with fuel supply, and with Megasquirt you can tune around whatever fuel pressure you set.  So you're probably wasting your money on the Aeromotive parts.

 

If you hadn't asked the direct question about your fuel lines somebody probably would have pointed that out.  You should be focusing on tuning your Megasquirt.

 

Edit - actually the site's activity levels are way down, and there was an outage a day ago.  So your thread isn't really getting much attention.  Plus your title isn't very informative.  No offense.   But an interesting title gets more views.

I do have an Ms2 and i do realise that i should be able to tune around it however with fuel pressure at 40 at idle thats 10 psi more than stock and when i lower my VE table numbers the car starts to lean out a bit but the numbers are way too low. at idle a VE number of  1 is still too rich. what does battery voltage have to do with it if the tested fuel pressure with and without the engine running is the same with the FPR connected?



#8 NewZed

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Posted 3 weeks ago

It's 4 psi higher.  Stock is 36.  Really not much at all.  Modern cars use 3 bar or higher FPR's.  43 psi.

 

You originally asked if the pump might be pumping too much fuel for the return line.  Volume increases with voltage for an electric pump.  Couldn't find a plot but here's a couple of charts, one at 12, one at 13.5.  For the same pressure you can see that output increases.  It's the increased output that will cause a backup if your return line is too small, and a pressure increase.

 

http://walbrofuelpum...7-fuel-pump-e85

 

Edit - you didn't say that you revved the engine when you measured pressure.  Most Z's and ZX's have low voltage at idle.  You have to increase RPM to see the max voltage, giving max pump flow, like you would if you were driving, not idling.


Edited by NewZed, 3 weeks ago.


#9 MerloZ

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Posted 3 weeks ago

It's 4 psi higher.  Stock is 36.  Really not much at all.  Modern cars use 3 bar or higher FPR's.  43 psi.

 

You originally asked if the pump might be pumping too much fuel for the return line.  Volume increases with voltage for an electric pump.  Couldn't find a plot but here's a couple of charts, one at 12, one at 13.5.  For the same pressure you can see that output increases.  It's the increased output that will cause a backup if your return line is too small, and a pressure increase.

 

http://walbrofuelpum...7-fuel-pump-e85

 

Edit - you didn't say that you revved the engine when you measured pressure.  Most Z's and ZX's have low voltage at idle.  You have to increase RPM to see the max voltage, giving max pump flow, like you would if you were driving, not idling.

Fuel pressure for a stock EFI Z is 37 at WOT! Fuel pressure at idle is supposed to be 30 not 36. with the engine running and the vacuum line plugged and disconnected my FPR should read 37 simulating 0in/Hg or WOT. when i connect wacuum fuel pressure should drop to about 30 psi because the engine idles with a vacuum of about 19 in/Hg. so 10 Psi of fuel too much not 4 psi. I did not rev the engine when i measured pressure i never thought about voltage however, my pump is pumping TOO much at idle so i would like to fix this before i fix getting TOO much with the engine revved up getting more voltage.



#10 TimZ

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Posted 3 weeks ago

Fuel pressure for a stock EFI Z is 37 at WOT! Fuel pressure at idle is supposed to be 30 not 36. with the engine running and the vacuum line plugged and disconnected my FPR should read 37 simulating 0in/Hg or WOT. when i connect wacuum fuel pressure should drop to about 30 psi because the engine idles with a vacuum of about 19 in/Hg. so 10 Psi of fuel too much not 4 psi. I did not rev the engine when i measured pressure i never thought about voltage however, my pump is pumping TOO much at idle so i would like to fix this before i fix getting TOO much with the engine revved up getting more voltage.

I agree that the fact that it's only dropping by ~4psi between zero vacuum and 19"Hg is a bit more troubling than the base pressure - where are you measuring the idle vacuum?  Could be a cracked/leaking vacuum connection to the regulator.  Do you see 19"Hg if you tee into the vacuum line directly at the regulator?

