I have low fuel pressure after 20-30 minutes of driving. It drops from 6.5psi to 2-3PSI and I feel serious loss of power.

first 40 minutes I drive like champion :-) but after that car runs like crap :-(

Car is ex 2.8 MPFI, now it have rebuilt 5.0 with roller cam and 600cfm double pumper, fuel lines was reused from 2,8

I switched to more powerful fuel pump http://www.ebay.com/itm/330371488771?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649 140GPH 9PSI and now for some reason I can't adjust fuel pressure lower than 7PSI but after 20-30 min of driving it drops to 3-4PSI again and I can adjust pressure from 4 to 7PSI

I have holley liquid filled guage. I have 3-port return style mr.gasket adjustable fuel pressure regulator.

Also 140GPH fuel pump caused flooding so I readjusted float levels and idle mixture screws with my wideband O2 but when fuel pressure drops then float levels became too low again.

should I switch to fuel pump with lower GPH such as Holley "Red fuel pump" 97GPH 7PSI ? http://www.ebay.com/itm/Holley-Red-Electric-Fuel-Pump-97-GPH-7-PSI-12-801-1-/390568568565?pt=Motors_Car_Truck_Parts_Accessories&hash=item5aefb366f5&vxp=mtr
My fuel pump placed under the hood, previous pump was placed under fuel tank but it had same issue. Also 140GPH pump is crazy loud, it makes me crazy )-: but actually first problem is fuel pressure and loss of power


If new fuel pump will not help then I want to switch to mechanical fuel pump.
My TBI camshaft is not supporting mechanical fuel pump. Can you advise me camshaft which supports mechanical fuel pump?

here is video
here is my setup

 

Either your gague is lying to you (liquid filled FP gagues are absolute crap- all of them) or you have a problem with the regulator.

I haven't used a Mr. Gasket regulator myself so I can't vouch for the quality. I have used the Mallory 4309 return-style regulator quite a few times, however, and it's a rock-solid piece of engineering.

It's also possible (though not likely) that the fuel tank is not venting properly and starting to draw a slight vacuum inside as the fuel level drops, but that's easy to check for- loosen the gas cap and the problem will immediately go away. I doubt your issue is that easy, unfortunately.

One thing I can tell you is that the problem is NOT your pump.

 

nevermind, I didn't fully read post above lol...

I think you're on the right track with fuel tank venting as I had the same problem last summer. unexplainable fuel pressure drop, also flooding after a 1/4 mile pass. Removed the rubber seal from the gas cap and the problem was solved.

 

thank you, guys. what if I will try to drive without fuel tank cap? I think i should get another guage (not liquid filled) and place it before fuel pressure regulator

 

You're probably FINE with the pump you have - those pumps however don't like to "pull" fuel - they're only real good at PUSHing fuel - that means that when you got it - there should have been instructions to mount it low and near the tank. I think everyone else has a good idea that it's the tank venting - to "test" you can simply run with the cap loose or off and see if you can reproduce the problems.

If the noise is bad (which that style is) then try rubber mounting the pump - an old strip of tire or semi-truck mud flap works well - make a long strip, mount 2 bolts from the back-side of the strip to mount the pump with washers and then make 2 'other' bolt holes through the strip to mount the rubber to the body - the metal of the pump never touches the body directly - which means you need to be sure to ground the pump to the body.

 

I checked my fuel tank cap, it was not vented style. I removed rubber gasket from cap today like Genopsyde said and pressure still drop from 7 to 4PSI. I removed cap when engine was at idle and nothing happen too. I still need non liquid filled guage.

Seems float levels ok. I adjusted idle mixture screws with vacuum guage and wideband O2. My highest vacuum I can get with this engine is 18Hg (both idle mixture screws out about 3/4 on my holley 600cfm double pumper). But O2 sensor read little rich fuel/air ratio - 12 to 12.9. If I will tune fuel/air ratio to 13-14 then I will loose vacuum to 16Hg and engine will try to stall and will begin to idle rough so I keep it at 12-12.9

When I drive vehicle with steady speed wideband O2 shows normal ratio 13-15

Another problem is that after 15 minutes flooding begins at red lights and full stops and wideband shows 11-10 air/fuel ratio and then drops to 9.

Is my 600cfm double pumper too large for my engine? Should I change jets to smaller size? I bought carb directly from Holley e-bay store but this carb was sold as "refubrished"

 

update:

I raised idle from 500 to 900 rpm and no more flooding and no more rich fuel/air mixture. Car runs and idles almost fine.

The only problem left is that periodically I still loosing power, BUT then POWER suddenly RETURNS. When it happen (loss of power) engine sound becomes like V6 engine sound, same feeling as missing cylinder(s) or when I push throttle pedal in neutral engine is bogging.

I was test driving my car 4 hours. 50% of time it have lack of power and other 50% power returns

I want try to switch from 6.5 to 8.5 primary power valve and move fuel pump to near the tank. I hope it will help.