P0171

My wife's '04 4.2L I6 trailblazer has started setting the P0171 lean on bank 1 code. I am trying to determine if this is too much air or not enough fuel. I used my reader to clear the code and look at fuel trims at idle with the vehicle warm. The LTFT is solid at 8.2% and the STFT oscillates between +5% and -3%. The RPMS are around 600.

While driving, the code will be set and the RPMS will go to around 900 at idle and will not return to 600. If I clear the code, they will immediately drop back to 600 at idle. I also noticed that if at idle with the code cleared, when I turn on the AC compressor, the code will immediately be set and the RPMS jump to 900+.

Any advice on how to determine if this a lack of fuel issue or too much air issue?
 

The_Roadie

Founding Member
Administrator
Welcome! I'm not the specialist on these, but the LTFT needs to go close to 25% to set the code. Looking at the numbers while driving or datalogging them for later analysis might be useful.

My memory seems to tell me that owners have more problems with vacuum leaks than leaky injectors, so you might slap a vacuum gauge on it. Intake manifold bolts are notorious for getting loose onthe I6, but they're also notoriously difficult to get to except by feel or through the wheel well. Also the brake booster diaphragm is a suspect for leaking - pull the hose and plug it off and see what changes. And loose, cracked or disconnected cavuum hoses, of course.

How many miles?

Any other maintenance history?
 

Robbabob

Well-Known Member
Welcome to the Nation!

Seems to be quite a few of these codes lately. Our platforms are getting old.

Several possibilities, easiest and common is that vacuum check. You're in good hands here, neighbor.
 

bobdec

Well-Known Member
Agree vac leak is most common. Usually a vac leak (if small) will have less impact as RPM and load increase. A lean caused by a fuel delivery problem (filter, pump, injector) will usually get worse under load. Also since MAP and RPM are used to calculate fueling in the '04 a marginal MAP could be a culprit. With key on engine off do BARO (barometric pressure) and MAP (Manifold Absolute Pressure) both read the same (usually 97-100Kpa depending on weather/altitude). At idle your PCM has to add 8% fuel to keep the ST Trims close to 0% (-3 to +5). As roadie says would be interesting to see what trims look like under load, 8% is not really that bad, not enough to set the DTC it must get worse at specific times or conditions.
 
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steve-in-ga

Member
I hooked up the scanner and drove the car, as RPMs got higher the LT trim got worse calling for more and more fuel. The O2 sensor seems to be operatong correctly. As you all stated, I suspect a fuel delivery problem over a vacuum leak at this point. I am going to get a fuel guage and and see what that reading is. I think I am also going to clean the throttle body and check the MAP sensor.

Does that seem like a sound plan with the readings I am geting at this point?
 

IllogicTC

Well-Known Member
KNBlazer said:
What about your Fuel Pressure Regulator? Sometimes the easiest things are overlooked....
I support this suggestion. FPRs are a common enough part needing swapped around here (they're not necessarily trouble parts, just tend to fail more often than other parts, relatively speaking).
 
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steve-in-ga

Member
I checked the fuel pressure without the vehicle started and it is 52 which is in the range. When I start the vehicle, it goes to 55 which according to the manual it should drop if the fuel pressure regulator is good. I pulled the vacuum line with the vehicle running and it made no difference in the fuel pressure. So I checked the vacuum at the resinator and there is no pressure. According to the manual, there should be 10-14 untis of vacuum. I hooked up my vacuum pump to the fuel pressure regualor and every time I hit it the fuel pressure drops.

I think the FPR is fine, I just have no vacuum at the port on the resinotor where the vacuum line connects to the FPR.

Where does the vaccum at the resinator come from that goes to the FPR?
 
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steve-in-ga

Member
My wife has had a few things done to it at a local garage so I do not know the answer to that question. What kind of parts are you referring to?
 

KNBlazer

Well-Known Member
steve-in-ga said:
My wife has had a few things done to it at a local garage so I do not know the answer to that question. What kind of parts are you referring to?
Just trying to build a timeline of when exactly it was that the problem was noticed...other members report that sometimes when using non oem parts create other issues as they don't work well with our trucks...
 
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steve-in-ga

Member
nothing has been replaced in a year or so.

not having any vacuum from the resinator vacuum port to the fuel pressure regulator is a definite problem as fuel flow is restricted. I need to figure out why there is no vacuum at that port.
 

Texan

Well-Known Member
If you look to the right from the port on the resonator, there is a port on the intake manifold. On my 03 short wheelbase TB that port has a rubber cover.
 

thisguy1985

Member
steve-in-ga said:
nothing has been replaced in a year or so.

not having any vacuum from the resinator vacuum port to the fuel pressure regulator is a definite problem as fuel flow is restricted. I need to figure out why there is no vacuum at that port.
i just seen this, it explains why theres no vacuum at the resonator.
http://gmtnation.com/forums/topic/10911-fuel-pressure-regulator/

im also getting the p0171 code and goin crazy tryin to figure out why :confused:
 

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