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Old 03-14-2020, 06:58 PM   #1 (permalink)
mr93wrangler
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CP4 pump help

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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I currently own a 2003 c4500 Chevy truck looking for help and advice for repairs and believe this is the best place for the help and information

2006 c7500 with the 7.8 isuzu engine
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Old 03-21-2020, 01:01 AM   #2 (permalink)
ScannerRavager
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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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Old 03-21-2020, 08:36 AM   #3 (permalink)
mr93wrangler
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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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2006 c7500 with the 7.8 isuzu engine
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Old 03-21-2020, 07:25 PM   #4 (permalink)
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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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Old 03-23-2020, 07:05 AM   #5 (permalink)
mr93wrangler
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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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I currently own a 2003 c4500 Chevy truck looking for help and advice for repairs and believe this is the best place for the help and information

2006 c7500 with the 7.8 isuzu engine
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Old 03-24-2020, 11:05 AM   #6 (permalink)
ScannerRavager
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the check valve is in the fuel filter head.
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Old 03-24-2020, 11:10 AM   #7 (permalink)
mr93wrangler
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So apparently it sounds like the problem is in my injection pump. Would you agree?
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I currently own a 2003 c4500 Chevy truck looking for help and advice for repairs and believe this is the best place for the help and information

2006 c7500 with the 7.8 isuzu engine
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Old 03-24-2020, 02:27 PM   #8 (permalink)
ScannerRavager
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probably, cp4's die all the time. only way to know for sure is to do a flow rate test.
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Old 03-24-2020, 02:28 PM   #9 (permalink)
mr93wrangler
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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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I currently own a 2003 c4500 Chevy truck looking for help and advice for repairs and believe this is the best place for the help and information

2006 c7500 with the 7.8 isuzu engine
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Old 03-25-2020, 06:34 AM   #10 (permalink)
ScannerRavager
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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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