2004 5.3l - Rough Idle, Misfires, And Lean Codes

Sierra770 · April 12, 2011

I started having a rough idle issue with my 2004 Chevrolet Silverado, 5.3L (T), and the check engine light is on. I took it to my local mechanic and he pulled the codes. The scan tool showed an O2 Sensor issue, multiple mis-fires on cylinders 1,3,5,7, and it showed a lean bank. He suggested I change plugs and wires, which I did. I noticed that plugs 2,4,6,8 had almost completely white insulators (lean), whereas plugs 1,3,5,7 were completely covered with black soot.

Changing plugs and wires made no difference. It seems to idle fine on initial startup, but after about 20-30 seconds, it becomes rough. It feels like it is running fine once off idle and highway speeds, but it feels like it's about to die when idling at a stop light.

Any ideas?

Eric

wrencher · April 14, 2011

Wild guesses are easy, no way in the world a crank sensor can cause your issue. I'll state as fact that your problem IS related to your bank 1 O2 sensor. You really need a scan tool to monitor the voltage of it to be certain as to whether the circuit has an issue or the sensor itself is the problem. If you have access to a scanner that gives real data and not just a silly code reader, you can look at KOEO voltage of the sensor and it should be near 450mv, if not unplug the sensor and if the voltage rises it is a certainty that you have a failed sensor. If you do not have access to a scan tool with datastream, just go ahead and replace the bank 1 O2( before the catalyst on the drivers side). The reason it runs ok on initial start is due to open loop fuel mode, once it warms up and fuel trim begins updating based on the input of O2 sensors, it is richening the bank to it's limit most likely since the O2 is indicating a lean condition via low voltage. An O2 that is stuck at low voltage will cause a lean fuel trim code because it cannot reach a median voltage where it thinks the fuel mix is correct and the O2 sensors are king in the PCM's programming, it responds to whatever they request.

I make a living actually diagnosing problems, not throwing parts at an issue. I do it freelance for many shops and individuals locally after working at dealers for 20 years. I've studied technology diligently for the past 25 years and take great pride in being pinpoint accurate when I recommend a solution. I'm fortunate to have a wife with an excellent job and it allows me to not have to tolerate working with incompetent people that exist in the industry. There are some good techs out there, but far more idiots sadly. About the only thing more misdirected is someone throwing wild guesses on a forum called "Ask The Technician".

Edited April 14, 2011 by wrencher

rdnkcpa · April 12, 2011

How many miles on the truck? Have you had to add coolant, but noticed no leaks?

Sierra770 · April 12, 2011

115,000 miles. No noticable leaks and I have not had to add any coolant. It just started doing this maybe two weeks ago, however, it was intermittent initially. This is the third time, and the longest it's been acting up (over two days now).

I've read that the fuel pressure regulator can be an issue, and to check the vacuum line. Where is the FPR on this truck? It's not on the fuel rails and it's not visible in the fuel line that I can see coming up from the rear of the motor.

Eric

GMstrong · April 12, 2011

all you have to do is locate the fuel rail which should be on the driver side above the intake mani.could be fuel pressure.might be your crank sensor going too.

Sierra770 · April 13, 2011

I've located the fuel rails...one on each side. The feed line comes up from the back of the motor, T's just above the driver's side fuel rail, and runs over the top of the intake, and plums into the passenger side fuel rail. There's no regulator anywhere on top of the engine. There are no vacuum lines running anywhere into the fuel system that's visible from on top. There's a hard plastic line that runs from the top front of the intake, down behind the back of the engine, and connects to a metal line that parallels the fuel line back towards the tank. I'd read that the Flex Fuel vehicles had the regulator in the tank. Mine is not a Flex Fuel, but I wonder...

I started it up today to check for leaks around the intake gasket (none). It idled fine for about thirty seconds and then picked up a really rough idle again. That's pretty much the norm.

Eric

rdnkcpa · April 13, 2011

Have you checked the fuel pressure?

[email protected] · April 13, 2011

Sounds like a vacuum leak on the passenger side of the intake to me, or either a crink or blockage in the fuel crossover pipe that takes the fuel from the drivers side of the intake to the passenger side.

