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1999 cummins isx 450. Fuel pressure drops after throttle…

Customer Question
1999 cummins isx 450...

1999 cummins isx 450. Fuel pressure drops after throttle input and stays at about 40-70psi

Mechanic's Assistant: What's the make/model/year of your truck? Engine type?

2000 freightliner with cumins isx.

Mechanic's Assistant: Are you fixing your truck yourself? What have you tried so far?

New fuel lines. Bew fuel pump. New pressure valves, new check valves. New fuel shutoff valve. New fuel filter.

Mechanic's Assistant: Anything else you want the truck mechanic to know before I connect you?

Basically rebuilt the entire fuel delivery system

Submitted: 8 months ago.Category: Medium and Heavy Trucks
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Answered in 21 minutes by:
11/22/2017
Technician: Daniel Wilson,
 replied 8 months ago
Daniel Wilson
Category: Medium and Heavy Trucks
Satisfied Customers: 6,825
Experience: Owner at DJ Wilson
Verified

Hi my name is***** will be glad to help. Any fault codes? Check for air on the suction and return side by using a clear hose or site glass to monitor for air bubbles. Let me know the results. We can follow up as needed.

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Customer reply replied 8 months ago
Supply lines are new. Also tried running direct from a external tank.After removing fuel pressure sensor and reinstalling it engine ran briefly at 260pai. After throttle input sudden pressure drop again to 60-70psi. Attached gage matches sensor reading. Pressure relief valve is new (250psi)
Customer reply replied 8 months ago
If i maintain throttle input engine will choke out and shut off.
Technician: Daniel Wilson,
 replied 8 months ago

Hi, That is not where I was going with it. I want to confirm no air on the suction side and see if there is air at the return which could lead to the fuel rail sucking air or injector problems.

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Customer reply replied 8 months ago
Faults - 0482 low fuel suply pressure at pressure sensor
0144 coolant temperature sensor high voltage
0422 voltage on sensors engine protect disabled.
241 lost vehicle speed signal - cruise disabled.Very high mileage truck. Over 3million miles
Customer reply replied 8 months ago
Okay. I will get clear hose and perform test as requested
Technician: Daniel Wilson,
 replied 8 months ago

Click on this link

ISX Troubleshooting

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Technician: Daniel Wilson,
 replied 8 months ago

Also VSS signal is important and will cause a cut back on RPM and performance. I would investigate that for a sensor or wiring issue,

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Customer reply replied 8 months ago
Supply no bubbles coming in
-Return line looks like there is bubbles
-Hooked up injector leak test tool, disconnected solenoids and disconnected shutoff solenoid. Cranked over engine. No bubbles coming from injectors.
Customer reply replied 8 months ago
Thats the front 3. I need to do the rear 3 injector leak test now. Will let you know soon.
Customer reply replied 8 months ago
Rear 3 injector leak test. No bubbles.
Customer reply replied 8 months ago
Sometimes the pressure stays over 250psi, idle is rough. When i throttle up slowly pressure rises as operation smooths out to about 265psi and than suddenly drops to about 60psi, engine begins to run very rough and will usually stay at 40-70psi. If i keep my foot on throttle engine will choke out. If i let off throttle RPM drops and stays at about 450rpm, pressure stays low in the 40-70 range. On random occasion will climb to 250psi usually after re-pressurizing the system.
Can any of the solenoid cause this?
Can injector cause this even if leak test passed?
Sucking in air? If so where?Truck originally came in because wouldn't start, found damaged fuel lines and debris on inlet, repaired. Performed complete rebuild of fuel manifold with new check valves and pressure valves and new shut off solenoid. I have some spare parts we tried to swap in.Originally when i installed a new 250psi valve engine overpressurised and shut down twice. Replaced 250psi valve again and we are at the current state. When i reinstalled the 250psi valve that seemed to have over pressurised the system we remained at current state as described above.
Technician: Daniel Wilson,
 replied 8 months ago

Thanks for the update. If the fuel supply pump and pressure is good then check the regulator for acting up or sticking causing the pressure to bypass and drop off. It is possible for an injector to intermittently act up. Take exhaust port temps for a cold cylinder. Also look for a weak injector with Insite. Cut out all except the one being tested, then engine should stumble before stalling. A weak injector will stall instantly. Let me know the results. Also 500K is the average life of injectors and the change interval recommended by Cummins so if you have at least that on them it is time anyway for a set.

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Technician: Daniel Wilson,
 replied 7 months ago

Hi, Did you get this resolved? Please take a moment to rate as that is how the site credits my time helping on the site or let me know if you need more help. 5 star and bonuses are always greatly appreciated.

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Customer reply replied 7 months ago
We performed injector leak test. Both front 3 and rear 3 injectors passed the leak test.Proceeded to remove all 6 injectors (they are mchanical) sent out for testing, 5 of 6 failed very badly.Also found debris in fuel delivery to injectors. Cleaned it out. In process of installing new set of injectors. Will update when complete.
Technician: Daniel Wilson,
 replied 7 months ago

Thanks for the update. It's a pleasure to help.

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Customer reply replied 7 months ago
Tested with new injectors. Starts without ether now and runs smoother.As you increase RPM fuel pressure rises to about 280psi.
At about 50-70% throttle there is sudden fuel pressure drop to 80psi. Slightly higher than before. Lots of white/gray smoke. Engine idle becomes rough.
Technician: Daniel Wilson,
 replied 7 months ago

Thanks for the update. Glad you got the hard start aspect corrected and for the drop monitor with a clear hose or sight glass between the tank and pump and the return for air bubbles. This will help determine a supply issue or an injector issue if bubbles found on return only.... Let me know, Pleasure to help

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Technician: Daniel Wilson,
 replied 7 months ago

Please take a moment to rate as that is how the site credits my time helping on the site or let me know if you need more help. 5 star and bonuses are always greatly appreciated.

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Technician: Daniel Wilson,
 replied 6 months ago

Hello,I see this is still open with no response from you. Would a call be easier for you? Ill send over the offer to get that set up. I am trying to close out my open questions and get them resolved. If you have resolved this please let me know. Im not an employee of the site and my time is valuable as is yours. The site only credits me a portion of the question value on accepted answers and positive feedback. I see neither has happened here. I took the time to post and research this information for you. Could you do the same for me?Take the time to either post a response with a status or leave a rating if your problem is resolved? It's been a pleasure to help.

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