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I have a 2000 chevy c2500 obs 5.7liter. I've been running lean for a while and I don't know how to fix it. I've replaced all o2 sensors, fuel pump, fuel pressure regulator, throttle position sensor, mass air flow sensor, and still running lean. Vacuum? It's possible but I did a compression check on each cylinder which all gave me about 180psi. Any ideas? Thanks.
 

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How do you know it's lean, is it throwing codes?
 

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A compression check, (to the best of my knowledge) won't tell you if if you have a vac leak. Assuming thats what you meant. You might have an intake leak at the lower intake section or upper, as well as a line or fitting. Make sure that the MAP sensor that sits in the upper intake plenum is sitting tight and not leaking. The grommet on those can degrade and cause it to suck in more air. You could also try taking some brake/carb cleaner and spraying it around different areas like intake flanges and lines when the engine is running. Pretty much anywhere there is a potential for a vacuum leak. If you spray a certain area and it causes the engine rpm to change, it's sucking in air at that spot. Do your lean codes refer to either bank 1 or bank 2? If it's only one bank thats being affected then I would look for a vacuum leak on that side. Bank 1 is usually driver side cylinders, and bank 2 is passenger side on GM engines. If both banks are getting a lean code then I would look at what can affect both cylinder banks for intake leaks. Usually this is something to do with the upper intake plenum since that feeds both banks before the air goes into the lower intake runners. It could also be a line that feeds into the upper intake too like a brake booster line.

It's also probably worth checking the fuel pressure still to make sure that it's within spec if you haven't already.
 

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It's also probably worth checking the fuel pressure still to make sure that it's within spec if you haven't already.
^^^X2 - when you turn the key on, engine off, what pressure do you see? Then when you turn off the key, how long does it hold pressure?
 

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Discussion Starter #6
A compression check, (to the best of my knowledge) won't tell you if if you have a vac leak. Assuming thats what you meant. You might have an intake leak at the lower intake section or upper, as well as a line or fitting. Make sure that the MAP sensor that sits in the upper intake plenum is sitting tight and not leaking. The grommet on those can degrade and cause it to suck in more air. You could also try taking some brake/carb cleaner and spraying it around different areas like intake flanges and lines when the engine is running. Pretty much anywhere there is a potential for a vacuum leak. If you spray a certain area and it causes the engine rpm to change, it's sucking in air at that spot. Do your lean codes refer to either bank 1 or bank 2? If it's only one bank thats being affected then I would look for a vacuum leak on that side. Bank 1 is usually driver side cylinders, and bank 2 is passenger side on GM engines. If both banks are getting a lean code then I would look at what can affect both cylinder banks for intake leaks. Usually this is something to do with the upper intake plenum since that feeds both banks before the air goes into the lower intake runners. It could also be a line that feeds into the upper intake too like a brake booster line.

It's also probably worth checking the fuel pressure still to make sure that it's within spec if you haven't already.
Thanks man. At this point it's in both banks. When I remove brake booster check valve I release air so I think I'm good there right? I appreciate your help
 

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The air that your hearing being released is the air thats inside the vacuum booster. All that means in the check valve is working by not allowing air to bleed back through into the manifold, and that the booster is holding a vacuum. It doesn't rule out a bad hose or leak at the fitting that goes into the manifold.
 
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