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Testing FI Fuel Pressure FAQ
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furgo
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PostPosted: Mon May 07, 2018 5:57 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

busdaddy wrote:
A bus will run with 0 fuel pressure via gravity feed and manifold vacuum, it'll idle but dies when you step on the gas. The injectors still "fire", they just don't provide much fuel and the spray pattern is likely bad too.


raygreenwood wrote:
[...]. They will open and admit fuel all the way down to near 0 pressure.


Perfect, that makes it clear, thanks!

raygreenwood wrote:
However...understand that with an operative pressure range of about 36-42 psi....every 1 psi of pressure is worth 2.94% to 2.38% of total fuel delivery.

So being 2-3 psi off can be significant.


In this particular case case we're talking about no running at all (unfortunately), rather than running poorly, but nevertheless a good point.

busdaddy wrote:
Have you tried priming it through a port on the S boot when you know it's going to be hard to start?, that'll decide if it's lean or has flooded itself while sitting. A trigger sprayer for household cleaners makes an excellent primer bottle.


Yes, that will be the next step, but before I want to check the injector solenoids and the injector pulses to rule out the electrical side of things.

In any case, my fuel pressure question has been answered. I'll keep diagnosing elsewhere.
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JimmieH Premium Member
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PostPosted: Tue Sep 11, 2018 1:37 pm    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

Does the fuel pressure normaly fluctuate like this with the engine running? The pressure is steady with the engine stopped and the pump running with the AFM jumped.
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Urglegrew
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PostPosted: Tue Jan 07, 2020 9:38 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

Quote:
"If you pinch shut the return fuel line SLOWLY FOR ONLY A FEW SECONDS(when you are by the starter), the pressure should climb to about 4 bars or 59 psi and that will show you that the fuel pump relief valve is indeed working correctly, but your regulator is "toast". If the pressure does not rise, there is a problem with the fuel delivery system."


If I get a spike/59 psi (my neighbor when testing said mine went up to almost 100 or so?) when pinching the hose behind the fuel pressure reg my fuel pump relief valve works but this shows that my regulator is toast? Or my regulator is toast if the pressure doesn't rise when I do this?


Quote:
"If the fuel pressure is too high, you need to shuff off the engine and remove the fuel return line from the regulator and direct it into a gas can. Run the pressure test agin and if the pressure is OK, there is something blocking the return line. Low pressure compressed air or a non-sparking wire might help clean it out. The gas pumping around the fuel rail is also what keeps the fuel pump cool."


Does this mean if I get too high of fuel pressure when I pinch the return line, or just overall when idling?
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busdaddy
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PostPosted: Tue Jan 07, 2020 10:50 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

Urglegrew wrote:
Quote:
"If you pinch shut the return fuel line SLOWLY FOR ONLY A FEW SECONDS(when you are by the starter), the pressure should climb to about 4 bars or 59 psi and that will show you that the fuel pump relief valve is indeed working correctly, but your regulator is "toast". If the pressure does not rise, there is a problem with the fuel delivery system."


If I get a spike/59 psi (my neighbor when testing said mine went up to almost 100 or so?) when pinching the hose behind the fuel pressure reg my fuel pump relief valve works but this shows that my regulator is toast? Or my regulator is toast if the pressure doesn't rise when I do this?


Quote:
"If the fuel pressure is too high, you need to shuff off the engine and remove the fuel return line from the regulator and direct it into a gas can. Run the pressure test agin and if the pressure is OK, there is something blocking the return line. Low pressure compressed air or a non-sparking wire might help clean it out. The gas pumping around the fuel rail is also what keeps the fuel pump cool."


Does this mean if I get too high of fuel pressure when I pinch the return line, or just overall when idling?


If you got 100psi the relief valve in the pump has issues, or it's a pump from some other car that makes too much pressure or flow. If the pressure is ~30 un pinched the regulator is working.

The second quote describes what to test if the normal idling un pinched pressure is too high.
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SGKent Premium Member
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PostPosted: Tue Jan 07, 2020 2:10 pm    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

I would think that the pressure in the hose between the return nipple and the FPR will equal whatever the fuel pump will put out which is about 100 PSI. When the hose is open the pressure between the pump and FPR should be about 32 - 36 - there is a chart. But once the return is sealed then the pressure will equal whatever the pump will do. I haven't tried seeing how high and hard I can push the pump, its sort of a destructive test pushing anything to its limit to see what it can do. All it tells you is what it did this time. Don't try it in a submarine.
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Caveat: I am not telling you what to, or what not to do. When a suggestion is offered, it is wishing you the best, and is based on my experiences as a mechanic, automotive machinist, and from racing in the era your bus came to life. My only goal is to share what has been learned on my path. Smile

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