fuel pressure dropping after prime with no start condition

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cadet18
Posts: 226
Joined: Sun Feb 05, 2012 9:46 pm
Car: 89 s13 coupe ca18det

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can anyone verify that after priming your fuel pump the pressure holds in the system. i just replaced injectors and went from 800 to 440. after installing them the car wont start. it will sputter but no idle. also after i prime the system and the prime cycle is over my pressure immediately drops to 0. all injectors fire when CAS is spun.


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float_6969
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The pressure should stay pretty high for a while. Technically there's a check valve at the outlet of the pump and technically the fuel pressure regulator should close and lock the fuel in the system, so technically the pressure should stay up indefinitely once the pump turns off. In practice it doesn't work like that. In every car I've ever put a fuel pressure gauge on, they hold pressure for a while (hours or even overnight) but it bleeds down slowly over time. That being said, yes, when you first turn the pump off, the pressure should hold at least for a few minutes.

You said you replaced the injectors and went from 800-440. I assume you mean injector size? How did you tune the ECU to deal with the decrease in injector size?

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Izento
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Joined: Fri Dec 12, 2008 12:20 pm
Car: RPS13

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I had massive fuel pressure drop. I would prime my fuel pump and my fuel pressure gauge would show an increase, but maybe like 20 psi. I would turn off the car from power before starting it and the fuel pressure would rise to 40, then I would start the car. My fuel pressure would bleed off extremely fast and develop hard-start problems. I replaced my fuel sending unit and it remedied the problem. I assume the same is for your car. The sending unit has a check valve and sometimes that valve can get messed up (for whatever reason). I bought that fuel sending unit from Zilvia too, thinking it would remedy another problem, but it just ended up causing more.

cadet18
Posts: 226
Joined: Sun Feb 05, 2012 9:46 pm
Car: 89 s13 coupe ca18det

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First off thanks for the input guys,

Float- yeah its definately dropping off immediately and i have the car running now which was a map error. i am using nistune and did go from 800cc to 440cc. i have martin from enthalpy working with me to get it tuned over email. (i never got the hang of it i guess).

Izento- you are referring to the part that the fuel pump hooks into if i am thinking correctly? the metal assembly that goes in your tank. i might pull that out and have a look at it. do you guys think that having the check valve not working will cause me any issues while i am tuning it? ie ill have to retune after fixing the sending unit?

Thanks again for the input guys! youre the best!

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Izento
Posts: 438
Joined: Fri Dec 12, 2008 12:20 pm
Car: RPS13

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Yes, that is the sending unit. The sending unit won't mess with your tune if this tune is done by email, then I assume you aren't actually on the dyno or doing a street-driven tune session. If you are going to be driving it while tuning, then yes, I think it will affect the tune. I say this because my sending unit was also making my car sputter sometimes under boost (very intermittent problem, gradually getting more common as I ran the car over time), so if your car ends up doing that, just know that it's probably still the sending unit giving you trouble.

cadet18
Posts: 226
Joined: Sun Feb 05, 2012 9:46 pm
Car: 89 s13 coupe ca18det

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Thanks for the info Izento! ill look into a replacement. i have reset the whole engine to stock now and am having issues still there is something hiding in my setup and ive gotta find it now. I am starting a new thread for this.

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sjbsuperman1425
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It probably wouldn't be a bad idea to have a look at the fuel pressure regulator on the fuel rail either. Place a rag around a pair of vise grips or channel locks and squeeze the fuel line on the fuel pressure regulator. It could be that it has failed and just won't hold the pressure anymore. It's a quick check that takes 5 minutes and you don't have to take anything apart. Just my .02 :)

cadet18
Posts: 226
Joined: Sun Feb 05, 2012 9:46 pm
Car: 89 s13 coupe ca18det

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Thanks ill check it out!


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