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New VP44 - Engine requires excessive cranking when hot

Leaky88
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Leaky88

Leaky88

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Some have responded that they have 7-12 PSI while cranking Engine.  AND they run as much as 18+ PSI  down the road.

With the FP Relay in PDC removed, my gauge shows a solid 15 PSI while cranking Engine. My FP stays a solid 17 down the highway.

 

Q1:I am not understanding why my cranking pressure is so high compared to the reports of others (I.e. 7-12 PSI)

 

Called FASS.  Was told some VP44 are finicky about the high FP and the HOT Start issue)

Was told to trim Spring to lower PSI.  I can do that, no problem.

Q2: If I trim the spring to reduce the “cranking pressure” will this not also reduce the 17 PSI I have available running down the highway? 

Thanks

 

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Mopar1973Man

Mopar1973Man

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Q1 - Mostly because the fuel pressure regulator is too high typically also some have opted for 3/8" fuel lines which adds restrictions also creates high pressure, but reduced volume. I've always suggested 1/2" from tank to injection pump. Including between the stock filter and VP44. I'm set for 17 PSI for idling pressure, WOT at 15 PSI. 

 

Q2 - Yes it will reduced both. Now if you running 1/2" line the drop should be very minimal. Running down the pavement cruise set at 65 MPH I'm 17 PSI no problem. Only time I see drop is under load (towing) or climbing grades. 

 

I do need to change fuel filters I'm now starting to see 15 to 16 PSI at at cruise state at 65 MPH about 60k miles on my filters.

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Leaky88

Leaky88

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On 7/10/2019 at 5:02 PM, Mopar1973Man said:

7 to 12 PSI during cranking is fine. Above 12 PSI and hard starts happen. What happens the ECM attempt to advance timing too quick so the lift pump pressure is reduced by cycling the power at 50% duty cycle so the pressure fall and the VP44 can advance timing till the RPM's come up. 

 

Just curious. Does this mean the VP44 must see at least 7psi in order for engine to start or No?  The reason I ask is because the HOT Start Mods to disable the Lift Pump would result in VP 44 seeing no pressure since the LP is disabled.  Correct? 
Or does taking the Lift Pump off line make the VP44 dependent on getting the fuel, which is very hard on it.

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01cummins4ever

01cummins4ever

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29 minutes ago, Leaky88 said:

Just curious. Does this mean the VP44 must see at least 7psi in order for engine to start or No?  The reason I ask is because the HOT Start Mods to disable the Lift Pump would result in VP 44 seeing no pressure since the LP is disabled.  Correct? 
Or does taking the Lift Pump off line make the VP44 dependent on getting the fuel, which is very hard on it.

In a simple Answer, No. The VP will fire off the injectors with O lift pump pressure as long as your system is primed. and engine cranking sufficiently. The ECM and lift pump do not even come into play until engine is started and idling,  This is why if you perform the bluechip VP test by basically hot wiring the VP to the battery it will fire off and idle while taking the ecm out of the equation. The lift pump is programmed through the ECM to give little bumps of pressure during pre starting situation but isn’t really necessary if engine fires off immediately, more less just to compensate for fuel being used to start the engine and then once started the lift pump will run full bore.  This usually happens so quick you really don’t notice it if your truck starts and fires off immediately like it should,  

@Leaky88  Does your truck fire off instantly when hot while the lift pump is disabled? 

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01cummins4ever

01cummins4ever

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2 hours ago, Leaky88 said:

 

01cummins4ever,

Thanks.

No.  it will NOT fire off instantly - when HOT- with the FASS 150 Lift Pump disabled.  

However, if I use a FASS DP02 Lift Pump, it starts HOT all the time/everytime. 

 

That makes things difficult  :think:

I would opt to run the DP02 lift pump until you get things ironed out with the 150, I know the hard starts can be hard to deal with while putting a huge strain on the starting system, Ive been their, I’m sure it will get figured out, we will just keep poking at it

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dripley

dripley

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The lift pump does operate during starting as long as it the oe pigtail is used for activating the pump or a relay. The ECM turns the power on and off quickly to lower the fuel pressure. I have seen mine on a long start issue and the gauge is bouncing a lot in the 7 to 12 psi range.

The engine will still start with no fuel pressure. Though when my oe lift died it would not start. I have had 2 pump failures with the AD I have on it now and both times the truck would start and I drove out necessity, I never knew when the pumps actually failed. I just looked at the gauge and saw at O psi. I have no Idea how far I drove with 0 PSI. I dont recommend driving with no lift pump, I did not have a choice at the time. 

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Mopar1973Man

Mopar1973Man

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On 8/7/2020 at 2:56 PM, dripley said:

The lift pump does operate during starting as long as it the oe pigtail is used for activating the pump or a relay. The ECM turns the power on and off quickly to lower the fuel pressure. I have seen mine on a long start issue and the gauge is bouncing a lot in the 7 to 12 psi range.

 

Exactly. This is proper ECM control of the lift pump it switches the 12V on and off at a about 50% duty cycle to reduce fuel pressure. This is so the proper cycle of the lift pump and should bounce the pressure at 7 to 12 PSI.

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Great work!

Great work!

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Those relays in the PDC can get old and finicky and don't always work when hot or cold or low voltage. Try swapping  the fuel system relay with one of the other ones and see what happens. Also try running a few feet of clear hose along the windshield wipers to watch for bubbles in the fuel.

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