2003 GMC Sierra Rough Idle

FAST92LT1RS

MEMBER
2003 GMC SIERRA

Engine problem
2003 GMC Sierra Four Wheel Drive Automatic 126950 miles

I have a 2003 Sierra 5.3L. My problem is that once the engine is up to temperature the idle will fluctuate. It will hunt down as low as 300 RPM. It does this while it is in park or in drive while stopped at a light. It does not stall but sounds like it wants to. I noticed that when it is hunting the volt meter drops below 14 Volts. Sometimes as low as 10V. I change the fuel filter once a year and it was changed less than 6 months ago. I checked the fuel pressure and it was 45-50 PSI. The check engine light is not on and I scanned the computer and did not return any codes. Plugs and wires were changed at 100K miles. It now has about 126K miles. What do you think?
Pete

5 Replies

MASTERTECHTIM

MECHANIC
4,749 POSTS

Unplug the vacuum line at the fuel pressure regulator and if there is any sign of gas then replace it. This is where I would start.

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Tuesday, February 19th, 2008 AT 11:59 AM

FAST92LT1RS

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13 POSTS

I did that and there was no fuel. Does this engine have an IAC Valve? I do non't think it does because it has the electric throttle body. I think the throttle body might be dirty or on the way out.

-Pete

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Wednesday, February 20th, 2008 AT 11:45 AM

FAST92LT1RS

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Fixed. A friend at a dealership said the most common cause of poor idling on all GM vehicles using the 5.3L V8 engine is a dirty throttle body. With the engine off remove the air ducting and spray throttle body cleaner on the throttle plate and inside the body. Use a soft tooth brush to scrub the edges and both sides of the throttle plate and around the inside of the throttle body. Spray again to rinse the carbon away. Use a soft cloth to remove the rest of the residue. The truck now idles strong and just as good as the day I drove it off the lot. I hope this helps out a lot of people. I am a Navy Diesel Mechanic and hate having to take any of my vehicles to the dealership.

-Pete

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+5

Monday, February 25th, 2008 AT 3:15 PM

MASTERTECHTIM

MECHANIC
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THANK YOU. IM SURE THAT IS GOING TO HELP A LOT OF PEOPLE. I HAVE NOT RUN ACROSS THAT YET BUT IM SURE I WILL.

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Monday, February 25th, 2008 AT 3:50 PM

JMADDOX703

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1 POST

I have a 2003 gmc sierra and I have replaced fuel pump and spark plugs and wires and it is still running rough will not idle right, runs rough like it is not firing on all cylinders