I bought a 2011 base cooper with a coolant leak that, unfortunately, ended up being a cracked block so I bought a low mileage used engine and can't get it to run smoothly. I'm getting a P0171 "Lean Fuel Ratio" code along with misfires across all cylinders. As I was messing around, I forgot to connect the MAF back up and it ran smoothly so I started to get excited. When I plugged the MAF back in, it started running rough again so I thought I had it solved. Put in a new MAF and have the same issues. I went back and looked at the data and when the MAF is plugged in, it reads about 3 g/s. When unplugged, the computer sets a MAF rate of 306 g/s which seems WAY too high, but it runs smoothly. Specs say it should read between 2 and 7 depending on RPM.

I've sprayed carb cleaner around all of the vacuum lines and intake seams and haven't gotten a response.

New O2 sensors, maybe one is bad? I dunno.

Any ideas as to what I'm missing?

 

A used engine... Oil level good? Correct oil in the engine? Sort of a long shot regarding the misfires but you want to make sure all bases are covered.

Intake leaks are possible even if you checked. You may have missed the leak. But also with an engine swap all the vacuum lines were disturbed. The vacuum hoses suffer from aging and from being disturbed during an engine swap can fail. They often fail on their bottom close to where the hose connects.

And of course with misfires naturally you have to consider plugs, coils or even deeper problems like cam chain/sprocket wear, Vanos issues.

But before jumping right to the cam chain with a used engine be sure the coolant sensor is reading correctly. If it reads wrong this can have the engine controller operating on the assumption the engine is warmer than it really is and fuel the engine incorrectly, probably too lean for conditions.

If you confirm the coolant temperature sensor is reading right pull the plugs and check the coils for good connections/good connectors and any signs of arcing.

Also, for completeness I'll add with the first engine troubles and now with a replacement engine the condition of the battery is suspect. And while you are "there" be sure all the connections/battery cabling is good not only at the battery but where the cables connect to the engine/car. And be sure you have all the ground straps present and accounted for.

 

Thanks, RockC!

Chasing down wiring is on my agenda for this weekend! I did swap out all of the vacuum lines around the throttle body and intake manifold, but I agree, it could be anywhere. (I did forget the grounding strap between the engine mount and engine and couldn't figure out why it wouldn't turn over for a hot minute!) I've replaced a lot of what you mentioned, but it's worth another look. I appreciate the ideas.

J-