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Discussion Starter #1
I've got a 2006 c7500 with the 7.8 isuzu engine that runs great as long as you spray it. If you try to start it without spray it wont catch. I have no pressure leaking from the lines to the filter. If I hook the outlet line up to the injection pump it leaks off to the tank with it not running. I unhooked the return line from the Injectors and the rail leaving it just hooked to the pump and with pressure from the tank it bleeds back to the tank.
 

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do you just have to spray it to get it going than it runs great? or does it die when you stop spraying? If you just have to use spray to get it going you need to figure out what injector is returning to much fuel. spraying it boosts the cranking speed just a little bit so the CP4 is able to over-come the excess return condition.
 

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Discussion Starter #3
It runs pretty good after getting it started and will continue to run until its shut off. I'm trying to get a scan tool to communicate to the ecm. I've got a launch millennium hd pro but I'm having trouble getting it to communicate to the engine computer. It does have a check engine light on now. The feed side holds pressure from the tank lines to the outlet of the last filter but if I try to pressurize the whole system it loses pressure from the return port on the injection pump. I thought it would have some type of check valve to keep pressure built up on the feed side when the engine is off.
 

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pull a vacum on the line that goes back to the fuel tank, see if it holds. if fuel bleeds off, repair the line back to the tank. that way you dont have to prime the truck and fuel system everytime you want to start it. the reason it wont actually start is one fuel injector is probably returning to much fuel. you need to do an injector balance and flow rate test on the injectors and find the bad one.
 

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Discussion Starter #5
With the return line disconnected from the Injectors it bleeds the pressure back to the tank from the injector pump. I would have thought there would be a check valve in the pump to keep the pressure built up.
 

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Discussion Starter #7
So apparently it sounds like the problem is in my injection pump. Would you agree?
 

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Discussion Starter #9
Bad think is it might have 50 miles on it. It's a replacement pump that was installed due to lack of power and smoking.
 

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Look untill you go thru basic troubleshooting you will keep chasing your tail. Flow test the pump, vac test the low pressure side of the fuel system for leaks and check the return rates of the injectors. or pay someone to actually do the diagnostics.
 

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Discussion Starter #11
Both feed and return hoses from the tanks to the frame are new I just replaced them. I know it's not on the lines from the tanks to the pump cause they'll hold air pressure. The injectors dont leak on the return side or in the cylinder or oil. I'm going to have it sent to the shop that remanufactured the injector pump to see what they can figure out.
 

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your not testing for injectors to leak on the return side, you have to flow test them to see if they return to much fuel while in operation.
 

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Discussion Starter #14
It runs good when it's running so I doubt it's an injector problem. To me if it was an injector problem it would leak into the oil, the cylinder or bleed back into the return system none of which it's doing. Like I said with the return line unhooked from the injectors it doesnt leak there when the feed side is pressurized.
 

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actually when the return is out of spec on injectors they can work perfectly fine when the engine is running, but return to much and make a no or hard start.
 

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Discussion Starter #16
I understand what you're saying but with the engine off it's not holding pressure. If I take shop air pressure to the feed line going to the engine and nothing but the return line hooked to just the injection pump the pressure feeds back to the tanks. Therefore it's not loosing pressure from the injectors fuel filter housing fuel rail or fuel lines. Its somehow bypassing it in the injection pump. I'm going to have it taken to the shop where the injection pump was rebuilt at to see what they say.
 

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Ive changed lots of injectors that have good balance rates but the return rate is way out of spec
 

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Discussion Starter #19
So if your right hows the pressure getting back to the return side of the pump with the return line not even hooked to the injectors??
 
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