Low fuel pressure on start

ninetyfive · Apr 28, 2026

We replaced the fuel pump and regulator two years ago and fuel pressure issues are back. It takes a while to build up 55psi. Thinking of replacing the pump and regulator again. Are there any components in the pump assembly that should be reviewed?

I ordered the regulator from Havik.
The replacement pump I am finding is a Bosch.

Thanks

ViperTim · May 6, 2026

Sounds more like the check valve is leaking.
If you stop it for the night and key on the next morning, is the pressure 0psi?
Because that sounds to me like the check valve is draining the fluid of your fuel system and your pump needs to refill & build pressure every single time. Thus I wouldn't believe it's actually a pump issue but more of an internal leaking issue.

But maybe the check valve is integrated into the pump itself, I don't know. I've never had this issue.

ninetyfive · May 9, 2026

From what I understand the check valve is in the regulator. We tested the pump, it is fine. We replaced the regulator. The issue did not go away. It is so weird. Get it hot, turn off and it starts right backup. Let it sit for an hour or two, long crank. Let it sit overnight, starts right up in the morning.

Goggles Pizano · May 10, 2026

You need to throw a pressure gauge in it and monitor it.

ninetyfive · May 10, 2026

Yes, ordered one. Will do some experiments.

99RT10GTS · May 10, 2026

Another fix would be to put an inline check valve to maintain the pressure. A common cheap fix for the Gen 1 cars that seem to have this issue.

MoparMap · May 11, 2026

ninetyfive said:
From what I understand the check valve is in the regulator. We tested the pump, it is fine. We replaced the regulator. The issue did not go away. It is so weird. Get it hot, turn off and it starts right backup. Let it sit for an hour or two, long crank. Let it sit overnight, starts right up in the morning.

My gen 3 has very similar issues, though sometimes even first thing in the morning it will crank for a while. Sometimes it will catch and run on basically one cylinder at a time until it gets enough rpm in it. Does it make any difference if you feather the throttle by any chance? I replaced my fuel pump/sender unit a while back when my gauge went out and I don't think it's ever been the same since.

I don't think it would make a difference, but I'd wonder if a clogged up fuel filter might do something like that. It would take a little time to get fuel through it, but once the line is at pressure you would assume it would hold okay. Could explain something being lazy to build pressure though.

ninetyfive · May 12, 2026

Filter was changed 10000miles ago. So i put a gauge on it. Morning it was at 0. Each key cycle built it up 10psi up to 48. Fuel in the rail, physics, and maybe longer injector pulse width when cold might explain starting up cold just fine, even at 10psi. While running it's steady at 48psi. Not sure if not reaching 55psi is another problem or if it's just another symptom of the pump trying to always catch up. Shutting engine off, pressure drops back to 0 within a minute. So next step is inline check valve like @99RT10GTS suggested. So where is the failed check valve then? Pump, regulator, somewhere else?

ninetyfive · May 12, 2026

Anyone know if a walbro gss342 pump would fit?

MoparMap · May 13, 2026

I believe there is a check valve built into the pump assembly (where the regulator is on a gen 3 at least).

Goggles Pizano · May 15, 2026

The regulator you bought they should honor replacing it.

Goggles Pizano · May 15, 2026

They was a guy in Australia (i think) who modified a regulator.

Dodge Viper fuel system shenanigans PART 1/2

This is not a 'how to', it's a voyage of discovery... I got this Viper a few weeks ago and found it's got some fuel system issues - the fuel pressure drops t...

