i have a rail mounted guage on my modded tpi iroc and noticed to day with the vacume line hooked up i have 40psi and with it unhooked 48psi but with the slightest rev drops a decent amount.if i rev it to 2500 it drops to like 25-30 psi and its kind of inconsistent meaning the pressure wont really hold at each rpm it jumps a few points up and down. is this normal

It does drop. How much? Im not sure. I believe it has the specs in the Haynes Underwear I mean the Haynes repair manual.

Yes

Where is your vacuum line connected to? Unless I'm mistaken, the pressure should go up as vacuum decreases (higher RPM).

First off, with the car off, as you turn the key, the fuel pump primes right. But then it stops after about 1 second or 2. Now when you crank the engine, the injectors are relieving pressure because of course the fuel is being released under a controlled condition. Now the fuel pump works by pulse, meaning that it turns on and off , on and off. When on, pressure rises, when off pressure is released via fuel injectors. Make sense? This is my best guess. Am I wrong anybody?

another thing i noticed is that when i cycle the key on and off letting the fuel pump prime the guage shows no pressure increase

To the stock port on the plenum.
Watch what yours does while driving and you punch it. It'll drop a few psi before coming back a couple.

how much should it drop. with the vacume line of the fpr at idle its about 40psi and while revving it in neutral if i hold it at say 3000-3500 the fp drops to like 25psi and stays there.

25 PSI may be correct because since you got a 350 block (LB9 engine), the fuel injectors are rated about 20-25 psi. I know for a fact that they are (injectors) higher pressure than the 305 injectors. Are you having issues with the car? Or were you just curious? If you want more fuel pressure, buy an aftermarket adjustable fuel pressure regulator.
http://www.amazon.com/gp/product/B000CC9AMA

That's too low.
Mine drops from 43 static to 39.

i have an adjustable regulator and 24lbs injectors i cant get it any highger. also do you have a stock pump and when you rev it on you car or hold it at say 3000 rpm whats it look like does the pressure jump at all

Don't remember what the pressure does. The car's at my other house so I can't check for you.
I have a high volume high pressure pump but that pump vs. a stock pump will make no difference at 3K rpm. The regulator does the work of lowering the supply pressure from the pump. If your regulator is fine and you can't get more than 25psi, then i'd venture to say that your pump may have bought the farm.

The pressure should be between 40.5 and 47 PSI or 41 to 47 PSI according to GM shop manuals. Some say it should be 42 to 47. According to Jon at F.I.C., it should be 42 at idle and 47 at WOT. I discussed my fuel with Jon as part of diagnosing why the BLMs on my car are at or near 160 after installing Bosch injectors I bought from F.I.C. I think the injectors are working properly. I do not know what the BLMs were with the old injectors. Two old injectors were shorting out and I had a bad O2 sensor so they would have been off before anyway. I did not have time today to do WOT with gauge taped to windshield and a passenger to read the gauge so I pulled the vacuum line to simulate WOT.

My car, a 1990 LB9 (305), has not been modified. The only fueling components replaced have been injectors and fuel filter. I probably should replace the fuel filter again based on age and not mileage. The car has 43K miles on it.

Here are my fuel pressures:
At key on while pump is on: 42 PSI.
At key on while pump is off before pressure slowly bleeds off: 40 PSI
Engine at idle: 38 PSI
Engine off idle (probably 1500 to 2000 RPM): jumps to 41 during tip-in and drops quickly to 31 PSI
Engine at idle with vacuum hose removed to simulate WOT with no vacuum: jitters between 42.5 and 43.5
Engine off idle (probably 1500 to 2000 RPM) with vacuum hose removed: 43 PSI steady
At ignition on while pump is on with return hose clamped off with Vice Grips: 64 PSI

Based on the above, I think my pump is okay since it reads 64 PSI with return hose clamped shut. Don't run the engine with the return hose clamped off.

I am not sure why it reads 31 PSI above in the one situation when it reads 43 with the vacuum hose removed. I may still do a WOT test. I may need to buy the 100-102 fuel regulator from TPIS.com. I hope I don’t need to drop the tank and install a new fuel pump.

so my car does the exact same thing ive always been told it was sposed to stay around 40-45ish if your hitting the gas or not mine will prime to 40psi and run around 42ish but when i get on the gas it will drop to 30ish. checked the diaphragm inside the pressure regulator and its perfectly fine i also have a brand new fuel pump less then 3 hours on it. what could be causing it? that was with 19lb injectors i have 24s now.

I probably need to rerun the test where I thought I saw 31 PSI. My later testing was with the gauge from AutoZone and I did not see what I wrote above when tipping in while parked. I would up the throttle from 0 to 10 to 20 % so RPM were probably 1500 to 2000. Leave it at the speed for a few seconds and let the throttle snap shut. When snapping back to 0% throttle, I did see the pressure drop but only monentarily. So this may or may not be an issue on my car and it sounds like on yours your pressure is staying low which does sound like a problem.

I did no testing while driving yet because I think I need to install a new pressure regulator based on my testing in the garage and because it is more difficult to test while driving--need 2 people and need to tape the gauge to the windshielf.

