1999 GMC Sierra


mike hunter 88

February, 7, 2008 AT 3:16 PM

Computer problem
1999 GMC Sierra V8 Four Wheel Drive Automatic 92,000 miles

When I start my truck in the cold it idels rough and dies untill it gets warm. The service engine light came on after that. The codes are PO171 Left bank runing lean. The other code is PO174 Right bank lean. I check the internet and found most of the sites suggest to clean the mass air flow sensor and replace the fuel filter. I bought that mass air flow senser cleaner, a bottle of fuel system cleaner, a fuel filter, and air filter. Replaced the iteams and cleared the codes. The next couple days it was around 50 degrees outside and everything ran fine. After a couple of days it got cold around 20 degrees. The truck was hard to start again and the codes are back. What dou you think. Should I replace the mass air flow sensor or is it something else. Thanks for any help you can give me. Mike


2 Answers



February, 7, 2008 AT 6:06 PM

First I belive you should change your coolant temperature sensor since it adjusts the fuel on cold starts, if it is bad it will make the vehicle run lean causing the hard poor start and the lean codes.


brian 1

February, 7, 2008 AT 6:16 PM

Might want to try this first, found a bulletin for this problem. Cold Engine Idle Misfire Or DTCs P0171 P0174 - keywords 4.8 5.3 6.0 air cold crack hesitate idle intermittent internal intake leak lean misfire miss P0171 P0174 P0300 power rough surge vacuum.99 through 05 GEN III truck engines may experience any of the following: A cold rough idle that diminishes as the engine warms up.
Possible DTCS P0171, P0174, or P0300.
Random misfires at idle.
The intake gasket seals may be leaking around the intake ports, causing a lean condition. The intake gasket seals may leak around the bottom side of the port, making it a challenge to detect the vacuum leak. The technician may also notice lower than normal MAF readings if the intake seals are leaking.

If leaking intake gasket seals are found, replace the intake gaskets. During the repair be sure to use the current GMSPO part number for replacement intake gaskets. The current GMSPO part number for intake gaskets will have a teal green intake gasket seal. The old version of the intake gaskets were orange in color. Use only the teal green colored gaskets for a repair.
The intake plenum should also be checked for excessive warpage that may cause an uneven clamping pressure of the seal. If the plenum is found to be warped it will need to be replaced. Service Information states, " An intake manifold with warpage in excess of 3 mm (0.118 in) over a 200 mm (7.87 in) area should be replaced". Intake warpage is measured at the gasket sealing surfaces, and should be measured within a 200 mm (7.87 in) area. This measurement is across only two of the intake runner port openings. Measurements taken across the entire distance of all four intake runner ports will lead to unnecessary manifold replacements.
Maybe this will help.

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