System too lean

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nlacomb

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Hey all,

I finally got my 2 new cats put in, as well as 4 new oxygen sensors. This obviously fixed the P0420 code, but now I'm dealing with some new ones. My code reader is showing a P0171 and P0174 which is system too lean on both the front and rear banks. The car does not idle very smooth like it once did, and there are some performance issues.

Twice now when I filled up the tank with gas the car failed to start afterwards. (The starter was turning over but no combustion) I'm able to get it to start eventually by giving it some gas. These are the only times I've had any starting issues.

The engine light will come and go every other day, always the same two lean codes. At this point I'm thinking vacuum leak but wanted to get some opinions on here as well.

- 2010 LaCrosse CXL AWD - 225,000 miles
 
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richardnewton

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Seems like vacuum leak to me. Check all vacuum lines and connections for cracks. Maybe one was dislodged. Do a smoke test if you can to help.

Possible gas cap is not connected properly or is bad or vacuum lines related to gas tank are bad.

For fun I would disconnect the battery for a few hours to reset computer. Also you could try simply unplugging the MAF.
 

HotZ28

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1996 Roadmaster Limited Collectors Edition 58k - 1996 PAU
Clear the codes with a scanner and see if they come back. Scanners that can read/delete OBD11 codes are very cheap nowadays.
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nlacomb

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I've cleared them several times and they do keep coming back. I think I'll check for a vacuum leak tomorrow when I do my oil change. I'll clean the MAF sensor as well for good measure.
 

HotZ28

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I've cleared them several times and they do keep coming back. I think I'll check for a vacuum leak tomorrow when I do my oil change. I'll clean the MAF sensor as well for good measure.
Thanks & keep us informed!
 

Rich B.

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Most everything covered. You don't have an acorn stash or mouse nest in air filter box?

Doubt, but if you installed a larger throttle body for more HP.... That's your problem, many of the posts sometimes keep you guessing. My Boss came with 80MM TB (about 3.15") and I installed an 84.5MM. Anything larger as 90MM and you have the symptoms, without fuel enrichment it won't work.
 
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richardnewton

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Did you try unplugging the MAF wires to see if car will run? I had similar start-up idling issue with 3800 engine that were solved with those fixes. Of course once I figured the MAF was an issue, I tested the electrics, determined the MAF was shot, picked a few OEM MAFs from the local junkyard (don't use aftermarket garbage as they just don't last). If disconnecting the MAF worked, you can search my posts for some tips to test the wiring to the MAF.

Did you try replacing the gas cap? I have had similar issues on several cars, particularly after a fill up. Caused by either bad gas cap, bad filler neck, poorly tightened gas cap. Inspect carefully.

Also, on the 3500 engine, there was an odd ball hard plastic vacuum tube at the back of the engine that was easily knocked off and the poorly designed friction-fit end was slightly worn once removed so would not seal properly. It could have been broken perhaps when changing O2 sensors or back spark plugs. I don't have a 3800 engine here now so can't look to see what vacuum tubes are in that region. And don't remember. But take a look around vacuum hoses that you might have knocked off or damaged during o2 sensor changes for this type of issue as well.
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