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Discussion Starter #1
I have a 96 12 valve with a 5 speed and 215 pump everything is stock besides the fuel plate. when the truck is under load especially in 4th and 5th gear my fuel pressure drops to 10-15 psi and loses power. it has an adjustable over flow valve which is set around 24 psi at idle. while revving in neutral at 2000 rpm the pressure goes up to 30 psi. i have tightened the adjustable ofv before and the pressure at idle would go past 30 but still drop to 10-15 psi when under load. i have changed the fuel filter, cleaned the pre filter screen, deleted the the fuel heater, tried 2 new cummins lift pumps, put in a new fuel line and a new sending unit in the tank but nothing has fixed or helped the low fuel pressure issue. I would like to keep the mechanical lift pump and not go to an electric pump but i cant find whats causing the low fuel pressure issue.
 

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Have you checked the FSS for full travel and adjustment? Any sharp bends in the fuel supply hose? An electric pump would be an expensive downgrade.
 

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You say you've "tried 2 new Cummins lift pumps ", were they stock replacements or hot rod lift pumps?? All the normal suspects sell a hot rod Cummins lift pump. You can also upgrade the spring in most Cummins lift pumps using a comp cams valve spring (part # 973-1).

What size fuel line did you use when you replaced the stock stuff?
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Discussion Starter #4
Have you checked the FSS for full travel and adjustment? Any sharp bends in the fuel supply hose? An electric pump would be an expensive downgrade.
ive checked the FSS multiple times and taken it apart to make sure nothing was inside preventing it from traveling all the way. the fuel line has no sharp bends or kinks.
 

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Discussion Starter #5
You say you've "tried 2 new Cummins lift pumps ", were they stock replacements or hot rod lift pumps?? All the normal suspects sell a hot rod Cummins lift pump. You can also upgrade the spring in most Cummins lift pumps using a comp cams valve spring (part # 973-1).

What size fuel line did you use when you replaced the stock stuff?
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ive tried a regular cummins lift pump from genos and a hot rod one from power driven. both gave the same amount of pressure while under load but the hot rod one made more pressure at idle. i used 3/8 fuel line
 

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Possible the OFV is sticking, malfunctioning, or going bad? Seems you have done everything else.
 

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Are you really "losing power" under load??? With 10-15psi still being supplied to the injection pump under load you really shouldn't be losing any power unless your turning the motor to some crazy high RPM were the barrels can't refill in time
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I say recheck the fuel line routing for kinks/ bends and if that's good, look at doing a sump or fuel draw straw with 5/8 line.
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Discussion Starter #8
Are you really "losing power" under load??? With 10-15psi still being supplied to the injection pump under load you really shouldn't be losing any power unless your turning the motor to some crazy high RPM were the barrels can't refill in time
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I say recheck the fuel line routing for kinks/ bends and if that's good, look at doing a sump or fuel draw straw with 5/8 line.
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it has stock governor springs so im not turning high rpms. It seems to run alright with 15 psi but when it drops below 15 the sound of the engine changes and i lose power. When going up a grade i can get it to stay at 15 psi but if i let it drop below 15 it starts running weird and wont accelerate. it'll hold its speed when the pressure drops but it wont go any faster. ive noticed it doesn't matter what rpm its at when i lose power its whenever the pressure goes lower than 15 it starts having some issues. i just recently put the fuel line in and have checked it for kinks and bends twice but i can check again. theres also been no change in fuel pressure when i went from the stock fuel line to the new one.
 

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How does the short piece of rubber fuel line that goes from preheater to lift pump look? That is a formed hose. Others who have tried to use a length of 3/8 fuel line don’t realize it has to bend too much and kinks there. I think someone here posted they got away with it by routing the hose differently. I think you can get a decent view of it through the fender well.
 

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Discussion Starter #10
How does the short piece of rubber fuel line that goes from preheater to lift pump look? That is a formed hose. Others who have tried to use a length of 3/8 fuel line don’t realize it has to bend too much and kinks there. I think someone here posted they got away with it by routing the hose differently. I think you can get a decent view of it through the fender well.
The one thats on it looks like its in good shape. I bought a new one anyway but havnt put it on yet.
 

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Discussion Starter #11
today i bypassed the prefilter and it didn't make a change in fuel pressure. i took off the hard line going from the lift pump to the top of the filter to make sure that or the banjo bolt wasn't clogged and they weren't. so then i took off the line going from the filter to the injection pump to make sure nothing somehow got passed the filter and clogged it and that line and banjo bolt was clean. With the engine losing power below 15 psi of fuel pressure i have egts over 1,200 and full boost (35 with boost elbow and over 40 psi with the waste gate plugged) so if im getting full boost and high egts it seems like theres enough fuel even with it dropping below 15 psi. the timing is stock at 12.5 degrees which i have checked and it hasnt slipped so what could be causing loss of power with high egts and full boost? i do think my fuel pressure is lower than it should be but should i be looking for something else thats causing my loss of power?
 

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What RPM does this loss of power happen at? And does it always happen at the same-ish RPM?? If that's the case, you may simply be running into the stock governor springs pulling out power.

For me that started around 2500-ish RPM and got worse as RPM went up. Made merging onto the freeway a bit of a challenge when the engine ran out of RPM in fourth gear (of an NV4500), but shifting into fifth gear dropped the RPM so much that I had fell off the turbo.

I fixed that with a set of 4K gov springs. Makes the truck just easier to drive now because I can hold RPM a little higher such that when I shift the engine stays at a high enough RPM that I don't need to re-spool the turbo after every shift. Also virtually eliminated the black smoke puff I used to get after every shift.
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Discussion Starter #13
What RPM does this loss of power happen at? And does it always happen at the same-ish RPM?? If that's the case, you may simply be running into the stock governor springs pulling out power.

For me that started around 2500-ish RPM and got worse as RPM went up. Made merging onto the freeway a bit of a challenge when the engine ran out of RPM in fourth gear (of an NV4500), but shifting into fifth gear dropped the RPM so much that I had fell off the turbo.

I fixed that with a set of 4K gov springs. Makes the truck just easier to drive now because I can hold RPM a little higher such that when I shift the engine stays at a high enough RPM that I don't need to re-spool the turbo after every shift. Also virtually eliminated the black smoke puff I used to get after every shift.
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It will do it at any rpm. its when i step on it or when theres a demand for power like when towing. for an example im in California so the speed limit while towing is 55. i will be going up a grade towing a boat which is maybe 3,000 pounds including the trailer. if im going up a hill at 60 mph in 5th i can maintain my speed but if i try to accelerate at all (barley pushing the throttle) the engine starts sounding funny and theres less power. if i down shift into 4th i can also maintain 60 mph but if i push the throttle any farther than what it needs to maintain 60 mph i lose power. its a very strange feeling when it "loses power" it feels similar to when the governor starts defueling. the engine doesn't lose or gain rpm when i have this feeling of lost power the rpms stay the same but its sounds funny and the truck doesn't pull forward. this also doesn't only happen when towing its just more noticeable and does it more often with the boat behind it. i always forget if the rear end is 3.54 or 3.55 but its not the 4.10. so 60 mph in 5th its turning kind of low rpms and in 4th i want to say its around 2,000 rpms at 60 mph
 
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