2003 Montero Limited 3.8L, 177,000 miles. Nickname: Silverback.

Purchased this mountain and it has these issues. Looking for guidance that I'm not thinking of so as I tear into the top of engine downward I inspect all possible failed components. Still a mechanic in training, lol.

Symptoms:
Throwing codes P0305 (#5 misfire) and P0431 (cold bank 2 or faulty catalyst/oxygen sensor).

Upon start it runs like it's missing cylinder fire. Sluggish but steady through all RPMs. I have another 2003 with same mileage and it's more responsive. This broken one is noticeably more weak. At about 2,100 rpm the miss isn't noticeable in the motor shake but the power is still weak. When looking at motor at idle it appears to shake like it has a single misfire.

Does not overheat and I've run it for all speeds and minor climbing for about 30 miles and 2 hours. Timing belt looks new. Doesn't lose any fluids. No oil and coolant cross contamination.

I was going to remove intake plenum and remove the intake butterflies (dam junk) and check spark plug wire and plug gapping. Then check coil pack for #5cyl and injector after plug.

If still a problem exists after fixing misfire code then I'll check tubes to Bank 2 and see if obstructed, then oxygen sensors for the catalyst error code. Not sure how to remedy if actual bad catalyst if it's bad Maybe just cut it out? I don't need it for emissions where I live. I'd rather simplify the SUV and remove stupid emissions crap.

Any suggestions on how or what to look at as I'm diagnosing this silverback mountain? Thanks!

 

If it was me doing the work, before tearing into the plenum to get to the spark plugs, I'd check the ignition coils. You can swap one of the good coils with the one controlling spark at #5 cylinder. Clear codes and see if you still get P0305 or if misfire moves to a different cylinder. If it moves -you got bad coil, if it stays on 5 - you have ether bad wire, bad spark plug, fuel or valve/compression issue on that cylinder.

If and when you do remove plenum, do a compression test on all cylinders (both dry and wet compression). This will give you some more info on what the internals of your engine look like.

If your Montero has iridium plugs - DO NOT gap them. They are not designed for that.

As far as catalytic converter goes - don't waste your money replacing O2 sensors. Since you are getting P0431 code it means all of the O2 sensors are working properly. They are not getting the signal they are expecting, but they are working.

I've dealt with this issue couple years ago. I installed spark plug non-foulers on the downstream O2 sensors and it fixed the cat converter code. Check the forum posting called "Montero XLS catalytic code P0431". There are links within it explaining how to install the spacers/non-foulers. Totally works.

 

Thanks man! Sounds like good advice on both. Hoping it's just spark equipment like plug or wire...not compression failure...that would really suck!

 

When was the last time you checked your spark plug wires? Once I had a small oil leak in one of the spark plug wells. Oil soaked the plug wire connector and the rubber end of it swelled and pushed upward. This resulted in loss of contact between the wire and spark and i was getting and intermittent spark (misfire on #5).

 

I'll check those too. I had one that was filling up with oil also, Lol. Stupid design. I'll have to still remove the intake manifold so I'll swap coil packs after I ohm the wire. Thanks!
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You can use a small mirror to peak under the intake plenum to see if the wire is seated all the way down (has an air gap above it), or if its pushed up and the plenum is holding it down on the plug