 

As far as fixing the pump pumping "too much" at idle -there is no fix for this, it's doing what it's designed to do.  The pump is going to pump what it pumps for the given voltage and line pressure.  It's the regulator's job to bleed off the excess flow to maintain the desired pressure.  

If you look at the flow chart below - if you are running at 12V (measured at the pump) at idle and want to have a 30psi system pressure, the total flow of the injectors plus the FPR's bypass flow  _must_ equal ~65gph.  For the same conditions and 13.5V the Total flow from the pump must now be 75gph.

gsl392.jpg


Edited by TimZ, 3 weeks ago.

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#11 NewZed

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Posted 3 weeks ago

The situation seems more clear now, and we have a new fact, the 19 inches of intake vacuum.  That wasn't stated before.  If you assume that all of the measured numbers are correct and properly taken, then the FPR alone seems possible as the problem. The FPR is just doing a balancing operation between fuel pressure on one side and air and spring pressure on the other.  Seems like the diaphragm would have to be abnormally rigid, stopping the movement of the valve to get the 40 instead of 34.7, like the diaphragm is not reacting to air pressure as it's supposed to.  They do get brittle with age, I've taken an old FPR apart and the diaphragm cracked and split in to pieces.  Not rubbery at all.  

 

So the uncontrollable richness could be from a damaged/split diaphragm letting fuel through the FPR directly in to the manifold.  Not uncommon.  Removing the hose and looking for liquid fuel in it will tell if the FPR diaphragm is damaged.  You can also check pressure leakdown.  Watch pressure when the pump is turned off.  It should hold at least 20 psi for hours or days.  

 

An old brittle cracked diaphragm in the FPR seems to fit all of the facts.

 

By the way, the Aeromotive FPR's leak down immediately when the pump turns off.  Very annoying.

 

 

 

 



#12 MerloZ

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Posted 3 weeks ago

The situation seems more clear now, and we have a new fact, the 19 inches of intake vacuum.  That wasn't stated before.  If you assume that all of the measured numbers are correct and properly taken, then the FPR alone seems possible as the problem. The FPR is just doing a balancing operation between fuel pressure on one side and air and spring pressure on the other.  Seems like the diaphragm would have to be abnormally rigid, stopping the movement of the valve to get the 40 instead of 34.7, like the diaphragm is not reacting to air pressure as it's supposed to.  They do get brittle with age, I've taken an old FPR apart and the diaphragm cracked and split in to pieces.  Not rubbery at all.  

 

So the uncontrollable richness could be from a damaged/split diaphragm letting fuel through the FPR directly in to the manifold.  Not uncommon.  Removing the hose and looking for liquid fuel in it will tell if the FPR diaphragm is damaged.  You can also check pressure leakdown.  Watch pressure when the pump is turned off.  It should hold at least 20 psi for hours or days.  

 

An old brittle cracked diaphragm in the FPR seems to fit all of the facts.

 

By the way, the Aeromotive FPR's leak down immediately when the pump turns off.  Very annoying.

My FPR does not leak down nor does it leak into the vacuum line but it is old. I understand that the aeromotives leak down, however why would this be an issue at all, the car doesnt need pressure when the car is off. My stock FPR holds that pressure. Vacuum is measured by my boost gauge and my MAP sensor they both read the same but i have not measure vacuum at the end of the line going to the FPR i doubt its leaking though. all three of these things are connected to the intake manifold.



#13 TimZ

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Posted 3 weeks ago

My FPR does not leak down nor does it leak into the vacuum line but it is old. I understand that the aeromotives leak down, however why would this be an issue at all, the car doesnt need pressure when the car is off. My stock FPR holds that pressure. Vacuum is measured by my boost gauge and my MAP sensor they both read the same but i have not measure vacuum at the end of the line going to the FPR i doubt its leaking though. all three of these things are connected to the intake manifold.

