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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3304
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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2007 Isuzu NPR 5.2. Constantly going into limp mode.

Customer Question

2007 Isuzu NPR 5.2. Constantly going into limp mode.
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Jeremy replied 1 year ago.

My name is ***** ***** I will assist you.

Do you have any codes on this vehicle for the issue that you are having? Have you checked the dpf filter on the vehicle? It can cause a repetitive limp mode. Were you just replacing parts or did you find an issue wi the vehicle?

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Customer: replied 1 year ago.
Truck doesn't have dpf filter. Just standard fuel filter which has been replaced as well as filter housing.
The issue with the vehicle is that it continues to pur itself in limp mode. Vehicle has to be turned off and restarted in order to get out of limp mode.
Expert:  Jeremy replied 1 year ago.

Low pressure on rail code. I see that all the time and it can be a number of things on these trucks. You already replaced all the fuel system parts, but the injection pump? Have you deadheaded the injection pump with a fuel gauge to see what the pressure is reading?

On the fuel tank to the frame, that rubber hose, have you checked it to make sure it isn't sucking itself inward? That is common on these vehicles. The hose gets worn out and they close up. I usually run the fuel from a bucket through a larger new line, then go around the block and see if the power holds or throws back to limp mode.

If back into limp, I then go through the chart.

One place to check is the suction on the fuel filter housing. Isuzu had a lot of problems with the filter housing on these engines. Bypass it temp and run around the blosk as well and see if the issue goes away. The 2005-2008, almost 2009 had the issues with the filter housings.

I'm attaching the diagnostic sheet for the code. Let me pull it up right quick.

Expert:  Jeremy replied 1 year ago.

Diagnostic Sheet attached HERE

Expert:  Jeremy replied 1 year ago.

Diagnostic Sheet attached HERE

Customer: replied 1 year ago.
Haven't dead headed pump but scanner shows actual pressure and desired pressure. Vehicle is never more than 5mpa's of desired pressure. Also vacuum on suction line is 5 inches of mercury at WOT.
Fuel filter and housing are brand new. Tank has been removed, inspected, and cleaned as well as the pick up tube and screen, every rubber hose replaced, and every metal line inspected. Fuel pressure sensor replaced as well as fuel pressure sensor pigtail.
Expert:  Jeremy replied 1 year ago.

Are the readings showing to be normal? I'm presuming you are using IDSS for the scanning?

I would deadhead the pump and see if it is building up or put a manual gauge inline. With you doing all that you have done, checking fuel lines, sensors, etc then I would look at the pump, injectors (but is no fuel mixing with oil, I doubt it is an injector), corrosion on ECM causing false codes. Sounds like short of replacing the pump, you have a new fuel system on the truck. :-(

Deadhead the pump and see if it is bleeding off in it or if the governor is acting up and bypassing the fuel at the pump.

Customer: replied 1 year ago.
All readings seem to fall in desired ranges. Have also removed ECM and inspected terminals. All seems fine. Truck won't go 50 feet without entering limp mode. What will dead heading pump tell me that scanner won't? Scanner shows me desired pressure as well as actual pressure. Always very close numbers.
Expert:  Jeremy replied 1 year ago.

The deadhead will give the high pressure off the pump. If the desired is 42, but pump deadheads 39, then the pump is bad. If it deadheads 50, then you are good. You can also see if the governor is bleeding off too much pressure by the pump building up to above 45 and then you show it dump down to below 42, then you will know the governor spring in the pump needs to be changed and not the whole pump.

Customer: replied 1 year ago.
Ok. Will check in morning and let u know.
Expert:  Jeremy replied 1 year ago.

I have attached the breakdown of the pump. The limiter is what the deadhead is going to check. That limiter gets stuck and will cause the pressure to get low and not build to desired.

Let me know in the morning what you find.


Customer: replied 1 year ago.
far everything checks out. Going thru the flow chart at step 15 is to check resistance at ecm. Do you have a numbered terminal breakdown so I know which pin is which?
Expert:  Jeremy replied 1 year ago.

ECM Pinout as requested HERE.

I'm sorry I was on late today. House had a plumbing pipe burst.

Customer: replied 1 year ago.
We got all pins figured out.
We went through entire flow chart and everything checks ok.
Dont have correct fuel pressure gauge to dead head pump. Workin on gettin one tomorrow.
Expert:  Jeremy replied 1 year ago.

Hmmm... Can I get a VIN so I can send a email to engineering? It is odd that everything is checking out on the flow chart and you are getting this code. I want to see what they have to say in regards ***** ***** issue.

Customer: replied 1 year ago.
Send first thing in the morning.
Expert:  Jeremy replied 1 year ago.

Ok. I want to see if engineering may have another flow sheet or if there is a TSB for the issue. I can pull up all TSBs, but it is easier to have VIN for vehicle and engine specific items.

Attached is a TSB for the issues you have described and the code where it states to replace the suction control on the pump. There are many others, but none other so far for this condition.

Customer: replied 1 year ago.
Expert:  Jeremy replied 1 year ago.

Per Engineering, if you did the flow chart and changed out the suction control valve with the same results, the fault will either lie in the injection pump itself or via and issue with the ECM. They stated that the ECM is a last resort and the most rare for this concern. However, per the complaints they have been receiving, the injection pump has resolved this issue 85% of the time after the control valve TSB has been completed and the issue remains. The other 15% has been fuel filter housing and/or rust in the fuel tank.

This is per Scott Randolph, Isuzu Engineer via the email I got back from them.

Expert:  Jeremy replied 1 year ago.

I couldn;t tell you what the rest of his title was for it was in Japanese.

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