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Cant seem to hold fuel pressure

Old 01-16-2019, 03:11 PM
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Cant seem to hold fuel pressure

Hey there, I have an 89 Trans Am with a 350 TPI and have an extended crank. I have narrowed it down to fuel pressure dropping off quickly after shut down. If I pinch off the return line pressure holds. I replaced the pressure regulator 3 times and cannot get it to hold pressure at all after shut down. Any thoughts?
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Old 01-18-2019, 05:32 AM
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Re: Cant seem to hold fuel pressure

Since closing the return line will force the system to maintain pressure, it would seem that the injectors are not leaking down. So long as that is the case, there should be no flooding or fuel pooling.

Even if the regulator is leaking pressure back to the tank, if the 2-second fuel pump prime is functioning, that should be more than sufficient time for the pump to pressurize the fuel rails and operate the injectors normally. The one critical item not reported is the standing (KOEO) fuel pressure. It would also help to know the running fuel pressure, with and without regulator vacuum applied.
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Old 01-28-2019, 08:23 AM
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Re: Cant seem to hold fuel pressure

Originally Posted by Vader View Post
Since closing the return line will force the system to maintain pressure, it would seem that the injectors are not leaking down. So long as that is the case, there should be no flooding or fuel pooling.

Even if the regulator is leaking pressure back to the tank, if the 2-second fuel pump prime is functioning, that should be more than sufficient time for the pump to pressurize the fuel rails and operate the injectors normally. The one critical item not reported is the standing (KOEO) fuel pressure. It would also help to know the running fuel pressure, with and without regulator vacuum applied.
I agree, a leak off after shut down should not be an issue if you have good KOEO pressure and pressure at idle. I assume you are checking with a pressure gage?

I had a similar issue and tracked it down to an O-ring leaking on the fuel rail to the 9th injector. Though, this prevented me from having KOEO or good idle pressure. Glad I found that leak quickly.
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Old 01-28-2019, 11:10 AM
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Re: Cant seem to hold fuel pressure

Originally Posted by bunky2004 View Post
Hey there, I have an 89 Trans Am with a 350 TPI and have an extended crank. I have narrowed it down to fuel pressure dropping off quickly after shut down. If I pinch off the return line pressure holds. I replaced the pressure regulator 3 times and cannot get it to hold pressure at all after shut down. Any thoughts?
Similar thing was happening to me recently, but it was intermittent. Try running your tests several times and at different operating temps if you can to make sure you are getting same results. When you pinch the return line and pressure holds, that is indicative of pressure leaking somewhere in the gas tank. You definitely don't want to go there if you don't have to. Check your fuel filter for leaks, and also connections just outside gas tank. Also, make damn sure it isn't your pressure guage leaking.
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Old 01-28-2019, 09:57 PM
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Re: Cant seem to hold fuel pressure

If pressure holds with the return line pinched, (assuming minimum 40.5psi.)this rules out: fuel pump, injectors, leaking pressure lines, leaking guage. It can only be a Regulator issue.That's the only thing you are isolating. This flow chart should help you.

Last edited by jczoka; 01-28-2019 at 10:01 PM.
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Old 01-28-2019, 10:15 PM
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Re: Cant seem to hold fuel pressure

Originally Posted by jczoka View Post
If pressure holds with the return line pinched, (assuming minimum 40.5psi.)this rules out: fuel pump, injectors, leaking pressure lines, leaking guage. It can only be a Regulator issue.That's the only thing you are isolating. This flow chart should help you.


sheet, your right. must have had concrete cores on my mind. but regulator already replaced. 3x. I'd still check it over various temp ranges if possible. Are you certain you got correct line for return line - pretty sure its the smaller one.
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Old 02-27-2019, 09:48 PM
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Re: Cant seem to hold fuel pressure

There was a member on this forum that reported the same issue. When you replace the fuel pressure regulator you get the diaphragm and sometimes the top cap peice. What is not replaced is the mating piece on the fuel rail. The mating piece on the fuel rail has a seat that mates to the disk on the diaphragm. The forum member said he found a crack in the mating fuel rail piece where the diaphragm seats.
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