Maintaining fuel pressure with the pump off is desirable for prevention of vapor lock in hot weather and for faster startup.

 

Is it that much trouble to tee into the line at the FPR and look?  "I doubt" is not helpful.  You're down to either the FPR is fine and you have a cracked vacuum line going to it (in which case your new FPR will do the exact same thing), or your FPR is bad.  Checking the vacuum at the FPR is what tells you which situation you have.  If this is too much work for you then we can't help you further.


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#14 MerloZ

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Posted 3 weeks ago

Maintaining fuel pressure with the pump off is desirable for prevention of vapor lock in hot weather and for faster startup.

Is it that much trouble to tee into the line at the FPR and look? "I doubt" is not helpful. You're down to either the FPR is fine and you have a cracked vacuum line going to it (in which case your new FPR will do the exact same thing), or your FPR is bad. Checking the vacuum at the FPR is what tells you which situation you have. If this is too much work for you then we can't help you further.



#15 MerloZ

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Posted 3 weeks ago

Who said its too much work for me are you joking lol. Ill check vacuum soon.

#16 NewZed

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Posted 3 weeks ago

The things that have been suggested are very well known, several to many threads are on this site about any one of them, and easy to check and fix.  That's the basic point.  This thread is kind of like an engine that we're waiting on to warm up so that it will start running right.  And really, we're mainly here for the puzzle, and a reason to go look at old information.

 

Looking back over the whole thing, you have to wonder if it's not just a bad gauge.  If you don't have fuel leaking in to the vacuum line, then the only thing that's really the most problematic to work around is the lack of correct pressure correction from the vacuum hose.  But even that shouldn't be an issue if you're using a MAP sensor, I think.  As long as it's consistent.  If it's consistent then you can just change the injector times based on your MAP signal.

 

Circling around to the beginning - is there really a problem here, besides tuning MS?  There doesn't seem to be one.  One thing you might do is make a table of fuel pressure versus MAP reading and see if it's consistent under a variety of conditions.  If it it's not consistent then you'll have problems.  If it is then there's really no physical/mechanical problem to be fixed that's going to make tuning easier.  You'll be in the same boat with the new parts.

 

Here's a reference about FPR's and MS.  They're not essential (he contradicts himself in the text, kind of funny) but they make life easier.  But you can tune around the lack of vacuum correction. Way down at the bottom.

 

 http://www.megamanua...ual/minj.htm#fb

 

 

Attached Files



#17 TimZ

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Posted 3 weeks ago

Here's a reference about FPR's and MS.  They're not essential (he contradicts himself in the text, kind of funny) but they make life easier.  But you can tune around the lack of vacuum correction. Way down at the bottom.

 

 http://www.megamanua...ual/minj.htm#fb

Well, yes and no - for a NA engine you can likely get away with it, but the lack of vacuum correction does limit your injector's dynamic range.  For boosted applications the dynamic range of the injector is so compromised that it would be a _really_ bad idea.


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#18 NewZed

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Posted 3 weeks ago

That's stated in the reference.  And now you're writing in vague terms.  Really-bad?  We're trying to be specific, I thought.  It's a convenience but it shouldn't be causing the complete inability to get his engine running well.  To the limit of the limited dynamic range.  He's running stock turbo injectors.

 

Kind of raises another question - how rich is rich?  He never really said.



#19 NewZed

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Posted 3 weeks ago

Maybe it's just a bad AFR meter.



#20 TimZ

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Posted 3 weeks ago

That's stated in the reference.  And now you're writing in vague terms.  Really-bad?  We're trying to be specific, I thought.  It's a convenience but it shouldn't be causing the complete inability to get his engine running well.  To the limit of the limited dynamic range.  He's running stock turbo injectors.

 

Kind of raises another question - how rich is rich?  He never really said.

Okay - 97.8% bad.  :mrgreen:


Edited by TimZ, 3 weeks ago.

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