1999 GMC Sierra Classic 5.7L - Check Engine Light, Hard Start, P0171 & P0174

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General Information: This customer came in with an A/C problem but also wanted to get the check engine light looked at while it was at the shop. They also did not want to spend much on repairs.

First thoughts: After pulling the codes P0171 and P0174, some general problems came to my head with this vehicle. I know these fuel pumps always go out, but it could be a vacuum leak. I also thought that I don’t need to spend too much time on it since the customer is not really concerned with the problem too much. So I’ll just focus on the A/C problem.

Diagnostics: I decided to check the live data on my small scan tool. Generally these codes, P0171 and P0174, mean that there is too much air in the air-fuel mixture. Now, that could mean roughly two things. Either there is too much air entering the engine through a vacuum leak, or not enough fuel because of a weak fuel pump or clogged fuel filter. Of course, there are other issues that could cause it but those are the most likely. I decided to check for a vacuum leak first with the scan tool.

1999 GMC Sierra Classic 5.7L - Check Engine Light, Hard Start, P0171 & P0174

Looking at the short term and long term fuel trims, you can see that they are elevated over 10. (Picture Below) Generally, when you total the fuel trims, anything over 10 is considered lean and anything under -10 is considered rich.

1999 GMC Sierra Classic 5.7L - Check Engine Light, Hard Start, P0171 & P0174

When I raise the RPMs, the fuel trims go down. (Picture Below) Generally that points to a vacuum leak.

1999 GMC Sierra Classic 5.7L - Check Engine Light, Hard Start, P0171 & P0174

I decided to not concern myself with it after that and take a look at the A/C, which is the customer’s main concern. But then I came across this. (Picture below)

1999 GMC Sierra Classic 5.7L - Check Engine Light, Hard Start, P0171 & P0174

Someone has been in here working before me and caused the check engine light.

I found the vacuum leak, but I run into a dilemma.

Do I fix it and charge the customer, and potentially get chewed out because he didn’t want to spend money on the check engine light at this visit? Or do I leave it and risk this crappy work by someone else fall back on me? I would be the last one to work on the vehicle so anything wrong with it would basically look like my fault.

It’s a simple fix, I decided to just put back together the correct way and not charge the customer. If it was something more complicated to fix, then I would have advised the customer of what the problem would be.

After putting the air duct together the correct way, the fuel trims started to go down and actually turned the check engine light off after driving up the road.

1999 GMC Sierra Classic 5.7L - Check Engine Light, Hard Start, P0171 & P0174

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