P0171 & P0174 codes
 

Everybody,
I am trying to get my truck ready for emissions here in Phoenix. I am receiving these codes on the computer: P0171 & P0172. They are Bank 1 (Driver's side) & Bank 2 (Passenger side) lean. These O2 sensors are before the catalytic converters. The vehicle is a '99 NBS Silverado 1/2ton, 4x4, 4.3 V6, & 5 sp manual transmission with 143k miles.
These are the items I have done:
1. Clean MAF & verified air filter was clean
2. Sprayed carb/throttle body cleaner around base of upper intake - no change in idle
3. Cleaned throttle body with throttle body spray
4. Replaced PVC hoses (1 elbow & short length of hose)
5. Used vacuum hose to brake booster to suck 1/3 pint of Sea Foam into engine
6. Poured remaining Sea Foam into gas tank
7. Poured another brand of injector cleaner into gas tank
8. Replaced fuel filter
A few months back I changed the fuel regulator because the truck would take a few seconds before it finally fired up on the first time of the day. This didn't help at all. I ordered an AC fuel pump today & will be replacing it when it comes in. Hopefully this will help with the hard starting problem & also the codes. This will be the 3rd pump I have had in it. The original & the pump I am now going to replace. The 2nd pump is an AC pump, also. After every change I have made, I have cleared the codes using the scanner from either AutoZone or O'Reilly's. Normally, the codes come back in anywhere from 5-10 miles. After the filter change today, it took 25 miles before they came back in. Unfortunately, I have to drive the truck 40-50 milles before the ECM is reading all the inputs & is ready for the emission test. I have NOT replaced the O2 sensors because I have read that if they both get the "lean" codes at the same time, then most likely, they are reading correctly. Also, the CCs were replaced a couple of years ago, also.
Any help is appreciated because I am very very frustrated with this, other than this, the truck runs great.

Chris

Re: P0171 & P0174 codes
 

Your MAF could be causing it. Smoke test the intake system. Its the best way to check for vacuum leaks. It does rarely happen, but both O2 sensors can fail at the same time.

Also, pull the spark plugs and compare bank 1 to bank 2. Is there a difference?

Re: P0171 & P0174 codes
 

Is there a way to test the MAF? how do you do a smoke test? I also heard about checking for vacuum leaks with propane. How is this done, also?

Re: P0171 & P0174 codes
 

Need to monitor Short Term Fuel Trims with a scan tool, If they are way positive while idling then go down at part throttle.....90% chance of a vacuum leak.

The bottom of the intake can leak, Intake gaskets on Vortec 4.3L's are a maintenance item. Use the Fel-Pro metal backed ones for replacement.

Re: P0171 & P0174 codes
 

Any recommendation on the model of scanner to use/buy? I was looking at Actron CP9575 or this one from Harbor Freight.

http://www.harborfreight.com/automot...ool-60693.html

Amazon.com: Actron CP9575 Auto Scanner...

Re: P0171 & P0174 codes
 

I replaced the fuel pump today & cleared the codes. They came back in at about 15 or so miles. Tomorrow I am going to check the base of the intake & see if I hear a rise in the idle. Then I have to decide if I am going to replace the gaskets myself or not......

Re: P0171 & P0174 codes
 

Intake gaskets are a common problem, I had to change mine a while back. That is exactly what was causing my engine to throw lean codes.

Re: P0171 & P0174 codes
 

Today I sprayed card cleaner around the base of the intake and base of the throttle body....no change in the idle. Is it worth it (about $100) to replace the O2 sensors? Or just replace the intake gaskets?

Re: P0171 & P0174 codes
 

I would do the intake before the O2 sensors.

Re: P0171 & P0174 codes
 

Need to monitor Short Term Fuel Trims with a scan tool, If they are way positive while idling then go down at part throttle.....90% chance of a vacuum leak.



I got a scan tool earlier today that reads live data. At idle (~700rpm), the STFT reads anwhere from 1.5 ot lower to 3.6 or higher. Most of the time it is around 3.1. At about 1500rpm, the STFT drops to about -.1 to -.5. Is this enough to indicate a vacuum leak? After changing the fuel pump, the codes come in at about 45 miles after resetting the computer.

Re: P0171 & P0174 codes
 

Symptoms of a BAD GM MAF Sensor
The most obvious one is that the CHECK ENGINE Light (CEL) will be on on your Instrument Cluster and driving you nuts. Here are a couple of others:

1.A MAF Sensor Diagnostic Trouble Code (DTC) stored in your vehicle computer’s memory.
•P0101, P0102
2.Lean and/or Rich Diagnostic Trouble Code(s).
•P0171, P01743.Fuel Trim Diagnostic Trouble Code(s).
4.No Power when you accelerate the car or truck.
5.Black smoke coming from the tail-pipe.
6.Your car or truck or SUV may idle rough and stall.


How do I test the MAF without a multimeter? I have one, but not one that reads in hertz.

Re: P0171 & P0174 codes
 

Intake gaskets. I have done a lot of them.

Re: P0171 & P0174 codes
 

I am very happy to report that I PASSED!! This was yestrerday. After looking over the engine again...I noticed the the tube that connects to the air cleaner tube from the PS valve cover was off. I must not have noticed it was off after I put the tube back on after I sprayed the the throttle body with cleaner. I went ahead and c/o the MAF sensor since it is easy to replace & not terribly expensive. I took it to Emissions the next morning. I had my scanner hooked up. It only takes approx. 15 miles for the computer to be ready, NOT 40-50 miles as I first thought. The truck now has 50 miles on it since I last cleared the codes and no codes have returned....:metal:
Thanks everybody for the suggestions & input!!

Chris