TPITuned Port Injection discussion and questions. LB9 and L98 tech, porting, tuning, and bolt-on aftermarket products.
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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
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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?
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
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.
I mean the Haynes repair manual. 
