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Not starting after warm

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jlbayes

jlbayes

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19 hours ago, JAG1 said:

Are you still powering your lift pump off the factory ECM lead? Jlbayes I like old technology..... water wheels for electricity and old covered wagons, but you need to get up to speed bud. Not poking here :punish:

 

Not sure what the ecm lead has to do with the fuel pump relay that is for the vp44. :doh:

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JAG1

JAG1

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The ECM lift pump power lead becomes an on off switch to activate an aftermarket Relay which then powers the lift pump off the batts.

 

Unless we talking about two different relays and understood wrong about which we were referring to. I know that you have a lot more hands on experience than I'll ever have. So I feel like a donkey trying to teach the professor. lol

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Mopar1973Man

Mopar1973Man

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I think you are confusing. relays maybe... 

 

There is a fuel pump relay in the PDC which is the power supply for the VP44. Then the stock lift pump lead from the ECM which does not come with a relay. Typically aftermarket lift pump have a power relay in the wiring kit. The only ones are the DDRP which typically do not have a relay kit which is very scary.

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KATOOM

KATOOM

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8 minutes ago, JAG1 said:

The ECM lift pump power lead becomes an on off switch to activate an aftermarket Relay which then powers the lift pump off the batts.

 

Unless we talking about two different relays and understood wrong about which we were referring to. I know that you have a lot more hands on experience than I'll ever have. So I feel like a donkey trying to teach the professor. lol

 

Correct...  The ECM fuel pump circuit simply tells the fuel pump when to pump.  Most recognized aftermarket performance diesel fuel pumps come with a relay which removes the amperage load to the ECM circuit by becoming that "on/off" switch for the aftermarket fuel pump.  Still though the ECM is dictating when the fuel pump runs.  As Moparman just mentioned, if the aftermarket fuel pump does not come with a new wire loom and relay then the ECM circuit must absorb the additional amperage to power that stronger fuel pump.

 

The delay relay mentioned is merely an "additional" relay added into the aftermarket wire loom which further interrupts the signal from the ECM to the fuel pump relay when the key is in the START position.  It does nothing else.....

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Callsign

Callsign

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Ok so IM getting 17-18 psi on FP gauge, when the issue occurs my tac moves while trying  to start (6 speed) this is a good thing I hear reguarding the ECM.  After trying to start if i go pull the fuse to my air dog, then try to start, truck starts right up like nothings wrong. This make any since?

 

 

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Mopar1973Man

Mopar1973Man

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Just now, Callsign said:

After trying to start if i go pull the fuse to my air dog, then try to start, truck starts right up like nothings wrong. This make any since?

 

Yes. Your cranking pressure is too high causing the timing piston to over advance and making it difficult to start. When you unplug the AirDog and then start there is no fuel pressure so the timing retards making easy to start. 

 

Cranking fuel pressure should be 7-12 PSI. Above 12 PSI typically is the problem of hard starts. 

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KATOOM

KATOOM

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1 minute ago, Callsign said:

Ok so IM getting 17-18 psi on FP gauge, when the issue occurs my tac moves while trying  to start (6 speed) this is a good thing I hear reguarding the ECM.  After trying to start if i go pull the fuse to my air dog, then try to start, truck starts right up like nothings wrong. This make any since?

 

 

 

Yes, makes perfect sense.  And to fix your problem you simply need to install that delay relay using the diagram I posted for you.  The VP does not like fuel pressure during engine cranking and you're feeding it 17-18 psi.....

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Callsign

Callsign

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1 minute ago, KATOOM said:

 

Yes, makes perfect sense.  And to fix your problem you simply need to install that delay relay using the diagram I posted for you.  The VP does not like fuel pressure during engine cranking and you're feeding it 17-18 psi.....

OK, im going to give this a try this weekend!

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Dieselfuture

Dieselfuture

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Would it be safe to say because my truck starts so fast why I and many others have no issues with too much pressure at start. And maybe why when it's cold out and it takes longer cranking times to fire engine up pressure is rising too high and on top of being cold it's also fighting higher pressure because of longer cranking where lift pump has time to build pressure. 

Another thing I heard that when cranking ecm cuts voltage to reduce speed of pump and pressure but with a relay if there is still enough voltage to energize coil pump will run at full psi, kinda confused here. I'm sure that's why the second delay relay is needed to avoid that, but... when is it necessary when cranking times get longer because of air in fuel cold weather bla bla bla.

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Mopar1973Man

Mopar1973Man

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8 minutes ago, Dieselfuture said:

Another thing I heard that when cranking ecm cuts voltage to reduce speed of pump and pressure but with a relay if there is still enough voltage to energize coil pump will run at full psi, kinda confused here.

 

ECM runs on a 50% duty cycle during cranking. This means 50% of the time there is a solid 12V at the pump and then 50% of the time there is 0 Volts. its a square wave pulse created by the ECM to reduce pump pressure during cranking. 

 

You can see the pulse in this old video.

 

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Dieselfuture

Dieselfuture

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@Mopar1973Man so it looks like it got to about 10 psi is that too much then and why you created relay mod to not have it come on at all. And that relay you pulled I'm guessing that is to kill power to vp so you could crank longer without starting. 

I know a lot of people don't have a delay relay at all like me for a long time, are we just waiting for trouble or we're just going to wear out relay sooner. 

And what I've mentioned before about cold weather starting, if we're at 10 psi cranking pressure seeing how your AD is behaving, it may contribute to harder starts. And if there was 0 psi on cold start it may fire up sooner because it wouldn't be advancing with pressure idk :shrug:

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KATOOM

KATOOM

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1 hour ago, dripley said:

I have at times heard my relay clicking on and off during an extended crank.

 

Everything's too loud for me to hear my relay click but I've confirmed with a 12 volt test lamp that the open pin on the delay relay will flash when cranking, proving the cycle of voltage to the relay during engine cranking.

Edited by KATOOM
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