Why do I need a Fuel Pressure Regulator ?

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jondee86
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Why do I need a Fuel Pressure Regulator ?

Postby jondee86 » Tue Jul 08, 2014 1:10 am

The AE86 GTS (and all fuel injected cars) has a Fuel Pressure Regulator (FPR).
On the GTS it is mounted on the firewall end of the injector fuel rail. Fuel enters
the FPR from the fuel rail via a bolted and O-ringed plug-in connection. Excess
fuel flows out of the FPR and back to the tank via a hosetail and low pressure
rubber hose connected to a hard line.

FSM figures are...
38-44psi no vacuum (engine off)
30-33psi with vacuum (engine idling)

Technically, the FPR is a vacuum compensated pressure sustaining valve, but
FPR is close enough. Its purpose is to maintain a constant pressure differential
between the fuel rail and the inlet manifold. Why ? The ECU pulses the injectors
for a measured fraction of a second, and adjusts the duration of the pulse (open
time) according to load and rpm. The ECU itself has no fuel pressure sensor, so
it relies on the fuel pressure differential across the injector remaining constant.

Now, one end of the fuel injector is in the fuel rail, and the other end is in the
intake manifold runner. A simple spring and diaphragm pressure regulator (no
vacuum compensation) can maintain constant pressure in the fuel rail. But what
happens if the manifold vacuum changes ? At WOT there is atmospheric pressure
(15psi) in the manifold. At idle there is a round 5psi in the manifold, and in both
cases there is around 40psi in the fuel rail.

Image

So at WOT the pressure difference is 25psi and at idle it is 35psi, which would
mean that the injectors are flowing less fuel per second of open time at WOT
than they are at idle. This is where vacuum compensation comes into play. To
correct the situation, manifold vacuum is applied to the top of the diaphragm
to offset some of the force exerted by the spring. This allows the valve to open
more, and drops the pressure in the rail. Now the rail pressure at idle will be
25psi, the same as the WOT pressure.

By varying the rail pressure to maintain a constant differential across the fuel
injectors, the FPR allows the ECU to accurately control fuel delivery to the engine.
That is why you need one :) So if one day your idle gets really rough, the engine
blows black smoke and struggles to stay alive, but your WOT performance is
fine... check to see if the FPR vacuum line came unplugged.

Cheers... jondee86
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saiGone
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Re: Why do I need a Fuel Pressure Regulator ?

Postby saiGone » Tue Jul 08, 2014 12:44 pm

Dope man. Lol i was actually looking at my FPR yesterday. Contemplating its existence.
Club 4AG member since 05 (Formally SaigonsSon)

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jondee86
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Re: Why do I need a Fuel Pressure Regulator ?

Postby jondee86 » Tue Jul 08, 2014 4:15 pm

There ya go !! The way I see it, if you have a problem that you are
trying to solve, it helps if you know how stuff works :)

Cheers... jondee86
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mad_86
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Re: Why do I need a Fuel Pressure Regulator ?

Postby mad_86 » Tue Aug 21, 2018 10:24 pm

Jondee

does the injector pusle fire more rapid at Higher RPM/load ?? That would allow more fuel to enter or is the
The Rate of injector pulse is constant and all levels of RPM.

Does the injector pulse differentiate between High comp ECU & Large port early model ECU USDM??

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Re: Why do I need a Fuel Pressure Regulator ?

Postby jondee86 » Wed Aug 22, 2018 12:22 am

Thefuel requirements of an engine vary with both rpm and load. If you can
imagine your car doing a steady 50 mph on a level road in 4th gear, the engine
is doing a certain amount of work and requires a certain amount of fuel. When
you reach an uphill grade you need to open the throttle more to maintain
50 mph. The engine is doing more work pushing the car uphill, and requires
more fuel, but the engine rpm remains the same.

For smooth running the combustion events need to be consistent, and for
this reason the injector operation is synchronized to engine speed. Essentially,
fuel is injected into the intake ports at the same point in the 720 deg engine
cycle each cycle, regardless of engine rpm So yes, the injectors fire more
often as engine rpm increases.

Injectors are basically solenoid valves. They are either open or closed. To get
them to vary the flow rate of fuel into the engine, the amount of valve open
time is increased. This is known as PWM or Pulse Width Modulation control. The
pulse width increases with the engine load. At low load the injector might be
open 5% of the available time per engine cycle, but at high load it may be open
60-70% or more. So yes, the injector pulse width increases with load.

