'05 Tahoe 5.3 / P0171 / Rough Idle / Misses

[Bwaite27]

Long story short (as possible) - I recently replaced the fuel pump/ sending unit, all fuel lines and nearly all evap lines because of a P0455 code. Dash kept telling me to tighten fuel cap, so I will likely be replacing the gas cap and fuel fill neck as well, cuz at this point, why not?! I also replaced the intake manifold gaskets. Yesterday morning, I replaced the vapor canister purge valve on top of the intake and cleared the code again. This morning, check engine light back on with new code, P0171. I've checked all the hoses for cracks, found nothing. Sprayed starting fluid around all possible leak spots with no fluctuations in idle. MAF sensor is reading temp accurately. She's idling pretty rough and seems to be missing a bit when accelerating. Anyone have any ideas on what I should check next?

 

[strutaeng]

Ouch! So the lean code started after you replaced the fuel pump or after you replaced the manifold gaskets? Does your scanner give you live data?

Maybe one of the intake manifold gaskets didn't seal properly. There's a pretty easy way to tell why watching fuel trims while accelerating the engine to rule a vacuum leak or not.

 

[Bwaite27]

Ouch! So the lean code started after you replaced the fuel pump or after you replaced the manifold gaskets? Does your scanner give you live data?

Maybe one of the intake manifold gaskets did seal properly. There's a pretty easy way to tell why watching fuel trims while accelerating the engine to rule a vacuum leak or not.

Thanks for the response! So the 171 code I've had since I got the truck about 2 yrs ago. I replaced the 2 upper O2 sensors (OEM parts) before I even put it on the road, it didn't change anything. Then it randomly started telling me to "TIGHTEN FUEL CAP" on the dash, so I replaced that. It kept happening so I checked the code and it was a 455, no more 171. About a month ago, I did all of the aforementioned work at the same time. Now my 455 code is gone and the 171 is back again.

 

[strutaeng]

The EVAP code and the lean code are 2 separate issues. Take it back to the shop and ask them if they could re check their work if the P0171 code wasn't there before you dropped it off.

 

[Ibustbravo]

Here is some info from alldata on that pid.. HPtuners can fix'er up.. And a picture showing it's bank 1. Looks like an air leak. You don't want to turn that pid off, you want to see the error but for testing or with a swap.. it's useful.

Conditions for Clearing the MIL/DTC​

DTCs P0171, P0172, P0174, and P0175 are Type B DTCs.

Diagnostic Aids​

• Allow the engine to reach operating temperature. With the engine running, observe the HO2S parameter with a scan tool. The HO2S value should vary from approximately 40 mV to approximately 900 mV, and respond to fueling changes.
• The normal Short Term Fuel Trim and Long Term Fuel Trim parameters should be between +10 and −10 % with 0 % the optimum, with the engine running at operating temperature.
• Any un-metered air into the engine causes this DTC to set. Thoroughly inspect all areas of the engine for vacuum leaks.
• A MAF sensor condition can cause this DTC without setting a MAF DTC. If there is a MAF sensor condition, the MAF sensor parameters will appear to be within range.
• Inspect the air filter for being the correct one for this application. Make sure that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.
• Certain aftermarket air filters may cause a DTC to set.
• Certain aftermarket air induction systems or modifications to the air induction system may cause a DTC to set.
• Certain aftermarket exhaust system components may cause a DTC to set.

 

[Bwaite27]

The EVAP code and the lean code are 2 separate issues. Take it back to the shop and ask them if they could re check their work if the P0171 code wasn't there before you dropped it off.

I did all this myself, so it would be me having to re-check all that work. I'm really not interested in going through all that again. Lol And yeah, I'm aware they're different, but it just seemed odd that one disappeared for a while during the time that the other was showing up. I've been driving around with a 10 day sticker from the DMV for 2 yrs now and it's time to renew my registration. But I cant until I get it inspected, which I obviously can't do with a light on. So it's down to the wire here. Lol I've put a lot of my time and money into this thing. It's a labor of love, though. I inherited this truck from my grandfather.

