3.8 engine swap - running rough and have an extra plug


New Member
I was helping a kid that lives in my neighborhood swap a 3.8 into his 2001 mustang after he blew his engine. His last engine failed due to low oil because it had a leak and he didn't check the oil. He bought a used engine from a junk yard with 76k miles that was guaranteed to run. After reassembly there is one extra plug I am not sure where it came from since I didn't unhook but its on the drivers side low right next to the next to the o2 sensor (green plug). Picture attached.

We started the car and he was able to drive it home but it did run very rough idle. The check engine light was one and the two codes were p0141 and p0161. There was also a strong rotten egg odor present when running. However before the previous engine blew it was running great so I find it odd that we now have 2 o2 sensor errors and a rough running engine. I am planning on checking plugs / wires / and some obv stuff in the AM but hopping someone could help with explaining the extra plug and toss any suggestions out.

Thanks in advance!


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Thank you for your assistance! I was super confused because I thought that was the main harness. I appreciate your help.

Sounds like the fuel system isn't doing it's doing job or your Catayltic converter is clogged. Also possible transmission fluid got into another system and caused that smell, but that's the least likley cause.

Check your fuel and emissions.