Engine/Transmission (1998.5 - 2002) VP44 Hard Starting

[Lightman]

qeustion for VP44 gurus

« on: February 4, 2003 03:45:18 AM »



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It's been almost 2 years since I've seen the insides of a VP44 and my memory is starting to fade. I'm stumped.



What would cause the VP44 to loose it's prime after sitting off for 10 to 20 minutes or so?



Here's some information.



Happens about 80% of the time when truck is on level ground

Happens more with an empty tank

Happens almost all the time when the front of truck is parked up hill

Doesn't happen much when the front of the truck is parked down hill

When it does happen, the engine will start after letting lift pumps run for 15 to 20 seconds

No codes set

No visible fuel leaks

Fuel bypass valve is OK (Cummins wants over $100 for a new one, by the way)

Fuel pressure after filter - 5-8 lbs during cranking - 22 lbs @ engine stopped 20 lbs @ idle 15 lbs @ WOT



Thanks

 

[LSMITH]

Pick up tube in the tank is cracked is my first guess, a line somewhere leaking is next. Fuel pressure as high as yours has caused some hard to start scenarios too, but it does sound like it is losing prime. I would drop the tank and have a look at the pickup tube if there is no visable leak anywhere.

 

[crawler]

Possibly the pusher.

Try this. Unhook the power wire to your pusher pump and crank the motor. It will probably fire right up. If it does then you might need a new relay for the pusher. Oh be sure to shut the truck off pretty soon because with the pusher disconected you will be restricting fuel flow. My truck did the same thing and for a temporary fix I put a switch between the power relay and the pusher. If the truck didn't want to start I would flip the pump off and like magic the truck cranked just like stock. Then you just flip the switch back on. I've since fixed the problem. PM me and I'll tell you about it. Oh, by the way I was using the power wire on the LP for my relay signal source and I was still having a hard start condition. But now I've fixed it.





Wes

 

[Ray Torresdal]

Hard start

I also just started having difficulty starting my truck. I just returned from a 3700 mile road trip, with absolutely no problems, 18. 36 mpg. average for everything except the last tank.



The morning after I returned, it cranked way longer than normal before starting. I also have a bad fuel pressure sending unit - that went out on my trip. Since this acted like previous times when the lift pump went out, I assumed bad lift pump. Put in a new one yesterday afternoon, and same problem. I haven't received the new sending unit, so can't verify the lift pump pressure. If I can't find leaks, then what???



I'll try disconnecting the lift pump - please explain why this helps??? Too much fuel pressure at startup???



Would there be anything related to all the slop covering everything after 300 miles of ice and snowstorm the last leg of my trip? It sure got grimy under the hood!



Thanks for the help, guys! I sure hope that I'm not in need of a new pump already - only 112,500 miles.



Ray

 

[crawler]

Yours may be something different like the fuel filter. The temporary solution that I suggested earlier was just for guys with a pusher pump. Re read my earlier post. You don't unhook the lift pump.





Wes

 

[A Johnson]

Re: Hard start

I'll try disconnecting the lift pump - please explain why this helps??? Too much fuel pressure at startup???"



Too much lift pump pressure at startup is correct. During the initial key on , lift pump pressure backfeeds ( for lack of a better word ) into the housing of the VP-44 and if it exceeds 5psi during cranking it holds the pumping plungers off of the internal cam ring, untill internal VP-44 vane pump pressure exceeds the lift pump pressure and exerts force on the opposite side of the plungers forcing the plungers out, onto the cam. This condition could be accentuated by a week internal vane pump or a cracked internal pulsation dampner.



A Johnson

 

[Lightman]

Thanks for your replies, guys.



Here is some more information.



The fuel psi is 5 - 7 lbs when cranking. The pusher pump is wired into the lift pump circuit through a relay. The fuel pressure guage is a mechanical Isspro tapped into the fuel filter outlet banjo, no isolator, and I trust it's readings (yes, fuel is routed into the cab -- no "flames" please).



Could the problem be air entering into the fuel return lines even though ther's no visable leaks; draining them and the VP44 out? Simular to what sometimes happens to 12vs.



Is there a way to check for a weak vane pump or cracked pulsation dampner?



When the hard starting happens it acts just the same as when the fuel filter has been replaced and the pump needs primed again. To get it started I bump the starter, let the pumps run 10 to 15 seconds, hit the starter again, the engine stumbles abit, then starts right up. Usually I have go through the start routine a couple of times.



Sometimes the truck will go a day or 2 with no starting problems at all. But it will ALWAYS start right back up within 5 minutes after being shut down. The condition has been getting progressively worse for the last 5 months.



Truck is an '01 ETH.



Thanks

 

[LSMITH]

5-7 PSI is what the newer programming is supposed to give you. Your lift pump pressure is not responsible for the hard starting.



Yes definately a leak in the return line would cause what you are getting. The hard line that comes off the back of the head runs down close to the lift pump and has a flexible section in it, if it is leaking it should be right in that flex section. If it is leaking bad enough to cause a hard start problem the line should at least be caked with dirty crud.....



No clue on the pump internals, hopefully AJohnson will stop back by this thread.

 

[A Johnson]

Checking for a weak vane pump is not an exact science, I can only go by what we have found in the field so here goes.....



If you dead head the inlet to the VP by putting a 12x1. 5 mm bolt in place of the banjo bolt at the fuel filter outlet, and then put a vaccum gauge on a tapped banjo at the VP inlet, and crank the engine it should pull 15" of vaccum or better within about 5 seconds of cranking. If it takes alot of crank time to get the vaccum up or it never comes up, you can bet there is an internal mechanical problem with the VP. Like I said there is no spec for this printed anywhere that I know of, so you kinda go by gut feeling, but the VP should pull vaccum relativly quickly as long as you don't loose a lot of fuel putting the fittings in. Unfortunatley the only way to verify what is wrong is to teardown the VP, which isn't really all that hard, but I have never had to have one run after I tore it down, and we don't have a pump stand to check them on either. Most of the time if a VP is hard starting do to internal problems it will be hard to start everytime.



I don't know how much affect air on the return side would be for you, I have never checked for it. The pressurizing valve does have a vent in it so any backward leakge of the fuel system could easily get in the VP.



I do recomend that you check for air on the inlet side of the VP. Do this by hooking a line with a ball valve on the end,onto a tapped banjo at the inlet to the VP. With the engine running, bleed off the lift pump pressure into a bucket and hold the ball valve under the fuel to look for foam or bubbles. If the engine wont start , do it the same way by bumping the starter, just be sure to bump the starter thru 3-4 lift pump cycles to make sure all the air is bled from the dump valve line. Some times you will get foam because of the turbulence caused by the fuel going from a high to low pressure when it exits the ball valve , but this usually clears up in a matter of seconds after cracking the valve.

Hope this helps in some way.



A Johnson