I'd like to run one more test on my fuel system. I want to "cap off" the return line and test the pressure with key-on/eng-off to confirm if the regulator or pump check valve is causing the system to not hold pressure.

I don't see any good places to pinch off the line, so I'm hoping to learn of a way to cap off the line at the point where the return line leaves the fuel rail.

Anybody ever do this ?

 

What youR trying to do is not normally done on a 2 pipe system.
Most pumps are capable of close to 100 psi. If you close off the return line expect to see the pressure increase to the pump's full capability.
Dead heading could blow a weak point apart.
To see if the pump holds pressure for at least 10 minute all you need to do is look at a pressure gauge on the fuel rail to see if the reduced rail pressure holds.
The return line is open ended anyway so the regulated reading at the rail is still indicative of the pump performance.
That is what is most important..
The system design is such that when each individual injector opens there is no significant pressure drop to the rest of the injectors. the regulator is pretty fast acting..
The other parameter is pump volume of flow, the more restriction in the system the lower the flow volume. It is important as the injectors along with pressure must have the volume feed to the rails or the motor goes Lean especially at high RPM.
Good luck.

 

I already know it doesn't hold pressure. I am trying to verify if it is the check valve on the pump or a bad regulator. If you know another test to find out please advise.

 

I described in some depth the system operation.
The only other fault that could cause a leak down of pressure is the fuel regulator vacuum diaphragm can leak fuel through the diaphragm directly into the intake causing a rich condition detected by the Ox sensors that will shift the long term fuel tables rich causing codes both banks to set 172/175.
In another case unrelated to the fuel pump or the regulator is leaking fuel injectors will bleed down pressure and can cause the above codes.
Look at the fuel tables to see if there is an excess fuel condition shown by shifted tables.
If ok there is a fault with the pump.
You have to go with system operations and not short cuts that bring about doubt in the long term after the work and expense has been done.
The very least that could be done is test fuel output from the line in an open condition. Even then volume delivery will not be proven adiquit for a closed system operation.
Also, dead heading a high pressure pump can cause the pump motor current draw to increase. It generally runs at about 5 to 7 amps+/- and could rise well over that range. The pump is a magnetic repulsion type that has an average load impedance. Once it is stopped under pressure, the impedance goes down, current rises. If the pump does not build any pressure, the current is likely to remains the same value seen or nothing if the pump or it's wring is open.
Good luck.

 

I got PO171/PO174 codes - lean mixture on banks 1 and 2.

THE FREEZE FRAME DATA IS:

Fuelsys 1 -- CL
Fuelsys 2 -- NA

Load Pct -- 62.0
ECT -- 149

Shrt FT1 % -- 4.7
Long FT1 % -- 28.1

Shrt FT2 % -- .8
Long FT2 % -- 24.1

RPM -- 1417
VSS (MPH) -- 23

-Checked vacuum lines/intake duct for loose/damaged lines - None found
-Ran a home smoke test - no leaks detected.
-Replaced PCV valve - no change
-171/174 codes only appear under load
-Replaced fuel filter - no change
-Vehicle is running smooth and normal right now.


-RAN FUEL PRESSURE TEST:

Key on/engine off
-goes to about 30 psi
-does not hold pressure

Key on/engine on
-30 psi steady @ idle
-snapping throttle increases pressure about 5 psi
-regulator w/o vacuum hose increases pressure to 40 psi steady
-no fuel detected in regulator vacuum connector

 

From your report, the long term fuel table reading are the cause of the 171/174 codes.
To acquire this, the OX sensors must detect excess oxygen in the combustion exhaust on an ongoing basis to cause the tables to shift.
These tables are not instant acting because they are more like peg count tables 'over time'. Only the short term fuel tables are near instant acting.
These two tables are quite different in the way they act.
.
The fuel regulator is acting normal as you remove vacuum to see the fuel pressure rise as it is supposed to do as the throttle is opened (vacuum reduces) in the intake tract to cause the rise.
No sign of gas in the vacuum line says the regulator diaphragm is good.
If the fuel pressure does not fall under increased engine RPM it boils down to an intake vacuum leak that is being missed, that is causing the 171/174 codes. As a long shot, don't discount the mass air meter as a cause as it also has a direct influence on the tables.
Not holding pressure likely is the fuel pump check valve.
This would be two different issues at the "same" time.
Pressure not holding very long after shutdown would show the check valve in the pump as not holding but good pressure while running does not depend on the check valve.
Check valve failure is more like a nuisance and may show up as a tendency to a slow start or excess long cranking than anything else because at key on it supports holding fuel pressure after the pump stops, then restarts at cranking to continue building pressure for fast starting the injectors need..
I think you have all the info you need to see the full operation and see the problems present.
You can get away with a check valve issue but the 171/174 codes need addressing first then the pump later.
Good luck.

 

Another note that is not applicable here but a demonstration of the effects of restricting the return line;
In some super charging installations of yester year, to get more fuel under boost conditions, a special boost sampled port controlled restriction valve was placed in the return line.
The effects of this raised the fuel rail pressure as boost rose.
This causes the fuel injectors to meter more fuel to support the power increase due to blower boost pressure increases.
Boiled down, the stock injectors meter fuel under the rated 35 psi standard or in pounds per hour and can support up to about 275 HP.
Beyond this level, more fuel is needed to be metered by the injectors by either raising the fuel pressure ...or... making the pulse width longer in the program..
When the fuel pressure is increased the fuel metering also increases through the injectors (at the same pulse widths) they are operated at.
There are limits to how far this can be done before a larger injector has to be used.
If this did not happen the engine would go lean and be damaged.
More power must burn more fuel.
Last point, larger injectors do not increase engine power by temselves. If larger injector are installed in a stock system the PCM narrows the pulse width out of program operation. This comes about because the Ox Sensors control the fuel tables and will shut down the pulse width trying to get back to stock metering.. This 'could' affect/cause rough idle do to being to rich and /or not metering fuel 'reliably' at such narrow pulse widths from larger injectors
This is how injector sizing is engineered for a specific engine design to satisfy both idle mixture and enough fuel at wide open throttle with some head room not to go Lean.