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Ask Gm Tech (Cam) Your Own Question
Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 1682
Experience:  GM Grand Master Technician 2007. 14 year experience.
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GMC 2500HD: Just placed both batteries on a LB7It will crank

Customer Question

Just placed both batteries on a LB7
It will crank but not start
It had sat for quite a while prior to battery replacement
I have an air dog on the truck as well as banks 6 gun
Submitted: 2 years ago.
Category: GMC
Expert:  Gm Tech (Cam) replied 2 years ago.
Hi there, I am Cam. Does it fire if you pump the plunger on the top of the fuel filter housing?
Customer: replied 2 years ago.
The plunger is hard, I can barely move it a fraction of an inch
Expert:  Gm Tech (Cam) replied 2 years ago.
Ok, how does it sound while cranking? Is it strong? IF so then with some help, do this. Unplug the connector on the top of the glow plug relay on the back of the drivers side valve cover, then have an assistant crank the engine, and WHILE cranking, spray no more than 1/2 second spray of brake cleans or start aid to see if that will help it. Small shots without the glow plugs will help prevent blow outs.
Customer: replied 2 years ago.
Just replaced both batteries, turns strong.
Just disconnected the heavy red cable from the ?glow plug? Relay.Spray where?
Expert:  Gm Tech (Cam) replied 2 years ago.
Oh sorry, into the air filter.
Expert:  Gm Tech (Cam) replied 2 years ago.
Yes, pulling the heavy cable to the glow plug controller does the same.
Customer: replied 2 years ago.
Ok, did that, and it word catch for a bit but not continue
Expert:  Gm Tech (Cam) replied 2 years ago.
I am thinking we have an injector issue. These use hydraulic pressure from the fuel system to operate the injector. SO a small amount of fuel is supposed to bypass to open the main part of the injector to make it operate. When these start to fail, this causes to much fuel to bypass and return to the tank. This resulting fuel loss will lower the fuel rail pressure and at a time when the pressure is needed most, during cranking, it takes longer to build the required 1450 psi to operate the injector. SO the test we do is this. Find the 2 fuel lines on the rear drivers side valve cover. Take the smaller one off and attach your hose to the metal pipe coming from the engine. Put the other end into the container and run the engine to fill that line. Then shut it off and remove the IGN 1 relay from beside the starter relay. If the engine will not start at that time, crank it to fill the line, then empty out the container and keep the line full. Crank the engine for 15 seconds fully and measure the amount of fuel that comes out of that line. More than 24cc means the injectors have failed and they need to be replaced.
Customer: replied 2 years ago.
I replace the injectors already
less than 10 thousand miles ago
Expert:  Gm Tech (Cam) replied 2 years ago.
OK, we still need to check if you can please.
Customer: replied 2 years ago.
I ran my hand around, and did not feel two lines.
There is a lot of wiring etc on top of the valve cover.
You may have exceeded my ability to follow your directions.
I also don't know where the relay is if I got that far.
Sounds like I needed to have some tubing/hose also to perform this.Not sure where to turn next
Expert:  Gm Tech (Cam) replied 2 years ago.
Yes we must start with this test.