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  #1  
Old 01-27-2009, 08:50 PM
123eddie 123eddie is offline
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Default Dodge 12 valve fuel pressure

What fuel pressure readings are normal for a 98 Dodge 12 valve. On diesel and wvo I idle around 15 psi and under full load around 20 psi. I have an Isspro mechanical gauge with a ball valve to temper the pressure. Lately the needle is bouncing around instead of being steady while idling.
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Old 01-27-2009, 09:49 PM
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VinDiesel VinDiesel is offline
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Do you mean needle valve? It would be very difficult to just crack open a ball valve enough to let a gauge read, but not enough to let pulsations through.

The book (Chrysler service manual) calls for Fuel Pressure: 18-24 psi at idle, 28-36 at 2000 rpm with no load. However, in "real life" what you're getting is about normal. The overflow valve gets weak and pressure decreases. After changing mine, the new one didn't hold up well at all. I ended up replacing the spring with a heavier one. My '96 now runs 25-28 all the time on diesel.
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'96 DODGE CTD 3500 SLT ext. cab LB
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Old 01-28-2009, 04:33 AM
123eddie 123eddie is offline
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Thanks Vin and yes I did mean needle valve. Do they run better at the higher fuel pressure?
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Old 01-28-2009, 06:27 AM
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cgoodwin cgoodwin is offline
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With a direct reading Snap On gauge mine reads 0-18 at idle and 0-35 at 2000. Yes I mean 0-35, the needle bounces so strongly that it is a blur - remember this is a piston pump. A while ago I was working with a well known diesel performance manufacturer and had access to the techs at thier "Skunk works", they advised that the stock pump be used but if it had to be replaced that a 45psi pump be used in it's place with sufficient flow to keep the inlet pressure above 25psi under load.

The pressure regulator in the return banjo should open according to cummins at 22psi if memory serves.
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Old 01-28-2009, 06:33 AM
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Quote:
Originally Posted by cgoodwin View Post
The pressure regulator in the return banjo should open according to cummins at 22psi if memory serves.
That sounds right - I spoke to Cummins about that a couple of years ago and they informed me that until you hit that point, there wouldn't be any fuel returning from the IP.

I've set my electric pump up to push 23 psi on WVO - and on diesel it bumps up a bit to ~ 26 psi. Turning on the controlled loop while on WVO eases the vacuum a bit and gives me ~ 25 psi on WVO. It's pretty consistent at idle through full throttle.
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Old 01-28-2009, 07:46 PM
123eddie 123eddie is offline
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I'm seeing 20 psi at 2,000 rpm with no load. The truck has 128,000 miles on it and seems to run fine. I have had the fuel pressure gauge in about 3 months. The banjo bolt with the regulator looks like it has never had a wrench on it so I think i will replace the regulator and see what happens. I was worried that my lift pump was starting to go and that was the reason for my low pressure.
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Old 01-28-2009, 09:34 PM
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VinDiesel VinDiesel is offline
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Quote:
Originally Posted by 123eddie
Thanks Vin and yes I did mean needle valve. Do they run better at the higher fuel pressure?
Not much reason to be above 25-30, but mine didn't really start running poorly until less than 10psi.


Quote:
Originally Posted by 123eddie
I'm seeing 20 psi at 2,000 rpm with no load. The truck has 128,000 miles on it and seems to run fine. I have had the fuel pressure gauge in about 3 months. The banjo bolt with the regulator looks like it has never had a wrench on it so I think i will replace the regulator and see what happens. I was worried that my lift pump was starting to go and that was the reason for my low pressure.
Your overflow valve is more likely the culprit. [I have a good used LP as a spare now, for this reason.] You can carefully remove the OFV (may have to file or grind just a bit on the corner of the head to get it out), disassemble, and stretch the spring a little. That will last for a while. Careful selection of another spring from the hardware store might work as well or better than a $45.00 replacement OFV.

BTW, a quick diagnosis for the lift pump is to use smooth-jawed vise grips (or a hose clamping device made for this) to temporarily clamp off the return line (rubber part ). Start the engine, observe pressure, be ready to shut it down. The piston lift pump in good condition can do 45psi+ very quickly.
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'96 DODGE CTD 3500 SLT ext. cab LB
BOMBed 12 valve Cummins Turbo Diesel
'94 3500 custom crew cab LB
stock 12 valve Cummins- can't keep a flexplate in it, not even a fancy/expensive billet one???
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  #8  
Old 02-01-2009, 05:45 AM
123eddie 123eddie is offline
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Quote:
Originally Posted by cgoodwin View Post
With a direct reading Snap On gauge mine reads 0-18 at idle and 0-35 at 2000. Yes I mean 0-35, the needle bounces so strongly that it is a blur - remember this is a piston pump. A while ago I was working with a well known diesel performance manufacturer and had access to the techs at thier "Skunk works", they advised that the stock pump be used but if it had to be replaced that a 45psi pump be used in it's place with sufficient flow to keep the inlet pressure above 25psi under load.

The pressure regulator in the return banjo should open according to cummins at 22psi if memory serves.
Chris's description above of hooking up a Snap On gauge is what started this post for me. My gauge is all over the place and making a tick tock noise. I just put 600 miles on it and I think I have solved my wildly swinging needle issue. My needle valve is opening a little on its own and then the gauge goes ballistic. With the needle valve properly adjusted I see 15 psi at idle and never over 20 psi under full load. I plan on checking the pressure regulator once I have my gauge issue solved.
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