Fuel Pressure

[LGAFF]

Originally Posted by GOLDCYLON

Id go with post #5's answer. He's prob forgotten more than most of us could ever know about the LT5

I generally agree, although the response focused on the high results with the vac line off.....was not sure if he noted the PSI drop note

As noted before, there are examples of this with other cars on the net doing the same; oddly no one posts their findings, just post the issue...many with new pumps or AM FPRs

[XfireZ51]

Originally Posted by LGAFF

I generally agree, although the response focused on the high results with the vac line off.....was not sure if he noted the PSI drop note

As noted before, there are examples of this with other cars on the net doing the same; oddly no one posts their findings, just post the issue...many with new pumps or AM FPRs

I think Marc did note both vac ref and non fuel pressure.

[Hog]

Originally Posted by XfireZ51

Hog,

The injector flow rate used in the calibration is the one you would have at idle with the FPR attached to a vacuum port. In a stock cal, its set to 22.7#. In older calibrations like the one I had for my 84 Xfire, you would calculate the BPC or Base Pulsewidth Constant. Here, the cal does it for you using the IFR.

Just discussion here:
Why wouldnt you use an IFR that is without the vaccuum source plugged in?

The purpose of a vaccuum refernced FPR is to keep the pressure differential across the fuel injector as close to a constant as possible. As pressure increases inside the intake(vaccuum decrease) like when goiing to WOT, the VRFPR(vac reference fuel pressure regulator) will increase fuel pressure differential equal. Lower plenum pressure(high vaccuum-throttle closed) requires less rail pressure, and higher plenum pressure(low vaccuum-open throttle) requires less rail pressure.

So if you input an IFR based off of actual injector flow that is reduced because of plenum vaccuum, your WOT fueling would be off.

I am just used to entering IFR based off of unrefernced fuel rail pressure. Such as when getting rail pressure at KOEO(Key On Engine Off) which will be the same rail pressure with the engine running vaccuum refernced detached.

It seems like using IFR based on unrefernced rail pressure, then let the VRFPR do its work when idling or cruising when plenum pressure is low(vaccuum is high).


Using Marc's pressures 43 psi should be observed at idle vaccuum source attached, or 51 psi at KOEO or vaccuum source detached.

Yes, no? Hmmmm??

[Hog]

Originally Posted by XfireZ51

I think Marc did note both vac ref and non fuel pressure.

Marc's numbers
Yes, faulty fuel pressure regulator.

On an LT5 the fuel pressure should be 3 Bar or 43 psi at idle with the manifold vacuum on the regulator.

With the vacuum hose off or at WOT the fuel pressure should be 51 PSI +/- 1 psi.

In the range of 45 to 55 psi each psi of pressure changes the AFR about .1. At 60 PSI at WOT you could be running an AFR point rich.

VS.

LGAFF's numbers

Idle with FPR connected 50lbs
Idle with FPR disconnected....60lbs

Free Rev with FPR connected drops to 40lbs
Free Rev with FPR disconencted 60lbs
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I find it concerning that a free rev causes you FP to drop down to 40psi, thats without load as well.

[LGAFF]

Originally Posted by Hog

I find it concerning that a free rev causes you FP to drop down to 40psi, thats without load as well.

I do as well, I just wanted to make sure there is no anomaly that could cause it, just as a car cannot build boost without load, maybe there is a reason for this....I am retesting the car today to see if there is anything shutting off the 2ndry pump

[XfireZ51]

Originally Posted by Hog

Just discussion here:
Why wouldnt you use an IFR that is without the vaccuum source plugged in?

The purpose of a vaccuum refernced FPR is to keep the pressure differential across the fuel injector as close to a constant as possible. As pressure increases inside the intake(vaccuum decrease) like when goiing to WOT, the VRFPR(vac reference fuel pressure regulator) will increase fuel pressure differential equal. Lower plenum pressure(high vaccuum-throttle closed) requires less rail pressure, and higher plenum pressure(low vaccuum-open throttle) requires less rail pressure.

So if you input an IFR based off of actual injector flow that is reduced because of plenum vaccuum, your WOT fueling would be off.

I am just used to entering IFR based off of unrefernced fuel rail pressure. Such as when getting rail pressure at KOEO(Key On Engine Off) which will be the same rail pressure with the engine running vaccuum refernced detached.

It seems like using IFR based on unrefernced rail pressure, then let the VRFPR do its work when idling or cruising when plenum pressure is low(vaccuum is high).


Using Marc's pressures 43 psi should be observed at idle vaccuum source attached, or 51 psi at KOEO or vaccuum source detached.

Yes, no? Hmmmm??

With the vacuum port disconnected, the FP will be as it would be at WOT or 100kPa. As stated earlier, the fuel pressure regulator changes pressure in a 1:1 ratio in order to maintain the same pressure differential across the injector. This in turn allows the injector to continue to flow at the same rate. So PFI uses a 1:1 ratio VAFPR to increase fuel pressure in order to MAINTAIN the same fuel flow regardless of MAP. Whether at 50 or 100kPa, fuel flow will remain at ~23#/hr per injector. Raise the nominal FP, and the flow rate scales upward.
It doesn't work like this for carbs or TBI systems where a VAFPR causes the injectors to flow more by increasing FP.
Accels, for example, are rated 21# at 39psi FP. Since our nominal FP is 43psi, the injector flow rate is ~23#.
All of this is critical in calculating what the Base Pulsewidth is. Based on the BPC, the ECM increases or decreases pulsewidth to meter fuel as needed. An inaccurate BPC affects the rest of the fueling.

[LGAFF]

Ok, here is what I have found;

*Key on, not running both pumps produce good pressure;

*Using the Tech 1, I can turn on the 2ndry pump....it primes and the car idles, however the tech 1 is showing only .2 volts....plug in the primary pump....13.4

Rev the engine in the test mode, fuel pressure drops, dies


So if all of this holds true a static test of pumps is basically worthless and only good to find an absolute failure...

[Schrade]

You ARE saying FPR VACUUM dis-connected, just to be absolutely clear... right?

Originally Posted by LGAFF

Idle with FPR connected 50lbs
Idle with FPR disconnected....60lbs

Free Rev with FPR connected drops to 40lbs

Free Rev with FPR disconencted 60lbs


Does the lack of load account for the drop?

Does FSM have spec tests? With vacuum ON, and OFF of FPR?

And if FPR isn't responding properly to vacuum [presence, or absence], could it be a vac leak?

What was pressure leakdown time? Or is it holding static pressure? (sorry if you posted that and I missed it...)

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Is this FSM material?

Originally Posted by LGAFF

Ok, here is what I have found;

*Key on, not running both pumps produce good pressure;

*Using the Tech 1, I can turn on the 2ndry pump....it primes and the car idles, however the tech 1 is showing only .2 volts....plug in the primary pump....13.4

Rev the engine in the test mode, fuel pressure drops, dies


So if all of this holds true a static test of pumps is basically worthless and only good to find an absolute failure...

[LGAFF]

No thats something I was testing, going to replace the secondary pump tomorrow....my only concern is that maybe the .2 is a reference and not the 2ndry pump voltage....

[LGAFF]

In the words of Frankenstein:

FUEL PUMP BAD!

Now it goes to 55+ when at WOT.


Old Pump read 1.9 ohms, new 63 ohms, old Callaway pump I had laying around 58.....those who know more about electronics might have a thought on this...