Apart from pulse width, the amount of fuel injected also depends on fuel pressure
and the size (rated flow capacity) of the injector. The high comp (smallport) engine
has larger injectors (around 235 cc/min) compared to the early bigport TVIS engine
(around 185 cc/min). The ECU has no way of knowing what size injectors it is
controlling, so it will open the injectors for whatever time its factory fuel map says
is correct for the rpm and load reported by MAP/AFM and rpm signals.

If you install smallport injectors on a TVIS engine, it will over fuel (run rich). If the
ECU uses an O2 sensor, it will try and compensate for the rich mixture by cutting
back the injection time, but there is a limit to how much compensating it can do.
Conversly, bigport injectors on a smallport ECU will under fuel. In general you should
use the ECU and injectors that match the engine for best all round performance.

Simply installing larger injectors on a stock engine offers zero performance gain,
and the car will probably not run as well as it did with stock injectors. Larger
injectors are only required when the engine has been modified to the extent that
it can draw in more air than the stock engine could. More air needs more fuel, so
yes, your 300 hp turbo 4AGE missile will need BIG injectors :)

Cheers... jondee86
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mad_86
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Re: Why do I need a Fuel Pressure Regulator ?

Postby mad_86 » Wed Aug 22, 2018 8:28 am

:lol: Right on the nail. Hahah


rebuilt smallport engine ,20v rod n crank, 0.5+ pistons,small port intake,
Smallport,injectors 250cc,40psi @rail
But on all Largeport electronics , i know its crippling
I have a Wideband getting full lean at 1/2 -Wot ,
It runs good, at idle,
At this point im trying to get the motor run a safe rich
While i save up for aftermarket ecu and tune

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jondee86
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Re: Why do I need a Fuel Pressure Regulator ?

Postby jondee86 » Wed Aug 22, 2018 2:46 pm

Doesn't sound right to me :? I would have expected that setup to overfuel if
anything. Are you running the bigport AFM ?

Annother thing to check is the resistance of the injectors. I think that the USDM
bigport ran low res injectors with a resistor pack, but JDM smallports ran hi res
injectors (no resistor pack required). Running the wrong combo can affect injector
performance.

Cheers... jondee86
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mad_86
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Re: Why do I need a Fuel Pressure Regulator ?

Postby mad_86 » Wed Aug 22, 2018 3:45 pm

Yes bigport AFM, & Yeah i have the Resistor pack jumped to allow the hi Res. Injectors , i was thinking is it possible that at high RPM the SP injector is not responding to bigport ECU signal or injector pulse speed.

And also cause high CR result of trd head gasket and also 4-1 headers but maybe possible air leak??? but I sprayed down intake manifold & TB with spoapy water check for leaks.no air leaks.

Maybe exhaust leak before wideband idk, these injectors were purchases new/reman ,

My next move to get the injectors flow tested to make sure its 250cc and/or try bigger if any change 265cc. 2.4 2ZRFE

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Re: Why do I need a Fuel Pressure Regulator ?

Postby jondee86 » Wed Aug 22, 2018 5:10 pm

- What happens if you put the bigport injectors back in ?
- What colour are the plugs ?
- Does the exhaust smell rich at idle ?
- When you pull the dipstick can you smell gas in the oil ?

Just checking :D

Cheers... jondee86
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Re: Why do I need a Fuel Pressure Regulator ?

Postby mad_86 » Wed Aug 22, 2018 7:01 pm

To put the big port injectors i need to re-solder the bigport injectors clips first.. for low res.

Plugs are dry, brown/normal, no white fouling
Idle , wideband 12.5-14

Exhaust smells normal , no fuel smell

I havent check the dipstick smell..

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Re: Why do I need a Fuel Pressure Regulator ?

Postby jondee86 » Wed Aug 22, 2018 9:54 pm

Hmm... something doesn't seem quite right... it's like you are running out
of fuel for some reason. If your WBO2 goes full lean the engine the engine
should no longer be running... the limit for lean combustion is around 18:1
and most widebands read to around 22:1 AFR.

Air-To-Gasoline Stoichiometry Balance Limits
8:1 = Most Rich before flame-out
18:1 = Most Lean before flame-out
12:1 = Start up Mixture for reliability
14.7:1 = Optimum Mixture for peak fuel economy


So if your engine is still running at full lean and hasn't coughed its lungs out
and died, your WBO2 gauge is on the fritz :)

Cheers... jondee86
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