Fuel pressure - Specifications

[Mopar1973Man]

Specifications

Here is a copy of the fuel system specifications from the Dodge Factory Service Manual.

Fuel pressure

Normal fuel pressure should float about 14-15 PSI running down the pavement. Should never fail below 10 PSI at all. Here is a colorized fuel pressure gauge to give you a clue of the fuel pressure span you want. You want the needle to stay in the green at all times. Yellow zone is a warning that you should check the fuel filter, lift pump performance, overflow valve, etc.

This is undocumented maximum pressure is 20 PSI. Now take notice to the overflow valve is set for 14 PSI so excessive pumping of fuel does nothing for performance except create more heat for the fuel, which in turns reduces the cooling ability of the fuel and adds more stress to the lift pump. So a little extra fuel pressure is a good thing because it insures the VP44 stays cooled and lubricated properly. But excessive fuel pressure will heat the fuel and reduced the cooling ability of the fuel.

Now always remember that the only lubrication the VP44 will see is the fuel itself so if you don't have enough fuel pressure it will cause damage to the VP44 injection pump. It's like the same as running the engine with low oil pressure it will continue to run but the damage to the engine will occur. Or in this case the VP44 which happens to be about $1,200 to replace.

Pressure Drop

Amount of fuel pressure drop between idle and wide open throttle. 2-3 PSI drop from idle to WOT is normal. But 5 PSI or more pressure drop from idle to WOT is pointing out a failed lift pump, plugged filter, and/or plumbing restrictions.

2-3 PSI Is NormalDropping more than 5 PSI is pointing towards a failed lift pump, plugged filter, or restrictive plumbing.Just to show you the difference in the stock plumbing which is 6mm ID plumbing and most performance pumps come with 1/2" ID plumbing. These pictures will give you a idea... First picture is a Big Line fitting over the top of the stock plumbing. Then the second picture compares the stock 6mm ID steel lines to a 1/2" ID Big Line hose.

Here is a short video of stable fuel pressure from 55 MPH to 70 MPH at WOT.

Cranking Pressure Test

Here is a simple test on how to do the test for cranking fuel pressure. Pull the fuel pump relay in the PDC and then attempt to start the engine. Pay attention to the fuel pressure gauge during cranking. Be aware this test will trip a P1689 code and light the CHECK ENGINE light. Excessive cranking pressure can lead to difficult starting problems.

Difficult Starting While Hot

This problem is caused by a VP44 with a torn diaphram internally. This damage is typically caused from a VP44 that was operated at below 10 PSI for extended period of time. So technically you should replace the VP44 to repair the damage diaphram but most rig up a oil pressure switch or a starter delay relay. Here is my idea...

This will cause the lift pump to remain off during the period of timing the starter is operating as soon as the key is released the lift pump will function normally and remain controlled by the ECM.

Suggested Lift Pump Replacement

You should replace the stock fuel system completely being that it cannot supply enough fuel for even stock requirements without causing damage to the VP44 injection or adding stress to the lift pump. The pumps I suggest are proven and come with a lifetime warranty. These pumps come with a full kit and replace everything from fuel tank to VP44.

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AirDog 100, 150, or 165

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Raptor 100 or 150

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FASS 100 or 150

I don't suggest these pumps... Because they don't address the pump location, plumbing restrictions, or come with much of warranty.

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Stock Carter or Carter Campaign Pump (4090046)

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Holley Blue or Holley Black

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FASS DDRP

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AirTex