PMO Reg Won't Go Lower Than 5 PSI

[Gordo2]

I hooked things up for a leak test today using a loop on the PMO regulator to recirculate the fuel.

With the PMO reg cranked all the way down - the fuel pressure remained pegged at 5 PSI.




Background

I've Cleared the Return Line...

Before I started routing any of the new fuel lines, I found I needed to clear a blockage on the return line. I found the blockage by accident - my plug in the return line inadvertenly fell out and no fuel came out of the line that was detached at the tunnel.

I found a blockage in the fuel tank return - you could remove the return line at the tank and nothing came out (while it had 10 gal).

I managed to clear the blockage following a few recommendations on the board (coat hanger etc), and was ultimately able to get compressed air to flow freely from the engine compartment return fitting, all the way into the fuel tank.

With the blockage cleared - the fuel seems to be recirculating through the return. The return line feels like fuel is flowing through it and the metal line feels cooler to the touch after the pump has been run for a minute or two.

Additionally, the fuel pump seems to be flowing the fuel, staying nice and cool.

Is the recommended 3.5 PSI supposed to be set with the engine running? Would that make a difference?

What Else Can I Do???

Thanks, Gordo

[timmy2]

Did you drain the tank and find what was blocking the return flow in the tank?
Perhaps it has found it's way back to block it again?

[BURN-BROS]

Regulators can only regulate a % of pressure. If your pump output too much then you cannot achieve desired pressure.

[Jack Stands]

What fuel pump are you using?

[Gordo2]

I'll hit these in order:

Yes, I checked the tank (through the fuel level sender) when I found the return was plugged - I was happily surprised to find it clean as a whistle for the most part, but it did have some rusty surface residue around the return assembly.

I'm running a Walbro GCL604 - which has specs/performance close to the original Bosch CIS pump:
- Operating Pressure: 73 PSI
- Minimum Flow @ Outlet: 56 GPH High Pressure

How big of a deal is 5 PSI vs. the preferred 3.5 PSI?

Gordo

[harvardma]

In your initial post you say you have the regulator cranked all the way down. You need to go up (counter clockwise) to lower the pressure.

[BK911]

I had the same problem on an 80. I disconnected one fitting at a time until I found the restriction.

Disconnect the return at the tank and dump into a gas can. Check pressure. If good restriction is at tank. If still too high go to next fitting back. Eventually you will narrow it down and find the restriction.

[toddu]

Gordo, are you doing this sans engine?

Todd

[Jack Stands]

Or switch to a low pressure pump like a Pierburg, making the regulator less necessary. It doesn't take too much pressure to overwhelm the needle valves on Weber carbs, I'd try to stick to 3.5 psi.

[911pcars]

Originally Posted by Jack Stands

Or switch to a low pressure pump like a Pierburg, making the regulator less necessary. It doesn't take too much pressure to overwhelm the needle valves on Weber carbs, I'd try to stick to 3.5 psi.

The fuel return line could dump excess fuel into a gas can and the system should still regulate the fuel pressure at the selected threshold. If it's different - tank vs gas can, there remains an issue in the return line.

Agree. Asking PMO's rudiimentary pressure regulator to drop 73 psi line pressure to 3.5 is probably an excessive demand. Try a low pressure pump (Pierburg, Airtex, etc. or equivalent). My 2.7 RS-spec engine managed nicely with a dead-head circuit (no fuel return) using an Airtex 4-5psi pump (mounted at the tank). With the usual pressure drop between the pump and carbs, the fuel pressure measured at the carbs was 3.5-4 psi.

Sherwood

[cabmandone]

Originally Posted by toddu

Gordo, are you doing this sans engine?

Todd

I know he has been doing the fuel pressure test without the engine in. We were discussing this yesterday and both of us thought the fuel to the carbs would change things and the pressure would drop when the engine is running versus just returning all fuel to the tank.

