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I have a 2003 gmc with 4.8 engine. Codes show lean on both…

I have a 2003...

I have a 2003 gmc with 4.8 engine. Codes show lean on both banks and replace mas. Have checked for leaks, smoke test intake gasket, checked fuel pressure and everything within spec. Start good on cole engine but as engine warms up, it starts running rough and will not idle. Please advise.

Mechanic's Assistant: Just to clarify, can you actually see the leak?

No leaks visible around intake or vacume hoses. Have checked fuel pump and filter and are good. I am am confused because it starts good and runs good on cold engine but when engine gets warm, it idles rough and starts missing badly.

Mechanic's Assistant: What is the model of your '03 GMC?

1500 sierra, 4wd

Mechanic's Assistant: Are you fixing your Sierra yourself? What have you tried so far?

Yes, had auto zone check on computer. Shows po140 and po143 and also msr replacement. I have replaced the mas and checked all of the vacum hoses for leaks. I had a friend help me smoke the engine and thete were no leaks anywhere. Checked the fuel with gauge and reads a constant 52-54. Trucks run perfectly on the road at 2000 rpms or more but when you stop at a stop light it runs very rough and rpms jump betweem 400-700 rpms . Steve

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

It started doing this overnight. Filled it with fuel night before and next morning it started running rough and engine light came on. Steve

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Answered in 38 minutes by:
1/21/2018
Gabriel Grey
Gabriel Grey, Auto Service Technician
Category: Chevy
Satisfied Customers: 451
Experience: Master Service Technician
Verified

Good afternoon, I hope you are doing great. My name is***** am a Chevy master technician, and I would be happy to assist you with this today.

I have a feeling it is something common that caused both banks to all of the sudden have an issue, usually you will not see a code affect both banks unless something obvious happened, such as a loose intake pipe, or broken vacuum line, but since you have already inspected those I do not think that will be our issue.

I am thinking it may be an issue with the new fuel you put in, if it is not a good mixture it will cause the engine to run rich or lean, and will set a code. Since it has not affected the driveability yet I would recommend driving it until you get the fuel low and then grab a bottle of fuel system cleaner to add next time you fill up. This will let us know if it is a fuel problem or engine problem

Gabriel Grey
Gabriel Grey, Auto Service Technician
Category: Chevy
Satisfied Customers: 451
Experience: Master Service Technician
Verified
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