Engine/Transmission (1998.5 - 2002) Fuel psi as start

[coreyallan]

Dose anyone know what the dynamics of the LP cycles at start up ie, when dose it cycle, for how long, what psi should I see on a stock system? Thanks for your time...

 

[JLandry]

Corey,



From the Dodge service manual:



"With the engine running, the pump has 2 modes of operation:



Mode 1: 100 percent duty-cycle with a minimum pressure of 10 psi, except when the engine is cranking.



Mode 2: 25 percent duty-cycle with a minimum pressure of 7 psi with the engine cranking.



The 25 percent duty-cycle is used to limit injection inlet pressure until the engine is running.



The transfer pump is self priming: When the key is first turned on (without cranking the engine), the pump will operate for approximately 2 seconds and then shut off. The pump will also operate for 25 seconds after the starter is engaged and the engine is not running. The pump shuts off immediately if the key is on and the engine stops running.



The fuel volume of the transfer pump will always provide more fuel than the fuel injection pump requires. Excess fuel is returned from the injection pump through an overflow valve. The valve is located on the side of the injection pump. It is also used to connect the fuel return line to the side of the injection pump. The valve opens at approximately 14 psi and returns fuel to the fuel tank through the fuel return line. "

 

[coreyallan]

JL, for those 2 seconds what psi should I see?

 

[JLandry]

Corey,



Going from memory, on my truck (with an Airtex gearotor type pump mounted in the stock location), the pressure might spike to about 9 or 10 psi before immediately dropping back to zero.



It seems odd Cummins or Chrysler would set it up so you have zero fuel pressure before starting the engine (while waiting for the "WAIT TO START" light to go out), but that's the way it works. I assume it works fine like this because as soon as you turn the key to start, the pump kicks back in again with the pump at the 25% duty cycle.



Regards,



John L.

 

[NoSeeUm]

JL, for those 2 seconds what psi should I see?

On my truck I just see a quick spike in pressure, maybe to 5 psi.



Jim

 

[dmagenau]

On my truck I just see a quick spike in pressure, maybe to 5 psi.



Jim

Mine is the same. I get a quick 4 or 5 psi spike and then back to zero. When it's below zero sometimes the guage barely moves at all for that couple seconds.

 

[coreyallan]

Hmm, I see 2-3 psi... . I use-to see a spike of 10+ b4 I had the problem described in my other thread "no start, no start, no start, start... " I'm trying to establish a base line for a fuel system understanding/upgrade and y my truck is acting the way it is... ...

 

[NoSeeUm]

I would think 2-3 psi is still OK. Just as long as you see the spike.



Jim

 

[16919]

Mine goes to about 10 or so... .

 

[DMcLeod]

I believe your software version has something to do with this. The LP on my 01 used to pump for 2sec (abuot 10psi and then fades when the LP stops) when you turn the key, but since a computer reflash in 06, Dodge's updated software only has it come on momentarily and the pressure gauge just barely nudges now. I understand this was a software correction to prevent starting problems that some trucks were having. I believe I read about the starting problems a few years ago on this site. That's all I know.

 

[coreyallan]

Hmm, interesting. My lp and starting problem started long b4 the reflashs I had performed. Weird that some people see 10 or so when others see 2-5. I know that might be inconsistency in the different systems or components but I wish someone could positively tell me what it is "supposed" to be. Jlandry, how do you like that airtex? What dose it pull down to at wot?

 

[JLandry]

Jlandry, how do you like that airtex? What dose it pull down to at wot?

Corey,



Let me start out by saying I was mistaken about what pressure I see during that 2 second cycle of the Airtex lift pump when the key it turned on. I just paid attention to it when I went to out to start my truck to at lunch. It spikes to 15 psi and immediately drops back down to zero.



Here are the typical pressures I'm seeing with the lift pump in the stock location:



Idle = 17. 5 psi

Cruise (Unloaded at 60mph over level terrain) = 13. 5 psi

WOT = 7-8 psi



When the fuel is cold (about 40 degrees F and lower), pressures tend to be about 1 to 2 psi lower. If it matters, I'm using B5 to B20 biodiesel.



So far the Airtex seems to be a great pump at a reasonable price. Only time will tell how reliable it is.



Best regards,



John L.

 

[SHobbs]

Key on mine goes to nearly 13 psi. After cranking at idle I have 11 psi and WOT never below 7 psi. Cruising at 65 mph I have around 9 psi.