LB7 Cranks but wont start - Chevy and GMC Duramax Diesel Forum
01-04.5 LB7 Duramax Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission Ect

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Old 12-12-2012, 08:25 PM   eBay Motors  #1 (permalink)
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LB7 Cranks but wont start

Truck seemed to be running fine. thought it may be down on power but couldnt really tell. Thought it was just in my head. Went in the pizza place for 5 minutes and came back out. It would just crank and crank and crank. So being stranded I sprayed some wd40 in the air filter and it started up and will run all day. May be down on power but not too terrible. Sometimes when i shut it of it will fire right back up. Most of the time it just cranks. Any ideas. I definately think im getting air somewhere. IF not getting air in the lines what else could it be.

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Old 12-12-2012, 08:28 PM   eBay Motors  #2 (permalink)
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More than likely with it being an lb7 it'll be bad injectors. What happens the bad injectors let fuel through when they shouldn't and it's able to build enough rail pressure while cranking. When you spray it it cranks fast enough to fire. Once it's running it's turns fast enough to maintain rail pressure. Get your balance rates checked.
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Old 12-12-2012, 08:33 PM   eBay Motors  #3 (permalink)
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ahh dont tell me that.. Just kidding thanks for the input. COULD it just be its sucking air somewhere?
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Old 12-12-2012, 08:37 PM   eBay Motors  #4 (permalink)
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Originally Posted by JHuber219 View Post
ahh dont tell me that.. Just kidding thanks for the input. COULD it just be its sucking air somewhere?
I would suspect it's sucking air. Is the primer soft? Does bleeding and priming help? If so, I'd suspect a leaking fuel filter head. Nothing an o-ring kit wouldn't fix.
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Old 12-12-2012, 08:39 PM   eBay Motors  #5 (permalink)
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i can pump it up and get it semi hard.... sounds bad, but when i walk around and crank it and go back to check it, its soft again. Last night i got it real hard and cranked for about 30 seconds and it started.
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Old 12-12-2012, 08:50 PM   eBay Motors  #6 (permalink)
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your getting air in the fuel it shouldn't get soft again. check the filter to make sure it is tight. worse comes to worse use an air compressor to blow down the fuel line from the filter head to see if there is a leak anywhere. Do smell diesel around the truck. I would check fuel lines and fuel head for leaks.
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Old 12-12-2012, 08:51 PM   eBay Motors  #7 (permalink)
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If it's losing prime it might just need the fuel filter head orings replaced.
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Old 12-12-2012, 08:53 PM   eBay Motors  #8 (permalink)
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ok well thats good news, hopefully i can get this figured out. Have you heard of getting rid of the stock filter housing completely and just running an air dog. I have one ordered it should be here friday.
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Old 12-12-2012, 09:01 PM   eBay Motors  #9 (permalink)
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Yep a bunch of people do that. The air dog filters fine.
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Old 12-12-2012, 09:08 PM   eBay Motors  #10 (permalink)
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is it pretty easy? Im still pretty new to diesels. They scare me a little bit. And yes this morning i could smell fuel but cant find where its coming from. Im sure its from the filter housing. Just praying I can bypass that with the air dog and it fires right up.
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