I am having an issue with my 2002 Mitsubishi Montero Limited that is making it run really rough after a few minutes or if I head up hill.

If I pull the fuse to reset the error code, the car starts up and runs fine for the first few minutes, then Service Engine light returns.

When the service engine light came on, my brother in law hooked up his diagnostic machine and it showed P0174 System too Lean (Bank 2).

Took it to my mechanic and they changed the Spark Plug wires and Spark plugs. Still ran rough. They replaced the coils with new ones, still ran rough.
That mechanic had no further clue, mentioned timing belt had slipped a notch???

Took it to another mechanic who has a better diagnostic machine, but he can't figure it out either. he says the O2 downstream shows running lean, then running rich when it starts running rough.

Any thoughts greatly appreciated?? Injectors leaking? Bad? PCM?

 

Can we get just a bit more information: Total mileage; Timing belt ever done; etc...

But generally, PO174 is a bit of a PITA. It usually means vacuum leak somewhere or sometimes, can be a dirty/malfunctioning MAP/MAF sensor. I would start by cleaning the sensor properly. If that is a no go, start testing vacuum hoses to find the leak. Of note, on rare occassions it can also be an intake manifold gasket leaking. I'm not sure on your motor but on my 3.8, there's like 3-4 gaskets in total and my gasket where it meets the engine was malfunctioning.

 

I agree with Ironchef. Lean condition means un-metered air is getting into the system. 1. Start by checking condition of all vacuum lines on the engine - a small crack near the connection will let air get sucked in.
2. Check to make sure all air ducts are intact (no cracks) and that they are fully seated and tightened.
3. It is possible that one of the plenum gaskets is bad and letting air in that way. It is more involved process but worth checking it.

Spark plugs and ignition coils will not cause lean condition. I think changing those was a miss.

 

Thanks for the input! I will check them out. Vehicle has 106,000 miles on it. Timing belt was replaced at 100,000 by previous owner. I picked it up two months ago and have put 1,000 miles on it.

Question, since the problem seems to be on the Bank 2 side, any vacuum hoses of particular interest that might affect just that side I should look at first?

 

New owner, freshly done timing job. I would almost bet that it's an intake plenum gasket.

 

I think you are right. A damaged plenum gasket will let air get into just one cylinder and thus Bank 2 lean error.
Pull the intake manifold and check/replace the plenum gasket.

 

Update: I was looking at some sites regarding the Mass Air Flow Sensor and when I unplugged the connection, my Montero starting running better. Based upon their suggestions, I bought and have installed a new MAF Sensor. Vehicle still idles rough, but the Service Engine light does not come on when in idle.

However, when I take the Montero up a hill, it throws P0300/Misfire code.

Any thoughts? Still possibly Intake Plenum Gasket? Could a bad battery possibly case the ECU to not operate correctly?

 

P0300 is multiple cylinder misfire. Since you had spark plugs and wires done, the first thing i'd do is to make sure the wires got connected to the correct cylinders. I'm not sure what ignition setup your vehicle has, I think it is 3 ignition modules on the top of the engine up front. Each one will have one wire go to Left Bank and one to the Right bank. When you are standing in front of the vehicle looking at the engine, the cylinders on the engine as follows: Passenger side front to back - No.1, 3, and 5. On driver side you'll have front to back 2, 4 and 6. The spark plug wires should be going from the 3 modules to the cylinders in following order (left to right) 5-2-1-4-3-6.
Check to see if any of the wires are not going to the right places and hope that is what is causing your rough idle and the misfire code.

 

Just following up on my issue.

Third mechanic was finally able to determine the timing belt tensioner had collapsed, making the timing belt loose. Loose belt caused the Engine Crank Sensor to go as well.

Looks like previous owner had replaced with "Made in China" kit

Luckily no bent valves or any other issues and Montero is running grand again!