Residual Fuel Pressure EFI

[stretch]

Installed a GM unit in tank, seems to work well. Going from external pump. What I’ve noticed with this new pump, once the engine is turned off, the pressure will drop from mid 40 psi to 20almost immediately. Over the coarse of the next hour, it will drop to zero. No problem with restarting the engine or anything. I just remember my external pump would hold 20 or so psi for hours or more. I’ve checked for leaking fittings on the fuel line, nothing found. Should I be checking into this further, or could this just be a function of the new pump?

 

[DirtDonk]

Without any external leaks in the plumbing, there are still three places that I know of that can leak down pressure.
One is the fuel pressure regulator if such exists in line, the next is one or more injectors, and the next is the pump itself I believe.

Does your GM module have its own built-in internal regulator, or is the regulator up on the engine?

 

[stretch]

Standard early 90s mustang setup. Can’t comment on the pump regulator. So I guess what I’m hearing is the line should be holding a residual pressure? That’s what I thought I remembered from the external pump. Unfortunately for me when I changed the pump, I also changed the fuel lines to NiCop a nd installed new injectors. I think Ill start with the regulator. Heck at this point it has at least 30 years on it anyway and go from there.

 

[DirtDonk]

Yes, it should hold almost full pressure for at least a couple of hours if I remember, and all is sealed up tight.

In your case the only regulator in use theoretically is the one up on the fuel rail on top of the engine. Plus you've got 8 injectors that could be dribbling off pressure slowly. Even one can do it, but if you have several that are just slightly leaking, that can drain the pressure pretty quickly I would think.
I believe most (if not all) pumps have a check valve that is supposed to be able to keep it from losing pressure and prime.

If the only thing you'd changed was the pump, then it would be suspect. If you changed multiple things and it only started bleeding down after, then they're all suspect!

Good luck. Tracking stuff down is fun, right?
Yeah, right...

Paul

 

[jmhend]

Installed a GM unit in tank, seems to work well. Going from external pump. What I’ve noticed with this new pump, once the engine is turned off, the pressure will drop from mid 40 psi to 20almost immediately. Over the coarse of the next hour, it will drop to zero. No problem with restarting the engine or anything. I just remember my external pump would hold 20 or so psi for hours or more. I’ve checked for leaking fittings on the fuel line, nothing found. Should I be checking into this further, or could this just be a function of the new pump?

Can you access the return line at all? Either at the tank or the engine?

The reason I ask is if you can disconnect it and add a hose to it and safely rout it to an empty container (it doesn't have to be huge). Prime the engine and tuen the key off. Look at the container (it should be empty). Let it sit as.is overnight and see if any gas is in the container if so you know where it is going when it loses pressure. Make sense?

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[stretch]

I can access it, and t that’s a really good idea. Thanks

 

[jmhend]

I can access it, and t that’s a really good idea. Thanks

Good luck. I am not saying you will get the answer that you are looking for but it is a place to start. My thought is that the pump may have more than 40 psi and it is pushing past the fuel pressure regulator or the pump its self is not holding positive pressure.

Just my thoughts. Can’t wait to hear the results either way.

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