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Old 04-27-2014, 09:10 PM
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fuel pressure problems

Questions about fuel pressure, on a 355 cu in chev motor with TPI. Fuel pressure test is as follows, turned the fuel pump on gauge went to 40 psi left the pump running for 10 seconds pump dropped to 32 psi. Started engine fuel pressure went to 42 psi increased rpm to 3500 and the psi stayed at 42. Should the pressure drop on the fuel pump before starting the motor and should the psi increase with added rpm

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Old 04-27-2014, 09:38 PM
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pressure will drop when started if the vacuum line is still connected. it shouldn't drop when not running however. does it also quickly go to zero when the key is turned off?
The proper way to test is to disconnect and plug vacuum line. you should have 43psi when at idle.
A drop when not running could indicate a leaky connector hose, from your pump to the pickup tube inside the tank. this assuming you're still using the stock type in-tank pump.
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Old 04-28-2014, 01:30 PM
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Thanks for your reply. Additional info. I don't have intank pump. Today Pressure test #2. Disconnected vacuum line from fuel regulator and plug the line. Turned pump only on and results were 40psi. Turned
pump off and after 5 min. the pressure dropped to 20psi. Started car ran at idle 43psi,, raised rpm to 3500 and psi stayed at 43, never could get more psi with greater rpm. turned engine off waited 5 min psi dropped to 30. Reason why I started with a pressure test is when driving my car engine runs strong.
As soon as the rpm start dropping like coming to a stop the engine acts like is either starving for fuel or and the engine stop running. when I slow down or come to a stop I must put the trans. in neutral and keep the rpm up then put it in drive to go.
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Old 04-28-2014, 02:57 PM
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. If you have the stock setup, fuel pump relay runs pump for about 2 seconds and shuts off... once engine starts, oil pressure tells computer to keep fuel pump running... if you have low oil pressure or bad pressure sensor, may have trouble keeping pump running...
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Old 04-28-2014, 08:59 PM
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My setup is not stock, I don't use a fuel pump sensor. when the key is on the pump operates until I turn the key off. Oil pressure is not a problem either. I have tried turning the idle up to a point when in gear the rpm about 850, out of gear rpm 1200. Sill the engine stalls when coming to a stop.
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Old 04-29-2014, 09:51 AM
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If you are ysing a vacuum referenced regulator pressure will be higher during pump running engine off, and when engine vacuum is high, the fuel pressure will be brought down.

Rpm will have nothing to do with with pressure, its all about manifold vacuum. You can be turning idle rpm, or 3500rpm and still run the same vacuum, therefore the same fuel pressure.

Are you running a VSS? Sounds like you may need a chip recalibration.

peace
Hog
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Old 04-29-2014, 10:01 AM
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If it's stalling when you come to a stop, it could be a bad throttle position sensor or, as mentioned above, a bad VSS or even just a VSS to ECM mis-match.

Are you running a lock-up torque converter? A TCC solenoid that is not disengaging would cause the car to shudder and even stall when you come to a stop. BTDT.
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Old 04-29-2014, 10:17 AM
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. Yes, as jmt mentioned, a going bad torque converter solenoid can also stall the engine... fairly common problem by now...

. Stock setups also have an accident safety feature that turns the fuel pump off in event of collision forces/engine fire, so the pump doesn't keep pumping gasoline on a fire while you're trapped in the vehicle...
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Old 04-29-2014, 07:32 PM
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Thanks everyone for your input. Little about my car, I have a 1978 Datsun 280Z coupe weights about
2700lbs soaking wet. My 355 cu.in motor block was built by a company making blown small blocks. The motor has Vortec heads, larger cam, roller rockers, oversize throttle body, plenum has been machined to adjust for the larger butterfly on the throttle body and is super charged. The trans. is a turbo 400 with a after market torque converter so their is no VSS or lock up torque converter, and I recently replaced the throttle position sensor even through the engine has less than 500 mi. The ecu was made for my cars profile. All electrical application is after market, all fuel lines replace with high pressure tubing.
Also the fuel pump was made by the same co., as the ecu.
My problem still remains when the rpm drop to about 500 the engine feels like its starving and stops running. Could the fuel pump be a problem even if it tests ok while the engine idles about 850 to 900rpm. As soon as the car is put in gear the idle starts dropping and starts running rough.
Or is this just the nature of the beast I built. PS this is not a drag car, I use it for car shows and weekend cruising.







. I have installed a
new throttle position sensor even through the motor has about 500 miles on it. Did not help, Im running
a turbo 400 so there is no vss sensor, not running a lock up torque converter. When I had my ecu made
the company took into consideration the motor and internal and external equipment , the turbo 400, weight of the car, gear ratio and 4 pages of additional information. The car does not have all the bells and whistles that a factory car would have.
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Old 04-29-2014, 07:45 PM
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The bottom paragraph was in error , I tried to edit my post but could not find the link.
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Old 04-30-2014, 12:10 PM
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. Sounds like when the engine idles down it falls off the ECM's fuel maps... might need your software maker to re-flash it/re-tune it a little... or maybe your vacuum sensor is going bad...
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Old 05-01-2014, 06:18 PM
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Thanks for the info. BuzzOL ., I did change the vacuum sensor no help. Next step I will check the voltage to the fuel pump and also contact the company that built my ecu and seek help
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Old 05-01-2014, 07:47 PM
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Have you tried shifting into neutral when you begin decelerating to a stop? If not I would give it a try to determine if the trans might be the cause. I have to say that I'm not familiar with how the ECM should respond with a supercharger regarding fuel maps, but I would be concerned with that also, as BuzzOL mentioned.

Are you running a stock fuel pressure regulator or an adjustable regulator? Do you have a fuel pressure gauge hooked up that you can see it from inside the car? If yes, what is the FP doing when you are decelerating?
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Old 05-01-2014, 08:07 PM
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it will still die when shifting into neutral. I will run a fuel pressure gauge to the inside of car to get results. and I still run a stock regulator.
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Old 05-02-2014, 02:48 AM
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Are you using a MAF or MAP TPI system? Are you using a blowoff valve and if so, is it blowing off when decelerating with the throttle plates closed?
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