Isuzu No Start

Michael Mobile Technician Utah Posted   Latest  
Case Study
Drivetrain
2005 Isuzu NPR 5.2L (6 4HK1)
Crank / No Start

I made a long trip to check out an Isuzu 4.5 hours South of my location. The customer had contacted me and told me that he had replaced the injection pump and the dealer said it needed to be programmed. I suggested that it did not sound right but I would bring all my programming and electrical test equipment just in case.

When I got there, sure enough, it was a no start. With a shot of Ether the vehicle would start and run fine. I chose to use the TEXA tool to help in diagnosing this one. In looking at the scan data, there were some things that did not look right. First, right after the engine was shut off the residual rail pressure dropped to zero. When running with the volume control valve disconnected, the pressure was at 36,000 psi at idle. When connected, it was 4600 psi which is very close to desired. At first, I thought maybe the sensor or wiring had issues. I checked the continuity between the PCM and sensor and all three leads were Golden. (I know a load test is better but I was being lazy) I checked the grounds and they were all intact. The sensor is a 3 wire sensor. I set the possibility of the sensor being a problem on the side for a moment and looked for other issues.

One thing I noticed was that the volume control valve (I don't know Isuzu's term) was reading current flow even when the vehicle was being started. This means that it is activated. I would think that it would full flow the pump until starting. The injection pressure when trying to start without the ether was only 1700 psi which is not enough to open the pintles. My customer told me 2500 psi was needed. So I set aside the Volume Control Valve because even when disconnected, the engine would not start without Ether. (I know it is not a good practice but you gotta figure stuff out) I am now thinking a mechanical problem. At this point I gotta shout out to Tim and Heather out at ATG. Even though I have not taken an Isuzu common rail course from ATG, the principles of operation are largely the same whether Bosch or Denso. Understanding the principles from the Ford and GM diesel classes, I determined we needed to see what the flow coming from the injectors was. We removed the metal line and connected a rubber hose to the return on the head. When running the fuel was pouring out. Well, there's your sign. The injectors were leaking. This explains the zero pressure when shut off. Most systems I have seen will retain some residual pressure.

When the customer called the dealer that was 2 hours away for suggestions, they told him 100% it is the injection pump. On one hand, I am sure they were just trying to help out. They did not want to tell my customer that he needed to send it down on a wrecker. On the other hand, nothing is 100% besides maybe a mid 90s GM fuel pump failure or a Nissan CVT failure. (Just a joke to keep things light.) What the dealer should have done is suggested my customer do some diagnostic tests before spending $3K on a pump. It is hard to hear but an 8 ball and a parts cannon can be a timely and expensive repair.

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Interesting
Thanks
Michael Mechanic
Texas
Michael
 

No fault codes?

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Michael Mobile Technician
Utah
Michael
 

Michael Rumfield, When I first connected there were no codes. We did generate codes by disconnecting sensor and volume valve wiring. The customer stated that the battery had gone dead. So there could have been some help with codes but it was all gone.

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Michael Mechanic
Texas
Michael
 

So, these are the original symptoms before the new injection pump was installed? And dealer probably said pump needs to be timed… which it does, but that shouldn't cause these symptoms, I don't believe. Can you start and drive watching live data and watch the fuel pressure with engine under load? It's been a while since I worked on 5.6, if memory serves there is a fuel pressure regulator in…

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Ryan Owner/Technician
British Columbia
Ryan
 

Good call Michael, The Denso HP fuel pump seems to flow a higher volume then the engine requires, compared to a Bosch system. Excessive injector return flow will show up with a 5.9L/6.7L/Duramax long before a John Deere or Isuzu will. A large majority of the Denso injectors we test will have high return flow but no customer complaint. That being said, when is gets to an extreme amount then they…

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Agree
Helpful
Jim Technician
New Jersey
Jim
   

You need to verify injector returns. I suspect you have a bad injector. Those pumps will generate the 6000psi needed to start in 1 revolution. I rarely see a bad VCV. Its usually an injector or the pump. Thge fact it will start and remain running with a snort, tells me it just needs to get over the hump to start. SO its close. My gut says injector(s)

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Michael Mechanic
Texas
Michael
 

I tend to agree on the rare leaking VCV, I only ran into 2 over about 8 years of regular working on these Isuzu… having said that, that is bringing a memory up of live data fuel flow while engine is running. I think the data could be compared while running and possibly noting a different flow rate on a poor injector… might could gather some telling info on injector cut out test as well. Someone…

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Jason Mechanic
Washington
Jason
 

Hi Michael, No start for high injector return is fairly common on these for us. Usually no codes. We pull the valve cover and the return manifold, screw some old banjo bolts with the heads cut off and plastic tubing pushed on as a quick dirty way to check returns. There is usually one or two that is returning a lot more than the others to confirm. Isuzu sells a kit with all injectors, high…

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