Does high fuel presure make the 24valve hard to start?

[juicedcummins]

I have done many uprgades to my fuel system including gravity-fed 3/8 lines from both my tanks and i would like to hear you're max #'s and how it starts. I was told by one of the top guys in the business that the 24valve doesen't want to start if you compound 2 fuel pumps.

Is this the case or is he full of crap!!I know from my own exp on chev's this was the ticket on the manual pumps if the truck was tricked out.

Thanks to any info.

 

[DAWG-1]

2 Pumps

I have a pusher pump and a HP lift pump in the stock location. At idle I have 23# and have never seen it below 15# @ wot. The truck starts just fine. I have the pusher pump wired with a relay to run off the stock lift pump signal.

 

[LSMITH]

juiced, I know that the Cummins fix for 98. 5-some 00 models not starting at high altitudes and cold temp includes a shorter lift pump run time before the starter engages and LESS fuel injected when starting. The 98. 5, 99, and some (not sure how many) 00 models will not start above 10000 feet if the temp is under about 15*. Your source of info might know of this and be referring to it. I have not heard of too much fuel available to the injection pump causing a hard start though, I think the problem is in the injection qauntities in a cold cylinder with little air. The bypass valve in the injection pump should let all of the "excess" go back to the tank with no harm IMO.

 

[Steve St.Laurent]

I've posted this previously when talking about the pusher pump setup that I'm running. I talked with both Cummins and D/C engineers about the pressure limitations of the VP44 pump when researching possible options and after adding the pusher pump to my truck. Both said that 20psi would be no problem but that one possible issue is that they've found that any more fuel pressure than 7 psi at the inlet to VP44 could possibly cause a hard start problem. If I turn the key on and immediately go to the crank position it takes a few rotations before the engine fires up (by a few I'm talking about 1-2 seconds). However if you turn the ignition on and then wait for the lift pump to shut down (2-3 seconds) and then crank it starts normally. In reality that's about how long it takes for the wait to start light to go out even when the grid heaters don't cycle. My first recommendation for any hard to start problem would be to make sure that your lift pump has shut down before you start cranking - yet another reason for a full time fuel pressure gauge. So far I have ~20,000 miles on my dual lift pump setup and I see 18 psi cruising and a minimum of 16 psi at WOT - previously I never got more than 16,000 miles out of a stock lift pump setup before hitting 0 psi at WOT, and at best I saw 4 psi at WOT. I'm now going into my first winter with the pusher pump setup (installed it in June) and our temps haven't been below 20 degrees thus far - will keep everyone aprised of my results in the future.

 

[Royal]

I was told by a Bosch Tech. Rep. that the reason DC has the computer limit the feed pump cycle for starting, is because the VP 44 does not like full pressure at cranking speed. He said that the pumping plungers in the VP 44 can not fully charge with fuel under high pressure at cranking RPM. Remember the Vp 44 only turns at 1/2 engine RPM. You may get by with higher pressure,but they engineered the low cycle 7 psi for this purpose. Good luck.

 

[Roger rodbolt]

Juicedcummins,



I've experienced hard or no starts with compound lift pumps. The cure for my truck: I run the aftermarket pump on a manual switch then though a Hobbs pressure switch tapped to oil pressure. This allows only the factory lift pump to operate when starting. Of course you must make sure that the rear pump is designed to flow fuel when it is not running. Dodge reduces voltage to the factory lift pump on start up, in order to prevent hard starts.



I'm sure that I'll get flamed for admitting this, but I idle at 27lbs with a fresh factory lift pump and 25lbs after it "breaks in" for a few thousand miles. HTH

 

[CoastyAV8R]

After reading my Dodge service manual, it said that the lift pump runs at 25% during the crank cycle to limit the amount of fuel to the injection pump at start up. This said, I would say exessive fuel pressure during start up woulld make your truck harder to start. It's in the section that talks about the lift pump operation.



Pat

2001 QC ETH 4x4

 

[D6Diesel]

The only modification I've done so far is pre and post filter fuel pressure gauges. So how best to use at startup? Choices are : 1) rapid turn of ignition key to engage starter motor (I'd think not the best way to start a diesel). 2) Let lift pump run to get pressure up to max (12 psi for me) and then start 3) allow pressure to fall of to 7 psi as per Steve and then start. With all the posts about low pressures from faulty lift pumps, I assumed the higher pressure was best for starting but there's not too much of a difference between 7 and 12 psi... or is there?

 

[LSMITH]

D6, just use the wait to start light. When it goes out fire it up. Your pressure should be 6 or 7 PSI when the light goes out