Sierra770 · April 14, 2011

Thanks Wrencher. I'm taking the truck to the shop today. I've printed your reply...maybe it will help them. At 115,000 miles, it was due for new plugs and wires anyway, so I'm not out anything YET. I miss the good 'ole days of carbs and a distributor. Technology certainly has given us more horsepower WITH greater fuel efficiency, but gone are the days of fixing it in your own garage.

Eric

[email protected] · April 14, 2011

Just let us know if a new o2 fixes it. As wrencher makes an excellent point.

Edited April 14, 2011 by 2001Silverado364

Sierra770 · May 4, 2011

Thanks to Wrencher and all the help on this forum. It was the O2 sensor. Problem is fixed, as is the annoying driveline "POP" upon accelerating from a stop (the typical bone-dry slip joint). I've been putting up with that noise for almost 5 years now and never could figure out what it was until I found this site.

Thanks for the help.

Eric

Koppchris822 · November 27, 2016

How many miles on the truck? Have you had to add coolant, but noticed no leaks?

I have this exact problem with a 03 Tahoe 5.3l. Bank 1 and 2 too lean and just noticed a leak while standing on my porch looking at the truck. Thinking it's the manifold gaskets because it has a rough idle when first started and cold but once it warms I think the gaskets expand and seal it and then runs halfway decent. Any ideas?

Edited November 27, 2016 by Koppchris822

Demonspawn29 · January 8, 2019

On 4/14/2011 at 2:29 AM, wrencher said:

Wild guesses are easy, no way in the world a crank sensor can cause your issue. I'll state as fact that your problem IS related to your bank 1 O2 sensor. You really need a scan tool to monitor the voltage of it to be certain as to whether the circuit has an issue or the sensor itself is the problem. If you have access to a scanner that gives real data and not just a silly code reader, you can look at KOEO voltage of the sensor and it should be near 450mv, if not unplug the sensor and if the voltage rises it is a certainty that you have a failed sensor. If you do not have access to a scan tool with datastream, just go ahead and replace the bank 1 O2( before the catalyst on the drivers side). The reason it runs ok on initial start is due to open loop fuel mode, once it warms up and fuel trim begins updating based on the input of O2 sensors, it is richening the bank to it's limit most likely since the O2 is indicating a lean condition via low voltage. An O2 that is stuck at low voltage will cause a lean fuel trim code because it cannot reach a median voltage where it thinks the fuel mix is correct and the O2 sensors are king in the PCM's programming, it responds to whatever they request.

I make a living actually diagnosing problems, not throwing parts at an issue. I do it freelance for many shops and individuals locally after working at dealers for 20 years. I've studied technology diligently for the past 25 years and take great pride in being pinpoint accurate when I recommend a solution. I'm fortunate to have a wife with an excellent job and it allows me to not have to tolerate working with incompetent people that exist in the industry. There are some good techs out there, but far more idiots sadly. About the only thing more misdirected is someone throwing wild guesses on a forum called "Ask The Technician".

Hello I am new to this message board and am having the same issue. I have the same starts fine but within 30 seconds of running it runs rough. If I try to back out of the drive way it will do so but as soon as it is put under a load it hesitates. I pulled the plugs and they were not the correct plugs for my 05 GMC Yukon. I did replace them with the recommended AC Delco plugs for my vehicle. Problem is still there I then after reading your post replaced the 02 sensor one bank one before the cat. After doing so it runs better but still not to the level I would've expected. I am wondering if I should then replace the sensor on bank 2 before the cat as I would assume that if one is bad the other is probably as well. Thanks for your reply in advance

Lancee4ls · May 30, 2019

Wrencher is %100 correct in his diagnosis for my truck. It is the o2 sensor causing my problem. I had misfires and raw fuel coming out of the passenger side tailpipe. Also loss of power. I’m very grateful for his advice.

prime · August 12, 2019

hellow out there call me prime time, been having the same issue with my gmc going to try two new 02 sensers, there not very expensive. ive tryed new plugs same thing again, ilve tryed cleaning them same thing start up fine than start ideling rough. when on the hiway the check engine flashes but at idle it runs rough, check plsugs there black again on all banks but num..1 so ill try 02 sensors, let u know.. thanks.