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Discussion Starter #1 (Edited)
I've been chasing this problem for 2 months now with no results. Even recently took it to the dealer to have my EBCM replaced due to my ABS light on and had they diagnose the P0171. They said they found the seal between the brake booster + master cylinder split (they said they heard the leak coming from there) but as fate would have it, no luck and the light is back on. Here's a little of everything I've done and the freeze frame data that shows up with the code.

1. Cleaned MAF and TB (was clean already due a previous cleaning)
2. Check for air leaks (smoked checked) - none found
3. Checked air filter - replaced with K&N panel filter, no change but wasn't expecting one
4. Check for air leaks, removed intake manifold cover (w/o removing manifold) - no luck
5. Replaced PRE + POST O2 sensors as a tune up (I know POST wouldn't matter to this cause)
6. Replaced MAF
7. Check for air leaks - none found
8. Dealer visit - new seal at brake booster with no luck

Before you start suggesting where to look, here's the situation where the code appears and only where the code appears, no matter how many times I clear the code. It honestly feels as if there is no issue, just a little worse gas mileage and an engine light in the cluster.

The truck reaches normal operating temperature with no issues (enters closed loop) and for example I drive on the highway for 1 hour. When I get off the highway and I reach where I was going, I stop and put the truck in park. If I wait 30 seconds the engine light will come on, P0171. Only "drivability" issue if you will call it that is when the truck is sitting for that 30 seconds, the idle will drop down to normal then dip maybe 50-100 RPM and come back up to around 650 RPM. I know the code occurs at HOT IDLE. So what's happening at hot idle? Vacuum would supposed to be at the highest amount because the throttle plate is closed. Pulse width on the injectors is obviously increasing but why and what's causing it?

Only options I've narrowed it down to is the following - remember I don't have drivability issues:

1. Bad TPS sensor
2. Faulty MAP sensor
3. Partial dirty injectors
4. Weak fuel pump at idle
5. PCM needs a reflash for whatever reason?
6. An unknown or seen/heard vacuum leak

Here's my FREEZE FRAME data:

FUEL SYS 1 CL (closed loop)
CALC LOAD % 13.3
ECT (F) 192
STFT B1 % 14.8
LTFT B1 % 29.7
FUEL PRES (PSI) 43.5
MAP (INHG) 8.3

ENG RPM 602
SPARK ADV (DEGREE) 7.5
IAT (F) 27
MAF (LB/MIN) 0.3
TPS % 14.1
AIR STATUS OFF
COMMAND EVAP % 15.3
FUEL LEVEL % 80.0
EVAP_VP (INH20) -3.050
BARO (INHG) 29.8
CAT TE 11 (F) 941
ECU VOLTS 13.240
LOAD VALUE % 11.4
EQ RATIO 0.999
REL TPS % 0.8
AMBIENT (F) 18
TPS B % 14.1
ACC PEDAL D % 19.2
ACC PEDAL E % 9.4
COMMAND TAC % 1.2

Let me know what you guys think! I've bolded what I think is important. My vacuum reading is low (normal operating conditions is 17-22 InHg?) - not sure about the fuel pressure, I couldn't find a specification. I think my TPS reading is OK, there are 3 different numbers so I don't know. Barometric pressure is normal for the elevation. Whenever the code comes on, my LTFT is always at least +30% so my injectors are increasing their pulse width 30% trying to compensate for the unmetered air between my MAF and my first O2. If the MAP sensor is faulty then it's giving the PCM a faulty reading, saying it's under load which would increase the fuel.

Please send help.
 

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Discussion Starter #3
Is the PCV fresh air hose (valve cover to manifold) a one-way flow tube? It doesn't have a check ball so it's a calibrated tube with some sort of valve? Or can air go both ways through? I've smoked tested the EVAP system it's OK. Had replaced the vent solenoid and updated wiring harness last year.
 

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i've been having that slight idle dip you speak of on occasion, only code i've had is an accel pedal position correlation, which hasnt popped up in awhile and when it did it always cleared on its own. if you track down that idle dip though let me know because things like that drive me nuts but i dont understand enough about vacuums on vehicles to mess with it
 

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Discussion Starter #5
An easy way to understand it in a basic sense is th accelerator pedal is an input through the PCM and an output through the throttle position sensor. The throttle plate won't shut 100% after you take your foot off the pedal because it'll suck the whole intake through the engine! So because it's an electronically controlled unit, it responds in accordance to vehicle speed and temperature to determine the correct throttle position coming to idle. My friend recently had trouble with a similar issues and he cleaned his TB which was super dirty (he had an idle code). Devices being out of correlation that are both electrically controlled is another issue that may just be slow response between the two asked on whatever is going on in the circuit. Hope it helps a little about how that part works. Any other info you can provide about your issue?I'll let you know though!
 

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Discussion Starter #6
I don't know the specifics but it takes a lot of inputs (load, MAP, MAF, etc) and determines what to do in the circuit. The TPS is also an input sensor to a lot of other circuits.
 

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Map sensor.

At idle you should be pulling closer to 20 inHG. The Map says only 8.
It only happens at idle because at higher loads the system is running off the MAF. Then at idle it goes to the MAP and it takes that long for the trims to max out.
 
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