Runs bad at an idle but not all the time

BBZ

MEMBER
2005 CHEVROLET SILVERADO
5.3L
V8
4WD
AUTOMATIC
112,000 MILES

I checked for codes cleared them and the same ones came back. The three I have is po131 po171 and po300. The exhaust is sooty. I have not checked the fuel pressure regulator yet nor have I checked fuel pressure. I also have not pulled the plugs. I did put plugs in the truck about 3,000 miles ago they had a good color to them then. Not full of deposits either.I have cleaned the mass air flow sensor.

8 Replies

JACOBANDNICKOLAS

MECHANIC
108,289 POSTS

P0171 - System too Lean (Bank 1)

P0131 - Oxygen Sensor Circuit Low Voltage (Bank 1 Sensor 1)

P0300 - Random/Multiple Cylinder Misfire Detected

Okay, the 171 leads me to believe there is a vacuum leak. Have you checked for that? The idea that you have a po300 can be the result of both other codes. Have you checked the wiring to B1S1? Have you checked the sensor itself? Do you have access to a scanner that can provide live readings of how the sensors are reacting?

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Friday, May 9th, 2014 AT 7:18 PM

BBZ

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5 POSTS

Looked at vacuum lines, used carb spray on lines and on intake manifold. Found no leaks. Someone said the intake gaskets are prone to leakage after about 100,000- 150,000 miles. They said the gaskets are an rubber o-ring type and they loose their shape and will cause a vacuum leak that will not identify with carb spray. I checked fuel pressure, has 55lbs. The pressure increases like it should when the throttle is snapped. I found no vacuum hose and no fuel pressure regulator on the fuel rails. I checked both rails a few times but found no regulator. Fuel comes in on the left side, there is a crossover tube going to the right side. On the right side is the schrader valve for checking pressure. But that is all except for each individual outlets for the injectors. Thanks for your help.

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Saturday, May 10th, 2014 AT 5:05 PM

JACOBANDNICKOLAS

MECHANIC
108,289 POSTS

They are pron to have problems. I've found many with coolant leaks. If the fuel pressure is good, then the regulator (that could be on the pump) is working.

You know, about a week ago, I ran into a problem with a Chevy van that ran bad at times at an idle. It would start running bad at times off an idle, but the idle was the worst. It would actually die. Other times it ran perfect. What I found was the coolant temp switch was bad and sending a bad signal (only at times) to the computer. As a result, the computer would run in a closed loop and dump fuel because the sensor told it the coolant temp was -40 degrees F. It was a a 20 dollar part and once replaced, it ran perfect. You are showing a lean mixture, but the coolant sensor could do the opposite telling the computer it is hot and lean the fuel mixture. It's just a thought. And yes, I was getting a PO300 from the vehicle. The O2 sensor codes you get COULD be the result of this type of issue.

The only way I was able to find the problem was to scan the computer with an active scanner that showed me what the ECM was seeing. Do you have access to a scanner like that that you could check such things?

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Saturday, May 10th, 2014 AT 6:33 PM

BBZ

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5 POSTS

Monday the 12th of May I will be hooking the truck up to scan tool. All I had before was a code reader. I will also be checking the o2 sensors. What has me puzzled is the black soot on the inside of my exhaust pipe and on the body. When I go to pass someone and I give it full throttle I also get the black soot smoke. That is why I thought it might be running rich instead of lean. But I might be mistaken. I might have the two conditions mixed up in my head. I'll get back with you Monday and let you know what I found and hopefully solve the problem. Thanks again Bob

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Saturday, May 10th, 2014 AT 8:01 PM

JACOBANDNICKOLAS