 

[Bwaite27]

Here is some info from alldata on that pid.. HPtuners can fix'er up.. And a picture showing it's bank 1. Looks like an air leak. You don't want to turn that pid off, you want to see the error but for testing or with a swap.. it's useful.

Conditions for Clearing the MIL/DTC​

DTCs P0171, P0172, P0174, and P0175 are Type B DTCs.

Diagnostic Aids​

• Allow the engine to reach operating temperature. With the engine running, observe the HO2S parameter with a scan tool. The HO2S value should vary from approximately 40 mV to approximately 900 mV, and respond to fueling changes.
• The normal Short Term Fuel Trim and Long Term Fuel Trim parameters should be between +10 and −10 % with 0 % the optimum, with the engine running at operating temperature.
• Any un-metered air into the engine causes this DTC to set. Thoroughly inspect all areas of the engine for vacuum leaks.
• A MAF sensor condition can cause this DTC without setting a MAF DTC. If there is a MAF sensor condition, the MAF sensor parameters will appear to be within range.
• Inspect the air filter for being the correct one for this application. Make sure that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.
• Certain aftermarket air filters may cause a DTC to set.
• Certain aftermarket air induction systems or modifications to the air induction system may cause a DTC to set.
• Certain aftermarket exhaust system components may cause a DTC to set.

Thanks a lot! That's some good info there. I actually did check some of that, other things I wasn't aware of, so I'll do some digging. Much obliged!

 

[strutaeng]

Understand.

In the future, the EVAP system checks aren't performed when the gas tank is less then 15% or more than 85% full. So you can clear the code, and keep the gas tank topped off and drive it like that and it won't set the code...just saying

 

[Ibustbravo]

Thanks a lot! That's some good info there. I actually did check some of that, other things I wasn't aware of, so I'll do some digging. Much obliged!

NP... Here's the torque specs for the intake.

Intake Manifold Bolt Caution: Refer to Fastener Caution. Tighten: Tighten the bolts a first pass in the sequence shown to 5 Nm (44 lb in). Tighten the bolts a final pass in the sequence shown to 10 Nm (89 lb in).

 

[Bwaite27]

Understand.

In the future, the EVAP system checks aren't performed when the gas tank is less then 15% or more than 85% full. So you can clear the code, and keep the gas tank topped off and drive it like that and it won't set the code...just saying

Understand.

In the future, the EVAP system checks aren't performed when the gas tank is less then 15% or more than 85% full. So you can clear the code, and keep the gas tank topped off and drive it like that and it won't set the code...just saying

I think I'm pickin up what yur puttin down. Wink wink, nudge nudge. Lmao Thank you kindly

 

[Bwaite27]

NP... Here's the torque specs for the intake.

Intake Manifold Bolt Caution: Refer to Fastener Caution. Tighten: Tighten the bolts a first pass in the sequence shown to 5 Nm (44 lb in). Tighten the bolts a final pass in the sequence shown to 10 Nm (89 lb in).

Yup, did that, too. Lol good lookin out

 

[Ibustbravo]

Yup, did that, too. Lol good lookin out

Your code is cleared?

 

[Fless]

I think I'm pickin up what yur puttin down. Wink wink, nudge nudge. Lmao Thank you kindly

If you do it like that and have emissions testing in your area, the evap drive cycle will show as "not run" and it could fail the emissions test.

Check the MAF airflow reading in g/s and see if it tracks with the engine displacement. Has the MAF been cleaned? Have a known good one to substitute?

When stone cold, compare the Intake Air Temp (IAT) or ambient temp and the Engine Coolant Temp (ECT) readings. Also see if they track somewhat as the truck warms up.

Any exhaust leaks on Bank 1?

 

[nonickatall]

I had the same issue on my truck and it was the vent valve, which is located behind the fuel tank. It cost $20 and is swapped in five minutes.

