Dropping Overnight Fuel Pressure

[chaseincats]

Quick question for you guys.

I have changed the fuel injectors, added an in-line check valve between the filter and fuel rail, and changed the fuel pump but I still get a drop in fuel rail pressure overnight.  I had a fuel pressure gauge on it and between last night and this morning it went from 36 to 20 psi and will probably be down to 0 within a day or so.

Not that this is a problem since with an electric fuel pump the car pressurizes quickly on startup but was just curious where else a check valve could be that might be failing.

[Jeff G 78]

Did you also replace the cold start injector?  If not, that could be the source of the pressure loss.

[chaseincats]

7 minutes ago, Jeff G 78 said:

Did you also replace the cold start injector?  If not, that could be the source of the pressure loss.

Sorry, I forgot about that one - yep it was replaced too.

[Zed Head]

Probably the fuel pressure regulator (FPR).  The valve seats are just flat pieces of steel.  Any rust or small particles can hang them open a bit.  I tested quite a few used ones when I had quite a few and most of them leaked down.  Took some trying to find one that didn't.

If the return line is still pliable you can clamp it and see if it holds.

[chaseincats]

1 hour ago, Zed Head said:

Probably the fuel pressure regulator (FPR).  The valve seats are just flat pieces of steel.  Any rust or small particles can hang them open a bit.  I tested quite a few used ones when I had quite a few and most of them leaked down.  Took some trying to find one that didn't.

If the return line is still pliable you can clamp it and see if it holds.

If it is the FPR, can those be serviced/cleaned or would I simply need a new one?

[Zed Head]

I don't know how it could be cleaned.  I think you'd need a new one.  Or a good used one.

Here's a  picture I took of one that I tore apart.  The fuel comes in from the side port and forces the valve open to exit down the hole in the middle of the center tube on the right.  You can see the wear marks on the seat where the exit port tube seals.

Edited August 26, 2022 by Zed Head

[EuroDat]

I worked out a method to test the system for leaks back when I was having this problem. It ended up being the check valve at the fuel pump.

Sounds like it could be your FPR, but new parts or rebuilt/refurbised parts can also leak.

It's here in Resourses --> Knowledge Base --> Fuel system