[BK911]

The pump will not create 73 psi unless dead headed. a completely unrestricted system will show near zero psi. The pmo regulator creates the restriction to produce the 3.5 psi system pressure. You need to go point by point to find the restriction. There is one somewhere.

[Gordo2]

Originally Posted by BK911

The pump will not create 73 psi unless dead headed. a completely unrestricted system will show near zero psi. The pmo regulator creates the restriction to produce the 3.5 psi system pressure. You need to go point by point to find the restriction. There is one somewhere.

BK,

Are you running a CIS / fuel injection pump, and if so, once you fixed the blockage how low were able to dial the PMO reg to (lower than 3.5 PSI)?

Thanks again,

Gordo

[mreid]

Here is my set up:



I am using the stock rear mounted CIS fuel pump with the stock return line to the tank. Rebuilt 1992 3.6 with PMO 50s. It is rock solid at 3 psi while the engine is running.

[BK911]

About 2 psi with no restrictions with stock cis fuel pump. My saga:

PMO fuel regulator

Post 19 is when it finally sunk in.

[911pcars]

Originally Posted by BK911

About 2 psi with no restrictions with stock cis fuel pump. My saga:

PMO fuel regulator

Post 19 is when it finally sunk in.

True. The pump will create whatever pressure is necessary according to the restriction imposed on it by a regulator under normal operating conditions. That's all and good. However, in case of an inadvertant restriction in the return line (e.g. collision damage, debris blockage or equiv.), an 80 psi pump could conceivably create higher or even max pressure. Exposing this amount of fuel pressure to the carbs that typically see 3.5 - 4 psi will result in fuel pressure overcoming the float inlet valves which could lead to collapsed floats and/or spill excess fuel into the engine which could lead to a fire or potentially hydro-locking one or more cylinders.

Using the existing high pressure fuel pump is convenient, but also overkill. YMMV.

Sherwood

[Gordo2]

Troubleshooting gone wrong.

Tonight I decided to disconnect the return line from the PMO reg and blow compressed air through the entire return line to the tank - it quickly turned into a holy crap moment.

Moments after removing the air nozzle I had a perfectly flowing 4 foot stream of fuel shooting back out of the return line

- If only Youtube were there to tape it

Obviously, if I were thinking clearly I would have removed the gas cap prior to blowing air into the tank.

Meanwhile it appears my fuel tank vent / vapor return lines are plugged or aren't capable of venting the pressure created with my air hose.

Alas, in spite of the excitement (and shower), I don't believe the tank vent blockage is related or could cause the higher than normal pressure I was seeing.

Gordo

[snbush67]

The PMO will handle 5 PSI, but the problem remains that you can not lower the pressure, I can dial mine down to 2 PSI with the pump running.

While making adjustments try tapping the dial lightly with the handle of a screwdriver, sometimes the dial can stick when making adjustments.

[cabmandone]

Originally Posted by Gordo2

Troubleshooting gone wrong.

Tonight I decided to disconnect the return line from the PMO reg and blow compressed air through the entire return line to the tank - it quickly turned into a holy crap moment.

Moments after removing the air nozzle I had a perfectly flowing 4 foot stream of fuel shooting back out of the return line

- If only Youtube were there to tape it

Obviously, if I were thinking clearly I would have removed the gas cap prior to blowing air into the tank.

Meanwhile it appears my fuel tank vent / vapor return lines are plugged or aren't capable of venting the pressure created with my air hose.

Alas, in spite of the excitement (and shower), I don't believe the tank vent blockage is related or could cause the higher than normal pressure I was seeing.

Gordo

I was on the phone with him when all this happened, he did NOT say holy crap. He did have to end our conversation very abruptly to put an end to the fuel flowing onto his floor.... I almost felt bad since I was the one who said "well check it" when he said he could put air to it and hear it bubbling in the tank.

[Gordo2]

To close this one out - I gave up troubleshooting the return line after my fuel dousing episode.

Purchased a Peirburg low pressure pump that I will be installing in the next couple of weeks.

Thanks for the suggestions and recommendations folks.

Gordo