The evap system is used to prevent gases from fuel from escaping into the environment. Of course, gas must be able to escape, because the air has to come out of the tank when the vehicle is refueled. That's why there is the activated carbon canister and the vent valve, which is open, when the vehicle is switched off.
If I now refuel the vehicle, the air can escape into the environment, through the activated carbon canister.
The vent valve closes when the engine is started, and at the same time the purge valve, which is located at the front of the engine, opens. Then these gases are burned. There is a pressure sensor checking if the system is tight. If the vent valve doesn't close, there is no negative pressure in the system and the message comes: "fuel cap open" because that is the most likely cause.

And because you then have a problem with air leakage at the same time, you have a rough idle

 

[Bwaite27]

Your code is cleared?

No. In fact, on Saturday, I plugged my computer back in and it's showing both codes now. 171 is the "system too lean" code, and 455 is the "evap system, large leak" code. I think they're related. I'm thinking the evap leak is big enough that it's letting too much air into the intake and throwing a lean code. I have some investigating to do. I've replaced so much stuff already and checked everything twice. I'm pulling my hair out at this point.

 

[Bwaite27]

If you do it like that and have emissions testing in your area, the evap drive cycle will show as "not run" and it could fail the emissions test.

Check the MAF airflow reading in g/s and see if it tracks with the engine displacement. Has the MAF been cleaned? Have a known good one to substitute?

When stone cold, compare the Intake Air Temp (IAT) or ambient temp and the Engine Coolant Temp (ECT) readings. Also see if they track somewhat as the truck warms up.

Any exhaust leaks on Bank 1?

I checked the IAT & ECT, but only after it was already warm. I'll check it cold later today, thank you! I did clean the MAF, but I don't have a known working one to sub out. And no exhaust leaks.

 

[29tomt]

When I bought my 2003 Tahoe in 2015 and for a while afterwards , it had a little roughness in idle. One day it crak and shutoff immediately and then it finally did crank and I booked it to the shop I use. Prior to that I had cleaned the mass airflow sensor and the throttle valve butterfly and area with the correct spray for each. After putting it on their engine analyzer, shop came back MAF sensor defective. They replaced the sensor. Made a huge difference in idle and overall perf of engine.

 

[nonickatall]

When I bought my 2003 Tahoe in 2015 and for a while afterwards , it had a little roughness in idle. One day it crak and shutoff immediately and then it finally did crank and I booked it to the shop I use. Prior to that I had cleaned the mass airflow sensor and the throttle valve butterfly and area with the correct spray for each. After putting it on their engine analyzer, shop came back MAF sensor defective. They replaced the sensor. Made a huge difference in idle and overall perf of engine.

The air mass sensor is a wearing part and will simply fail at some point. But its accurate measurement of the amount of air is incredibly important for the engine to run. That's why it's just as important that the intake system is airtight and the O2 sensors are working correct.

 

[taj1991]

2005 chevy tahoe Z71 (5.3 gas)
Need a little help here men. In May this year, I needed the head gaskets replaced. Unbeknownst to me, the repair shop installed 799 heads off a 2012 Suburban instead of the original 706 heads. Long story boring, since getting the truck back the CEL has popped every 2 weeks with P0171 and P0174 codes and I've been chasing the problem it seems. My question is, would those different heads be causing the PCM trouble to map the air flow thus throwing those codes? Also, should the PCM have been flashed/tuned for those heads or would the milling have been enough? Any thoughts are welcome.

 

[nonickatall]

If you have a lean mixture on both banks, I would first check all the related parts, regardless the cylinder heads.

And first I would check for air leaks, that is most likely, probably the intake assembly is not mounted correct, or they forgot a hose, for example from the PCV,

Additionally check the air mass sensor, very important check the fuel pressure and check the o2 sensors.

And you can measure the compression. If the compression is equal, the engine should have the same amount of air in the combustion chamber and should inject the correct quantity of fuel, so you should have a correct combustion.