Fuel Pressure Regulator and Vacuum pull

[timo78]

my issue
Hesitation on acceleration. I start by pressure testing the fuel rail, and things get interesting.

-At idle, I am seeing roughly 30psi.
-I rev up, and fuel pressure drops and bobbles right around 26psi which is the injectors cutoff pressure, and would explain my 'st st stutter'.
-I then remove the vacuum line to the Fuel Pressure Regulator. Pressure goes up to 36psi. I then rev the engine and pressure goes up to 40+psi.

It seems ok if the FPR is disconnected. I believe the FPR is made to open when vacuum is applied, so I would then believe I am getting too much vacuum pull.
Any ideas?

[RSorak 71Westy]

It seems to me your FPR is working backwards....usually with less vacuum FP goes up as less vac means wider throttle opening.

[Amskeptic]

my issue
Hesitation on acceleration. I start by pressure testing the fuel rail, and things get interesting.

-At idle, I am seeing roughly 30psi.
-I rev up, and fuel pressure drops and bobbles right around 26psi which is the injectors cutoff pressure, and would explain my 'st st stutter'.
-I then remove the vacuum line to the Fuel Pressure Regulator. Pressure goes up to 36psi. I then rev the engine and pressure goes up to 40+psi.

It seems ok if the FPR is disconnected. I believe the FPR is made to open when vacuum is applied, so I would then believe I am getting too much vacuum pull.
Any ideas?

I think you might have a vacuum leak and casual calibration on your FP gauge and maybe an electrical issue if the above increase in fuel pressure occurs when you rev the engine with the pressure regulator out of the picture.
You seem to be getting the proper vacuum only briefly when the engine coasts down from closed throttle rpm spike.

Manually apply vaccum to a dedicated hose to the FPR while reading the gauge. What is the drop in pressure under vacuum? Does the fuel rail hold pressure when you shut off the engine?

IIRC, injectors cut off at 14 psi.
Colin
(please edit your subject header and post to let us know model year and engine size)