high end stumble, fuel pressure readings...

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?

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.

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?

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.

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?

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.

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.

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?

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

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. :cry:

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.

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.

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.

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.

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?

I only mentioned a plugged cat since so many people see something wrong and run out and buy tons of expensive parts before truly making sure what there problem is. Take it one step at a time. When was the fuel filter replaced? If it needs done its under $20 and takes under 20 minutes if you know what your doing. Then you want to go on to step two.

Took me five minutes and 16 bucks to do it two days ago, when I was suspicious of low fuel pressure. That was before the car died of course.

A plugged cat would not cause the fuel pressure to drop to 28psi.

So I guess the only thing there is to do is replace the pump and/or fpr. No one commented on my post on whether the racetronix pump-only kit was what I needed. For the fpr, AZ good enough?

edit: I ordered a new fuel pump kit from Granatelli:
http://www.granatellimotorsports.com/product_display.php?gid=153
It comes with a Walbro 255 lph pump. I should get it tomorrow, $128 shipped.

These cars are know for having several problems at once so maybe its a weak pump along with other issues.

These cars are know for having several problems at once so maybe its a weak pump along with other issues.

Yeah, I've learned that unfortunately. All I know is that the pressure is too low, so once that is cured it will be on to step two, if required. I'm crossing my fingers.

Fuel pump install was straight forward. Pump primes, fuel pressure is back to normal now for koeo.
Only thing is, the car won't start. :mad: It's doing the same thing as it did when I tried to start it after it died on the freeway. It cranks, and when I stop cranking it, it feels like its still trying to fire for a few seconds afterwards. The only abnormal thing I can see is a burned wire going to the oil pan, above the oil plug, something like the oil switch. But I was told that wire isn't necessary to start my car.
I did the spark test using the icm terminals, everything checked out. Checked the grounds by the coil, they seem to be ok. Checked the fuses real quick, didn't see anything there either. I don't know what's up. :confused:
What could cause a car to instantly lose all power during WOT?

Have you scanned it for codes? Try that first, then clear the PCM by pulling the PCM BAT fuse for 30 seconds. It's possible that the low fuel pressure has caused elevated long term fuel corrections, and the engine is flooding on cold start. Did you try starting it with the accel pedal on the floor, to put it in "clear flood" mode?

When I installed the fuel pump I had the battery disconnected, that has the same effect as pulling the pcm bat fuse correct? I tried to scan for codes but I am not getting any. Can the car set a code without the car being able to run? I tried part throttle while cranking, but not WOT, I will do that.

Your codes and fuel trims have been cleared. There are certain codes that will set when the engine is trying to start... like the Opti high and low res pulse codes, and the ingniton control codes. A lot of the other stuff like EGR requires the engine to be running, under very specific road condtions.

So I guess I just have to keep troubleshooting...

How do you test for spark at each plug? I heard about a little tester with a blue light, or should I just put something in the boot and see if it arcs to the block?

Is there a way to test the coil, besides using the spark test? I was told that AZ can test coils, can a 97 coil be tested? The one on shoebox looks different.

I am getting adequate fuel pressure, is there a way to test if the fuel is actually going past the rails into the engine?

Should I open up the valve covers and look for anything out of the ordinary?

When the car died, and I forced it to limp off the freeway, it was missing, backfiring, and the manifolds were glowing, which I was hoping was just a failed fuel pump. Could a failed fuel pump damage something else? Should I take off the exhaust to see if its clogged? Will it run with only exhaust manifolds?

If anyone else has other troubleshooting ideas, let me know. If it comes down to it, I guess I will pull the opti and see if my newly installed piece crapped out on me.

I installed a new coil and had the ICM tested and it passed. Also tested the battery. I removed spark plug #1 and it looked normal, not fouled. I had a friend turn the key and I could see the plug sparking (shocked the s*** out of me too). I can turn the engine over by hand, I was trying to see if it was locked. I sprayed some starter fluid in the throttle body, no change. The engine cranks strong, turns over, but doesn't start. Checked all the fuses under the hood, all good.
I don't know what else to test. :confused:

Sounds like you are stuck. I changed the fuel pump and cranked and cranked for 3 days and gave up. My girlfriend went out 1 week later when the tow truck showed up to take it to a repair shop and thought she would try it for the hell of it and it fired right up. Give the keys to someone else or turn the key with you opposite hand if all else fails. Sounds stupid but it worked for me more than once. You figure;)

Two week old opti...and it failed. I didn't have an E4 torx socket when I had ordered my new opti and I forgot to thread lock them...:( Needless to say, I found my rotor complete disconnected, both screws had back out. I fixed it and the car fired right up. I'm gonna put the rest of the stuff back on tomorrow; alt, wp, etc. I'll take it for a spin and post the results.

Well I took the car for a spin, it runs, doesn't seem to pull like I would like it to. Seems to have flat spots, and some slight stumble. I don't know if the opti was damaged when the rotor was loose. I haven't taken pressure readings with the new fuel pump yet, I will. No codes yet so I'll have to wait and see.

I took off the damaged opti and installed a new AC Delco opti. Took it for a spin and found out that I also had a burned #7 wire. Replaced it and the car runs great now. So far I've put on 120 problem-free miles with no codes either. Hopefully it stays that way. :D