Question about fuel pressure!

eclipsyco · Jul 26, 2014

So guys searching around I've found a few threats about this but not one with a direct answer so a few months back I installed a Fuelab AFPR and a walbro 400 pump along with 6an lines (feed and return) and aftermarked fuel rail.
The problem I'm having is that fuel pressure drops fast after I shut off the car. As I turn off the key pressure will go to about 20psi which I read is normal and from there in about 6 minutes drop to 0psi. Is that normal? Is here where I have my doubts. I have read that some will take hours or even over night to drop to 0. Most of them have the aeromotive AFPR but AFPR is AFPR doesn't really matter the brand does it, I mean is not like mine is a cheap AFPR anyway.
Cause my main problem is that it will take 3 or 4 cracks to get the car started when cold, once it builds pressure it starts at the first crank and I didn't have this problem before the AFPR and pump install.
I did bought a brand new install kit for this pump but I didn't lube the o-ring on the pump could this be the problem? Or is normal the pressure drop on the AFPR?

1SloEvo · Jul 26, 2014

I think they all should drop to 0. Why keep the line pressurized at all times when not in use? It would just create additional wear and tear on the fuel lines. That's what the whole reason for the pump 'priming' the system when you turn your key to the run position prior to cranking it over. You could just turn it the the start position a couple times without actually trying to start it. This would build a little more pressure I would think.

I actually ran that same setup on a -6 feed and then used the factory feed for a return line, with the 400lph pump and didn't have any issues. This was with an ebay AFPR.

athlete3344 · Jul 26, 2014

Is the pump rewired? It may help give it a bit of a boost during cranking.

98gstsmoke · Jul 26, 2014

Normally on all fuel systems regaudless of what it is their should be pressure unless it sits for long periods of time. The reason it takes so long to start when cold is you are priming the system all the time. This actually is hardder on the pump than it primed. You need to so a fuel pressure test with a gauge and see how fast it falls at the rail.

If it is quick then it is a pump issue. Or a leaking injector pass the seat. If it is simi slow then it will more than likley be a line issue.

eclipsyco · Jul 26, 2014

Yes guys I know it should drop to 0 and it does but I don't know if 6 minutes after shut off is normal or is too fast! And yes it takes a few long cranks to turn on specially after a couple days off.

eclipsyco · Aug 1, 2014

Hey guys so because I like conclusions and also be helpful to others I'll update this thread!
The problem of the pressure dropping so fast was the o-ring on the pump now I still loose pressure after shut down but it will take up to 15 minutes. BUT I still have hard starting issues and is just getting worse actually today car didn't start at all and drained the battery cranking for to long the problem now might be the injectors leaking I'm pretty positive about this so a removed them and send the for cleaning and flow test they will be back until next week also I will change the fuel filter. Hopefully this solve my issues of random going super rich (I started another thread about this) and hard start and also it might help with the pressure as well. I'll keep you guys updated.

Tiki2777 · Aug 1, 2014

Well to clear some stuff up in this thread...
For starters, the fuel pressure dropping to zero fast is perfectly normal for a car fitted with an adjustable fuel pressure regulator, it bleeds off the pressure.
turning the key to on will NOT prime/run the fuel pump, you will need to wire it to do that yourself if you want to build fuel pressure before you start it.

And yes, please check for leaks in your fuel system in the engine bay...no need for fires
You can usually smell it too

eclipsyco · Aug 1, 2014

Yes! true that the pump only runs when you crank it is for safety reasons! I have no external leaks what so ever and I don't smell fuel at all but sometimes it runs so rich is unbearable to even stand close to the car, in the 9.x AFR range which is damn rich even for WOT so due to that and to backfires when I crank it, plus the endless cranks just to make it start and it stalls from time to time also I get very slow throttle responce I came to the conclusion that some or al the injectors are leaking... And I'm changing the fuel filter just for peace of mind. Gas tank is pretty clean tho and fuel pump filter is brand new.

boost97gst · Aug 1, 2014

Off point but am I the only one that thinks not priming the pump is dumb? Why did mitsu do that? The 420a's primed, along with 99% of all cars. I dont see the big safety thing in wiring it like that. It just causes longer cranking which is ALWAYS bad on an engine with 0 effective oil pressure.