Did you check the pressure supplied by the pump? You could do it like I did or with the Autozone pressure gauge kit that would allow you to connect the gauge inline in the fuel line. The kit has many, many adapters to fit many cars to allow one to T-into the fuel line.

BTW. TPIS said they would ship out my order on Thursday of last week but their online order system was down and they did realize the problem existed until Saturday. So my order would was not shipped out until Monday. They were nice enough to change the shipping method to overnight delivery after I told them I planned to work on it at the begining of the week because I had some time off work. But UPS failed to deliver it by Tuesday, the last day I could work on it before the holiday. It was finally delivered Wednesday afternoon shortly before I left home for the Thanksgiving at a relatives house.

well ive got a decent gauge with a bunch of adapters but id need to chop up a flexable line to put it inline i dont have any metric fuel line adapters for it and anyone that tried to get a metric to an fuel line adapter knows how much that crap costs.

my car does the same thing has bbk afpr and 24# accel injectors.and i have since installed a walbro fuel pump and hot wire kit. still does the same thing.

Totally wrong;
injectors are rated by the flow rate (lb / hr ) at a set pressure ; in these cars @ 43psi

Schrader valve fitting is -4AN.Take the valve out and screw on a s/s braided line to the gauge
On a TPI easier to use one of these so line comes out at right angles and fitting can be installed without taking intake apart
http://www.summitracing.com/parts/RUS-670290/

You need a -4AN blanking cap to close the unused end

vetteoz does it agian. I stand corrected. I thought that the injectors sprayed at that pressure. Baffled.

Injectors spray is at the pressure (psi ) of the fuel rail as set by the regulator.
The computer controls how long the injector is open regulating the fuel as required; shorter time for idle - longer time for WOT
If you increase the fuel pressure ( more psi ) then more fuel will flow in the same amount of time (think garden hose; the more you turn it on ; the more water will come out in the same time)
If you go to a bigger ( more lb/hr ) injector (keeping the pressure the same ) then more fuel will flow in the same time ( think big garden hose vs small garden hose)

This is why you must retune the computer when you change injectors because there is nothing else to tell it the injector size has changed
It will hold the injector open based on what size injector it "thinks" it has ; not is what actually installed

While nothing tells the ECM that the injector size changed other than the constant/calibration the ECM does read the oxygen sensor voltage. If the sensor indicates it is running too rich or too lean, then the BLMs (Block Learn Multipliers) will be adjusted down or up over time to compensate. The compensation is limited to 160 (a multiplier of 160/128) on one end to richen up the mixture if lean and 108 to make it more lean if the mixture is too rich. This learning compensation only occurs in closed loop fueling mode. So the closed loop fueling mode will attempt to compensate if one adds a higher flow set of injectors but it would be best to "tune" the software for the higher flow injectors so the BLMs stay near 128 (multiplier of 128/128 = 1) so the BLMs do not max out. Reading the oxygen sensor voltage and BLMs with a scan tool or PC software and cable are one of the best ways to determine whether or not the fueling of the engine is correct. Adding an additional oxygen sensor, a wide band sensor, will provide even better informaton as too how rich or lean it is running.

my car does this and doesnt have a tune. would a tune for the 24# injectors fix this.

Ideally we want the BLMs to be within roughly 4 counts of 128. We definitely do not want BLMs to be pinned to 160. If the integrator is above 128 and the BLM is 160, then the ECM was unable to compensate enough. I have had some BLMs at 160 with integrator at 135--integrator is high enough to indicate the ECM would have gone higher than 160 if not limited to 160.

The 350 TPI engine came with ~22 lb/hr injectors and the 305 TPI came with ~19 lb/hr injectors. Using larger injectors with a stock engine will do nothing to improve performance and may cause problems with idle and with pinned BLM cells. A modified engine that is able to take in more air and produce much more horsepower than a stock engine requires injectors with higher flow rates but will likely require tuning of the ECM for best results. They are numerous threads that discuss this topic with postings from individuals who know about tuning than I do--at least until I am able to order a Promulgator and start tuning my own engine.

I installed my the TPIS AFPR which includes the housing, gaskets, and Torx bit but it does not include a new diaphragm/spring kit so I bought one from AutoZone for $40. The O'Reilly/Murrays kit was in a BWD box but appeared identical down to the o-ring package, directions, and additional note on separate piece of paper. Car Quest kits are from Standard Motor Products. I think the Napa kit is also a Standard product. So I think all four of these stores sell the same Standard Motors Products kits but some charge up to $80. The Standard Motors product diaphragm is nearly identical to my original part.

Some posters here say the Accel/Lucas injectors are not the best choice for our TPI engines. The Bosch Design II was considered to be a better choice years ago and now the Bosch Design III is considered by most to be the best injector for our TPI engines. My car idles as good it ever has since new and seat of the pants performance is also as good as ever. I think I can now actually shift up when the light tells us to in the 5-speed for best fuel economy and the engine actually does not bog done nearly as bad as it did before. I went uphill at about 20 MPH in 4th and uphill at 35 MPH in 5th without much complaint from the drivetrain and exhaust. Tipins where also without the issues I would have expected before changing to the Bosch injectors from www.fuelinjectorconnection.com and the AFPR housing from TPIS.com.