I drove around with a fuel gauge taped to the windshield and at WOT all the way up to a shift it never got passed 40 psi. At idle with the vac compensation line removed it was about 43.5. Dying fuel pump? No problems with bleed off or starting. No fuel in vac compensation line either.

 

My stock fuel pressure regulator doesn't seem to be very accurate either...

 

The pressure should be higher at wot I think. On mine it reaches almost 50 psi. Don't know what the minumum is.

 

You want the pressure at WOT to be as close to 43.5psi as possible. The pressure is reduced in several ways. If the pump can't supply enough pressure is one. Faulty fuel pressure regulator is another. Third is an obstruction in the air inlet system that causes excessive vacuum (low MAP) at WOT. Something as simple as a really dirty air filter can reduce air flow, and maintain a bit of vacuum at WOT, and the vacuum causes the FPR to pull the pressure down. Check your MAP at WOT and compare it to the barometric pressure. Both those data points can be obtained with a scanner or scan software.

I'd look for at least 40psi at WOT. At that level, your injectors are flowing 4% less than the programmed value. It going to run lean.

 

I would say if you disconnect the FPR hose at idle and it shows 43.5psi then keep it disconnected and floor it and watch pressure, if it stays at 40psi from say 3000-6000rpm then it's your pressure regulator.

If you disconnect the FPR hose then floor it at 3000psi and pressure starts around 43.5psi and gradually reduces to 40psi at 6000psi then it's not the regulator but is the fuel pump or clogged filter (or something else limiting max fuel flow like a kinked fuel line)

When your at WOT and RPM increase from 3000-6000 so does the fuel flow, regulator only regulates pressure not flow, if pressure is constant but low then it's likely your regulator, if pressure starts ok then drops then likely it's your pump or filter that cant keep up with the demand (or a fuel line restricion). A regulator could get "stuck" slightly closed and create an artificial high pressure condition under very low flow/demand conditions (like idle - mimicking symptoms of a bad/weak pump/filter/line) but I've never seen this happen though I did see one stick wide open creating 0 psi and ALOT of flow, resulting in what looked like a bad pump according to a pressure gauge.

 

[QUOTE=Injuneer;5858663]Faulty fuel pressure regulator is another. Third is an obstruction in the air inlet system that causes excessive vacuum (low MAP) at WOT. Something as simple as a really dirty air filter can reduce air flow, and maintain a bit of vacuum at WOT, and the vacuum causes the FPR to pull the pressure down. Check your MAP at WOT and compare it to the barometric pressure. /QUOTE]

Searched how to check for a faulty FPR. One post you had suggested pulling 15" vacuum on it and see if the diaphragm holds, it did. Also no fuel in vac comp line as well as an increase in pressure when removed. I believe it passed all of the op tests.

At 100% TPS, all of my MAP readings in Tunercat were 1.00-1.01 which I believe means 100-101 KPA. My baro reading was 1.02-1.03 at those times. Are those readings close enough?

Fuel filter was changed 500 miles ago.

Fuel pump has 140K on it as far as I know.

 

Yes... those MAP numbers are very close to BAR, as long as those were under full engine load, at very high RPM - no problem. If it was WOT at low RPM, or simply sitting still under minimum load, doesn't mean anything.

 

In the 5000 rpm range it is around 12-14 kpa difference. (.92-.9 map reading in tunercat). Baro today was 104 kpa. Still close enough?

 

barometric at my elevation is 86kpa

 

Fuel pressure should be around 43.5psi while cruising. As low as about 39.5psi (-4psi) under hard decel, and up to about 47.5psi (+4psi) at WOT.

Same idea as with a 1:1 fuel pressure regulator for boosted applications. For every psi of manifold pressure you add (boost), you need to increase fuel pressure by 1psi to maintain the same pressure difference between the 2 pressure regions.
This is why my fuel pressure climbs to about 54psi under full boost. 54psi minus the 7psi boost, minus the 4psi increase from cruising to WOT (but no boost), shows a pressure dirrerence of about 43psi across the injectors.

Under hard decel (~20 in. hg.), the vacuum will 'suck' the same amount of fuel at a lower fuel pressure, think pressure difference, not just fuel pressure

If fuel pressure stayed completely constant, the engine would run rich at low manifold kpa, and lean at high manifold kpa. (I assume) this is why vehicles with an in-tank regulator (that hold constant pressure) also have an 'injector slope' in the tuning, so you can tell it the injector changes flow rate versus manifold vacuum.

 

Sounds like way too much vacuum at WOT. You are losing more than 10% of the available pressure in your intake system, between the air filter and the intake manifold plenum. That means you are giving up 10% of the power you could be making without that pressure loss through the system. Something is wrong. Also means the vacuum at WOT is pulling the fuel pressure down by about 1.5psi. You need to find out what is resricting your intake air flow.

GM pressure spec for the LT1 is 43.5psi with the vacuum compensation line removed. Acceptable range is 41-47psi. With the vacuum line reconnected, you're going to see a 6-8psi drop in the fuel pressure. Less drop than that with a beefy cam. You are losing 3.5psi at WOT, when you shouldn't be.

 

I have a similar vacuum issue at high RPM. I have a difference of 8KPA compared to BAR. Took the CAI off and no improvement. If I take the elbow and MAF off and run speed density, I do cut that about in half.