I just might wire in a toggle for that very reason to prime on start.

OP it is very normal for pressure to immediately drop from 43 to 20 and then taper off pretty quick. There is something that acts as a check valve of sorts in the fuel pump assembly but i cant remember. However, it shouldnt stop you from starting the car.

eclipsyco · Aug 1, 2014

Yeah guys thanks I'll have the injectors back on Monday and I'll post back, reading through a bunch of threads a came to realize that yes I'm worrying to much about the pressure and is normal actually is unusual to pressure stay longer than a hour!
Now about the safety reason I guess is in case you are involved in an accident or something, I don't really know LOL

there must be a reason tho!

OWL · Aug 3, 2014

If you want the pressure to stay at the rail you can always install an inline check valve to meet your inlet fuel flow rate.

boost97gst · Aug 3, 2014

^^^I assume you mean a check valve in the return line. The feed line would still allow pressure to bleed through the afpr back in the tank. The only thing about this is having constant pressure on the injectors while sitting. One wouldnt think that would cause a problem but if they were leaky at all you would have one flooded cylinder(or more) when you went to start it. A quick 1.5second prime would be ideal. Simple to wire in to a relay and bypass the stock one. Or just a fuel pump switch. Great antitheft device that has the added benefit of quicker starts!

eclipsyco · Aug 9, 2014

Ok guys! so problem is partially gone after injector cleaning, I'm still having starting issues tho the car died on me at a gas station after push stating it, I got back home and died on me again a couple blocks from home I had to crank it like 3 times and last one was a really long crank at least 5sec crank to get it running again! So now what? Assuming injectors are ok now and pump too what else could be!?
On a sidenote pump is no rewired! but with a 400lph pump I thing is not really necessary since I'm not even close to the power level to max it out! Also I think my battery could be dying that or either the alternator in not properly charging it could this also be a problem? I mean it crank strong but after 4-5 cranks it drops the voltage to 11.x which is not enough to crank again!

briansmith · Aug 12, 2014

Like boost97 said sounds like ur regulator has no check valve. Went u turn the key off fuel pump stops and a siphon effect starts on ur pressure line. U need either A: one way check valve or B: a way to prime the fuel system before crank. Ostrich can let u prime by pushing the gas petal to wot before crank. It's on YouTube or u need to run a switch to power the fuel pump but that's a pita too many switch for this or that one always get left on by accident

eclipsyco · Aug 28, 2014

Hey guys! So I'm finally updating cause I still have the same problem and is driving my nuts!
I know this started as a fuel pressure problem thread! But I have checked all fuel related stuff,
I cleaned the injectors which made it better a little bit, AFPR is absolutely not leaking! test it a few times fuel bump is sealing great no gas smell at all, when is on!, runs great idle, cruise and WOT good A/F ratios on wideband and link shows +/-0.5 fuel trims so now I'm thing maybe is voltage related problem!
So please check the following and feel free to chime in i'm open to any ideas and opinions

No gas smell under any condition (idle, cruise, WOT)
Injectors have been cleaned still show symptoms of bad injector tho I'll have to test the myself but they told be they were ok
Fuel pump is not rewired
AFPR is not leaking and holds pressure for a few minutes
Even if a prime the fuel pump still no start
Voltage on battery is 12.3v with ignition on
After 5 long crank battery voltage drops to 11.8v - 11.9v
If a bridge start it with other car it will always start after second crank or if I push start it
At idle no load voltage on battery is between 13.8v - 14.1v
At idle with load voltage drops to between 12.6v - 12.8v (A.C. wipers, rad fan, radio)
If I start it, it will work fine for the rest of the day but if I leave it overnight same problem again
I did a parasitic drain test and voltage draw is 0.08 amps (probably due to aftermarket alarm)
Before fuel mods a had absolutely no problems
Not OEM alternator with 2 wires only

That's all a can thing of right now, battery is around 9 months old and I haven't done a load test yet! This is my third battery since a owned the car (3 years in October)

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Question about fuel pressure!

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