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Thread: Fuel pressure 86 svo

  
  1. #1
    16 PSI Boost
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    Fuel pressure 86 svo

    Hi guys i have a 86 svo with some mods my prob is fuel pressure. When my gauge is hooked up i turn key on gos to 40psi them strats to fall guickly . Bad regulator leaky injecter. Car does run and advise thks..

  2. #2
    OVER-BOOST!! gbeaird's Avatar
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    Usually, a bad regulator will leak through the diaphragm. Pull the vacuum hose off the regulator and see if it's wet. If it is, there's your leak. Otherwise yank the plugs and see if any are darker than the others. Dark/black plug ends points to a leaky injector. I think there's also a check valve in the pump assembly that can fail.
    Gene Beaird,
    86 2R SVO, G Stock,
    Pearland, Texas

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    what is normal fuel pressure for a 86 at idle, beside the bleed down i got 40psi at idle pull vac off goes to 45psi. what should be pressure at prime with key on. i will pull plugs tonite. the pump is walbro 225hp less then a year old.

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    OVER-BOOST!! GAboySVO's Avatar
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    Pressure looks correct, maybe just a little high, but it can depend on your cam and how much vacuum you have at idle. Another place that can leak pressure quickly is the hose at the in tank pump. A loose clamp or split hose could cause the issue.
    Mike S

    '86 SVO 9L Leather
    '86 SVO 9L Road Warrior
    '96 300ZXTT

  5. #5
    OVER-BOOST!! svono50's Avatar
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    "Normal" fuel pressure is right around 40psi give or take a few psi and pressure will go up when you pull the vacuum line as you are seeing. I would say 45psi is a little high for a stock regulator, but nothing to really worry about. Do you have an idea of what the pressure is doing when driving? It should climb 1psi per 1psi of boost, so if you run 15psi boost add that to the 45psi base w/o vacuum. You should be seeing ~60psi fuel pressure at full boost.

    If you don't have a leaky injector, I would start to suspect the check valve in the Walbro. Unfortunately there is a flood of counterfeit Walbro pumps on the market, which have suspect quality and lifespans. A couple of years ago I had a 255lph Walbro die on me when I was running the track day at Barber during the Reunion. Luckily it would run the car, but wouldn't support full boost runs...not good when out on the track. I am now running a 340lph pump, since that is all Paul had laying around in his shop. I still have my 155lph Walbro pump I swapped out for the 255lph pump in my 'backup' parts box.
    Ted
    86 SVO Mustang
    17 Cooper S Clubman ALL4

  6. #6
    OVER-BOOST!! gbeaird's Avatar
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    Quote Originally Posted by svoturbo View Post
    what is normal fuel pressure for a 86 at idle, beside the bleed down i got 40psi at idle pull vac off goes to 45psi. what should be pressure at prime with key on. i will pull plugs tonite. the pump is walbro 225hp less then a year old.
    Sorry, I guess I should have been specific. Pull the vacuum hose with the engine off and check it and the nipple on the regulator for raw fuel. Although, if it is leaking, I'd expect fuel to drip out of the regulator with the engine running/key turned on.
    Gene Beaird,
    86 2R SVO, G Stock,
    Pearland, Texas

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    hi guys this crazy , got off work last nite put fuel gauge on car turn key on held 39psi . no drop what the hell happened?

  8. #8
    OVER-BOOST!! gbeaird's Avatar
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    Quote Originally Posted by svoturbo View Post
    hi guys this crazy , got off work last nite put fuel gauge on car turn key on held 39psi . no drop what the hell happened?
    The chunk of dirt holding the check valve/pintle open finally passed?
    Gene Beaird,
    86 2R SVO, G Stock,
    Pearland, Texas

  9. #9
    OVER-BOOST!! svono50's Avatar
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    I think Gene's theory is a decent one, you may just have to keep an eye on things to see if it continues. You could always start with getting a set of freshly cleaned/balanced injectors in there and go from there. Good to hear your back to a holding pressure.

    As for reading your 'base' fuel pressure, I was told by a Master Ford Mechanic (who is also an SVO owner) that the proper way to read the fuel pressure is actually with the engine off. I raised my eyebrow too when I was told this. It is the only way to ensure the rest of the fuel system is at a 'fixed' state (i.e.- no opening of the injectors). Makes sense when you think about it. It also ensures that the fuel pump resistor is not in play as well, since there will be no vacuum to it. To run the fuel pump, you just need a length of wire to jumper two pins in the EEC (see pic) and have the ignition in the KOEO (Key On, Engine Off). Your fuel pump will run at full power and should give you a nice steady reading. It is in this 'state' that you should read in the neighborhood of 39 psi.

    One reference I have gives 'delivery' and 'working' pressure ranges, but I am not 100% certain as to their definitions to what those conditions are, so take that into account. My personal guess is that 'delivery' is without the engine running and 'working' is with the engine idling, YMMV. The 'delivery' pressure should be 35-55psi and 'working' pressure should be 39-49psi. It also states that the minimum pressure right after de-energization is 30psi, but doesn't mention anything as to how long it should hold or a rate of pressure drop after shut off.
    Ted
    86 SVO Mustang
    17 Cooper S Clubman ALL4

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