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Discussion Starter · #1 ·
Hey guys/gals. Hopefully someone can help me out with this, as I cant figure it out. I have a 2008 Colorado, 2wd, 5sp, with 105k miles. The other day I was driving and I noticed my truck would buck/stutter when I would begin to accelerate in each gear. This mostly happens in lower rpms, 1800-2200. The check engine light came on and I had it scanned, P0171 "lean bank 1". I decided to change the air filter since it had been awhile and the code went off. Unfortunately it is still stuttering/bucking. I have since cleaned the mass air meter, replaced sparkplugs, and even tried a new mass air meter. Nothing seems to work. After doing some research I found that this is typically caused by a vaccum leak. Therefore I took a can of carb cleaner and sprayed on all intake connections, the crankcase vent, etc and noticed no change in the vehicles idle. Any ideas for me to try next. I was thinking to check fuel pressure to see if maybe the pump/filter are faulty? Maybe check 02 sensors?

Cliff Notes:
Truck threw P0171 code "lean bank 1". Light comes on and off.
Have tried:
cleaning mass air
replacing mass air
replaced sparkplugs
checked for vacuum leaks.

Any ideas on where to look next? Should I check fuel pressure? Maybe 02 sensor?
 

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I don't know if carb cleaner will do any good (since we don't have a carb). I only say that because some chemical cleaners are made for specific applications for a reason. Also, spraying it into the intake doesn't do much as it just gets sucked into the cylinders and burned up instantly.

I don't think it will correct your issue, but a routine procedure is to clean the throttle body. It requires removing the throttle body from the engine and cleaning with Throttle Body Cleaner (not carb cleaner). That usually cures erratic idle and stalling, but not a stumble.

A lean condition is generally a result of air entering the engine after it has been metered by the Mass Air Flow Sensor. Air is read, computer makes adjustments as necessary and you're done. However when the system is read that it's lean it means air entered the engine but fuel was not increased to match the air.

Some say to replace the front O2 sensor. Maybe that could be your next attack.

Has any work been done lately, service/maintenance? Or was this an out of the blue thing?

Do you know how to remove and clean the throttle body?
 

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Discussion Starter · #5 ·
I don't know if carb cleaner will do any good (since we don't have a carb). I only say that because some chemical cleaners are made for specific applications for a reason. Also, spraying it into the intake doesn't do much as it just gets sucked into the cylinders and burned up instantly.

I don't think it will correct your issue, but a routine procedure is to clean the throttle body. It requires removing the throttle body from the engine and cleaning with Throttle Body Cleaner (not carb cleaner). That usually cures erratic idle and stalling, but not a stumble.

A lean condition is generally a result of air entering the engine after it has been metered by the Mass Air Flow Sensor. Air is read, computer makes adjustments as necessary and you're done. However when the system is read that it's lean it means air entered the engine but fuel was not increased to match the air.

Some say to replace the front O2 sensor. Maybe that could be your next attack.

Has any work been done lately, service/maintenance? Or was this an out of the blue thing?

Do you know how to remove and clean the throttle body?
Thanks for the help. I wasn't spraying the carb cleaner to clean the intake. I was spraying it around the intake and air inlet to check for vacuum leaks. If there is a leak the cleaner will be breathed in causing a higher idle.

I'm guessing I'll throw a fuel pressure gage on it.
 

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Discussion Starter · #7 ·
Unfortunately I was not able to find any vaccum leaks, which would have been an easy fix.

The frustrating part is this has happened out of no where. I haven't performed any maintenance recently and the vehicle is completely stock. I purchased the truck with 70k on the odometer and it now has 105k, this is the 1st time I've had any issues.
 

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Discussion Starter · #9 ·
first o2 sensor could be shitting the bed, other than that or a vacuum leak all that's left is the fuel pump or injectors. Your fuel pressure regulator is built into the top of the pump since it is returnless.
Thanks. Is there a place to check fuel pressure on the fuel rail? I figure I will check fuel pressure to help diagnose. I will try the O2 sensor as well.
 

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yeah see the black round thing below the feed line(a little tough to notice)that is a cap for a standard gauge fitting. That faces the front of the motor so look near the front of the intake manifold and you should see it.
 

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Before you get too far into complicated diagnostics, did you check the vacuum hose at the rear part of the resonator box/intake manifold? That one often gets overlooked when replacing the spark plugs (as the resonator has to come off), and if it's not reattached properly it'll throw that code.
 

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I get that code every time my air intake hose gets lose or moves
on theTB
 

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Discussion Starter · #16 ·
Well here is an update, I was driving the truck the other day and it went into reduced power mode and was stuttering, misfiring pretty bad. Bad enough i thought i popped it. I turned the vehicle off, unplugged the maf sensor and plugged it back in and the truck ran ok again. When I mean ok, I mean that it still stutters at low rpm but other than that it runs like a top. Any further suggestions?
 

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Discussion Starter · #19 ·
I checked my fuel pressure yesterday and that is also fine. I thi k I finally figured out what is wrong. Now it's just getting to it. I believe there is a vacuum leak under the black plastic intake cover. I think it's the line going from valve cover to the mani. Looking online I think I have to take the intake manifold off to get inside that cover. Any ideas? That seems like a lot of work.
 
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