Resolved Spyder Unusual Fuel Pressure Problem

[DogWhistle]

We could use your help on a frustrating mystery. ‘99 4G64 Spyder.

The fuel pressure holds steady @38psi at idle and any driving up to 3k rpm.

While driving, if you rev over 3K, the fuel pressure drops in half and *stays* there even when pulling over and returning to idle.

If we crank the pressure regulator up to 38 again, the problem is the exact same.

The fuel system is stock pump (less than 5k miles on it), new stock fuel filter, new stock injectors (less than 5k miles; all ohm the same), with a Fuel Labs AFPR with gauge.

We just installed a new regulator diaphragm and the exact problem is happening. The old diaphragm had no tears or holes.

When the regulator was apart, the tension springs was loosened all the way and gradually tightened to get the right psi.

Also, the fuel pressure drops immediately upon shut off, but can’t find any leaks.

A fuel pump rewire is next on the to do list, but we’re wondering if the return line has any components that might contribute to this?

Any insight to help narrow this down will be much appreciated.

 

[pauleyman]

We could use your help on a frustrating mystery. ‘99 4G64 Spyder.

The fuel pressure holds steady @38psi at idle and any driving up to 3k rpm.

While driving, if you rev over 3K, the fuel pressure drops in half and *stays* there even when pulling over and returning to idle.

If we crank the pressure regulator up to 38 again, the problem is the exact same.

The fuel system is stock pump (less than 5k miles on it), new stock fuel filter, new stock injectors (less than 5k miles; all ohm the same), with a Fuel Labs AFPR with gauge.

We just installed a new regulator diaphragm and the exact problem is happening. The old diaphragm had no tears or holes.

When the regulator was apart, the tension springs was loosened all the way and gradually tightened to get the right psi.

Also, the fuel pressure drops immediately upon shut off, but can’t find any leaks.

A fuel pump rewire is next on the to do list, but we’re wondering if the return line has any components that might contribute to this?

Any insight to help narrow this down will be much appreciated.

Leak at the fuel pump inside the tank.

 

[DogWhistle]

Leak at the fuel pump inside the tank.

Thank you. It sure seems like something other than the regulator is deciding what the pressure should be.

Do you recommend a new fuel pump while we’re in there?

 

[pauleyman]

Thank you. It sure seems like something other than the regulator is deciding what the pressure should be.

Do you recommend a new fuel pump while we’re in there?

This is an educated guess. Think about it though.
Doubt you need a new pump.

 

[DogWhistle]

This is an educated guess. Think about it though.
Doubt you need a new pump.

You may have been on to somthing. The fuel hose segment above the pump has become very brittle and looks like it shrunk.

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[pauleyman]

You may have been on to somthing. The fuel hose segment above the pump has become very brittle and looks like it shrunk.

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If you're going to connect it like that make sure you're using fuel hose.

 

[DogWhistle]

Here's the update. We are going to declare it resolved. It was a combination of problems. TL/DR: Less-than-tight hose connection from fuel pump to the sending unit housing and probable pin hole in the rubber section of the fuel return line.

New fuel pump and strainer installed. New tight -- fuel hose -- connection to the sending unit. Feed and return lines disconnected on both ends and lines blown out with compressed air revealing traces of sediment, but nothing major that should have impeded flow. Flexible sections of rubber fuel lines were replaced. Although not pressure tested by itself, there may have been a pinhole leak in the return fuel hose section, screwing up the return circuit.

Once the new lines were connected, we ran the engine and adjusted the fuel pressure (regulator) to 38 psi with the vacuum line attached. The system actually holds 20psi of pressure for a while after shutting off the engine.