youtu.be

Goggles Pizano · May 15, 2026

FUEL PRESSURE REGULATOR
A fuel pressure regulator is used on all gas powered engines. It is located on the top of the fuel pump module. A separate frame mounted fuel filter is used.
Fue l Pressur e Regulato r Operation : The pressure regulator is a mechanical device that is calibrated to maintain fuel system operating pressure of approximately 379 kPa (55 psi) at the fuel injectors.
It contains a diaphragm, calibrated springs and a fuel return valve. The internal fuel filter is also part. of the assembly.
Fuel i s supplied to the filter/regulator by the electric fuel pump through an opening tube at the bottom of filter/regulator.
The fuel pump module contains a check valve to maintain some fuel pressure when the engine is not operating. This wil l help to star t the engine.
I f fuel pressure at the pressure regulator exceeds approximately 56 psi, an internal diaphragm closes and excess fuel pressure is routed back into the tank through the pressure regulator. A separate fuel return line is not used wit h any gas powered engine.
The pressure regulator is a mechanical device that is NOT controlled by the PCM or engine vacuum.

GTS Dean · May 17, 2026

I have never had one of these apart, but found the patent online and it did include a non-serviceable filter inside. If the production Viper regs were set up this way, it's no wonder they are failing over time. The regs should pass minor particles to an easily serviceable primary filter so the regs don't choke off. I know that many fuel pumps I corporate a check valve in the discharge nip ple to hold pressure, but not all.

Goggles Pizano · May 17, 2026

But it would be the particles getting stuck in the check valve that would make it not close and leak back if the spring still has the same load rating.

I would be interested in back flushing it and also disassembling one.

maverickagm · May 18, 2026

I have a gen 1 that also has a long crank. I know my fuel tank has been out before, but I'm not exactly sure why.

I never liked the idea of dragging out the tank to replace the regulator, only to have the problem come back. Interesting note about particles in the check valve. I'm curious if adding an external check valve after the filter would help, and last.

I have some an6 hose and fittings. I ordered a Radium check valve and some 5/16 efi quick-connect to an6 adapters. They won't be here for a few weeks though. My plan is to have the filter male end adapted to an6, loop around with the inline check valve, adapt an6 to a male 5/16 quick-connect. Then the stock hose that usually goes into the filter will go into the new hose. This way if it doesn't work, it's easily reversed. I'm not sure how much of an impact it will have having the check valve aprox 30 inches from the regulator. Hopefully it doesn't look like a caterpillar of adapters....

I've heard the suggestion of adding an inline regulator before, but I've never heard anyone mention what actually worked for them, what parts they used, and where/how they mounted it.

GTS Dean · May 18, 2026

That sounds like a very logical course of action. Here is what a typical check valve in the pump discharge nip ple looks like. The pump on the right has it, the left one does not. I guess it's possible for the thing to come apart, get into the regulator and lodge the bypass window partially open. That would lead to both low pressure and multiple key-on cycles to get enough to start. Also attached is the patent diagram for this style of regulator.

Goggles Pizano · May 18, 2026

@maverickagm no effect. the check valve can be 30 inches or 30 feet from the regulator.

Adding an inline regulator is not a good idea unless it has a fuel return line port. The back pressure on the pump will kill the pump.

maverickagm · May 18, 2026

No no, not an inline regulator. That would be a bad idea. Just a simple one-way check valve to help keep the line pressurised (or at least keep fuel in the line) once the ignition is turned off.

In the youtube video you posted above, the guy does the same, but puts his check valve above the gas tank, immediately after the regulator.

ninetyfive · May 23, 2026

@maverickagm what do you think about the check valve causing rail overpressure, leading to leaking injectors. Don't we need a way to relieve increasing pressure?

Goggles Pizano · May 23, 2026

ninetyfive said:
@maverickagm what do you think about the check valve causing rail overpressure, leading to leaking injectors. Don't we need a way to relieve increasing pressure?

Unless the regulator fails and the pump can pump upto 90-100psi, nothing really to worry about. But check whatever injectors you have max pressure spec. Some bosch top out at 75psi

That's why I said not to waste time with a check valve or that stupid timer someone was selling and/or showing people to wire in. Just cycle key to build pressure is the cheapest work around until you can fix it.

Or just wire in a push button switch to the fuel relay to run the pump.

Low fuel pressure on start

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