ss.slp.ls1

My car has a stumble/hesitation/miss at 5000-5500 rpm.
Replaced recently:
opti - 1 week ago
coil - few months ago
wires & plugs - few months ago
fuel filter - few months ago

I was thinking icm or fuel pump.
Hooked up a pressure gauge tonight and got these readings:
koeo - 45
idle - 36
vacuum to fpr off - 46
driving at wot - fuel pressure drops to around 28 before/at stumbling.

Do I need a new fuel pump?

mrmint69

It sure sounds like a new pump is in order. Volume being down usually points to one on the way out and will probably get worse if not replaced.

ss.slp.ls1

At the suggestion of someone, I drove the car around with the vacuum hose disconnected from the fuel pressure regulator.
The stumble was still present at 5000-5500 rpm.

Fuel Pressure readings (all with vacuum to fpr disconnected)
@ idle - 46 psi
WOT in neutral - drops to 28-32 psi @ 5000 -5500 rpm
WOT under load - drops to 28-32 psi @ 5000 -5500 rpm

Does this eliminate my fpr? Do I need a new fuel pump?

Injuneer

No, it does not eliminate the FPR. Something could be preventing the regulator spring from closing off the flow completely. At WOT in gear, you have the same effect as removing the vacuum compensation line, so you won't see anything different by removing it. Why would you run your engine at WOT in neutral - that does not simulate any sort of real world operating condition? Not sure that your test of removing the vacuum line proved much of anything.

ss.slp.ls1

Someone suggested it, and said that it would eliminate my fuel pressure regulator. Tested pressure again today: 34 @ idle, 28-30 @ WOT around 5000-5500 rpm. Stumble seems consistent at around 5200 rpm. What is the fuel pressure supposed to be @ WOT? Is it supposed to drop?

mrmint69

Pressure should not drop and if it does you need to fix it and be careful not to ruin your engine letting it run lean for any period of time. Unplugging the vacuum line simulates wot so if it reads 46 psi that would be what you would see if your had no fuel issues i believe.

Injuneer

The "correct" fuel pressure for the LT1 is 43.5psi without the vacuum line connected. GM specs allow anything from 41-47psi as acceptable. The purpose of the vacuum compensation line is to hold the DIFFERENTIAL pressure across the injectors (rail pressure minus manifold pressure, remembering that vacuum is negative) at 43.5psi. If you have 20"Hg vacuum (= minus 10psi gauge) at idle, the regulator compensation line would pull the fuel pressure in the rails down to 33.5psi, so there is always a DIFFERENTIAL pressure of 43.5psi pushing the fuel through the orifice in the injector.

At WOT/max air demand, if you have an efficient air intake system, manifold vacuum should be very low, with manifold pressure approaching barometric pressure. In that case, the gauge pressure in the manifold would be 0psi gauge, and the rail pressure would have to be at 43.5psi. If you have the typical 1 to 2"Hg loss in the inlet air track, your rail pressure would end up being 42.5-43.0psi.

Point is, 28psi is wayyyyyy too low. At 28psi at WOT, the injectors are only flowing 80% of the fuel they are supposed to flow, so a well tuned 13.0:1 WOT target A/F ratio would end up running at a very lean 16.2:1 = misfires, and possible engine damage from detonation.

ss.slp.ls1

So we are all in agreement that the pressure is too low. What could be possible causes? Is it limited to only the fuel pump and the fuel pressure regulator?

Injuneer

Failing pump, damaged lines in the tank, dirty sock in the tank, dirty/incorrect filter, crimped supply lines, faulty FPR.

ss.slp.ls1

So do I go about checking and replacing those things in order of cheapest to most expensive until the problem is fixed? If one of the plastic fuel lines was bent or crimped anywhere, is there a way to repair it without replacing the entire line?

Thanks for the previous replies, I just wanted to get those questions answered before I broke the bad news. The car fell on it's face yesterday afternoon, died right as I entered the freeway. I was able to get it going, sputtering along at 5 miles an hour (couldn't go faster), horrible missing and backfiring, struggling to stay alive. I don't know what happened.

mrmint69

Changing the fuel filter if you suspect its dirty is a cheap and easy thing to do. The fpr is next and there are many threads on how to check it. All the other stuff besides visual inspections in in the gas tank and the would requiring dropping the tank or making a trap door which are also on here alot.

ss.slp.ls1

I pushed the car home. The car won't fully start. It keeps cranking strong and when I stop trying to turn it over, it sputters barely for a few seconds like its trying to run. While its sputtering I can hear the fuel pump priming over and over.
With koeo, the pump primes but fuel pressure only goes to 32 psi now. It keeps this pressure, no bleed off. While its sputtering, the pressure flickers at around 30 psi.

P.S. While chugging along to get off the freeway, the manifolds got really hot and glowing and I burned a wire going to what I believe is the oil level sensor (wire goes into a big nut looking thing screws into the oil pan right above the oil plug). Does this have to be repaired to get the engine running again? I want to do priorities first.

ss.slp.ls1

What brand should I get? I saw this online from racetronix:
http://www.racetronix.com/RX-FLT1-FPKG-2.html
Would I just need the fuel pump only kit for $150 (bottom left)? This was cheaper than the kit from autozone which was $250.

mrmint69

The glowing manifolds usually mean plugged cats. This could cause serious performance issues. Make sure this is not your problem before you go into the fuel pump. Maybe theres more than one thing wrong.

ss.slp.ls1

Cats should be independent of fuel pressure shouldn't it? The car intantly lost all power at WOT, can a clogged cat do that? I didn't notice any restriction besides the stumble at 5200 rpm. The manifolds were only glowing orange after I forced the car to pull itself off the freeway while barely running.
I will have to get my fuel pressure back to normal before I can test to see if my cats are bad, correct? I've seen lots of mentions of the trap door, but no write-ups on how to drop the tank. I want to review both methods before I cut my car up. I don't want to start a debate, I just want to see write-ups on both, including how to disconnect everything after one of the two methods is used.
To test the cats, once I get the car running, do I simply pull off the exhaust after the exhaust manifolds and see how it runs? Will it run with the exhaust off?