Howdy. A little background info...New Montero to me. Ran fine for about 6 months and then started to do this. Has about 171,000 miles. Air cleaner is new and motor oil/filter change. That's all I've done so far. I also notice it seems to run kind of rich smelling or possibly exhausts something that smells like incompletely burnt combustion gases. There aren't any leaks or visible damage to anything I can see. Gets about 13mpg fuel mileage in town.

It has this code that may be cause of the rich/strange unburnt fuel smell? I've reset it with code reader and it returns and comes and goes.
"P0431, bank 2"
This code supposedly can mean these things: Faulty catalytic convertor,
or exhaust leak near rear HO2S,
or faulty rear HO2 sensor.

The real problem symptoms now...
Sometimes when cold or warm (as in engine temp) it will start right up and then die about 2-3 seconds later. I noticed after 3 to 4 key cranks it always started and then stayed on, until a few days ago. It started to die and not want to stay on at all for a couple of days. It did this if I gave it gas or not, but giving gas seemed to help some (but it would still die usually). It sat for a few days and now seems fine again.

Could it be the $30 Fuel Pump Relay which is on driver's side by the battery? It's the same as the MFI Relay...which I'm unsure what that relay does entirely, please inform me if possible. Can I switch those two identical relays to test operation? If the Fuel Pump Relay is bad, will it damage anything being swapped with MFI Relay to test relays?

I have a 2003 Montero Limited also that uses same Fuel Pump Relay. Should I swap and see how that does for a while, or use the MFI Relay route from the 2001?

Thanks much! *****

 

P0431 is the bad cat converter code. Your O2 sensors are working fine (that's why they can detect a dead cat). I don't think the symptoms you describe relate to this code. Most likely you have something else going on. If you get a chance, pull the freeze frame data for any of the codes (including the P0431). You should be able to do this when you read the code with the code reader. Post FF data and perhaps we can figure out what is going on.

 

Can the code readers from Autozone and O'Riley's do the ff info you spoke of? I took it there and had codes read. Thanks much.

 

Meant to ask also, will a dead cat effect my my Montero in any negative/harmful ways? Is it ok to drive it like that until i replace the cat?

 

The code readers at the parts store will pick up freeze frame data. The folks there often do not look at them because they just look at the code and try to sell you the replacement part. Just ask them to make sure they print the FF data for you.
As far as having a bad cat converter - no it will not harm your car. It does not affect your fuel mileage or anything like that. The only thing that is affected by having a bad cat converter is the emissions. If you live in the area where you must pass the emissions to get vehicle registered - you will not be able to do so until you replace it.

 

Attached are frame info for it while at 832rpm idle and then again at 2,039rpm, revved while transmission in Park. First link has only 1 picture, that is the first frame at idle. The second link has the remaining 3 photos in order, top to bottom. Does this tell us anything useful? These were taken after it was warmed up for about 22 miles on interstate and in town.

There weren't any stored codes at the time of the frame, but last night the check engine light came on after I removed the breather tube and cleaned around the intake air port and the large butterfly valve that I could flip, and I cleaned about 2" past that too, where I could reach with a thin rag and cleaner. So I'll go back maybe Saturday/Sunday and re-read the codes. My guess it will be the same code I specified previously. After I cleaned the intake area I could reach, it started hard (3 attempts) and then ran just fine on 4th (not the same way I described before on my first post). Maybe a little smoother now, not sure.
Imgur: The most awesome images on the Internet

montero freeze frames - Album on Imgur

 

I'd suspect that you have a vacuum leak. The combined fuel trim levels (STFT + LTFT) at idle are about 17-23%. In plain English this means that PCM calls to add more fuel to the system than expected at that load. The demand for fuel drops when you rev up the engine (10-18% combined fuel trims). When you have a vacuum leak you'll have higher fuel trim levels when at idle and lower at open throttle. What is somewhat suspicious is that you have significant difference between the banks. This may indicate a bad seal on the manifold gasket. You can try to test for vacuum leaks with starter fluid (spray into different areas around the manifold with the engine running at idle). If you hear a change in the RPM - you found your leak. But on the Montero it is very hard to get down to the area where manifold bolts down to the engine block. I'd try to replace manifold gasket and see if this will improve things. Also check all random vacuum hoses on and around the manifold to see if any of them are cracked or fell off.

 

Great. Thank you very much. I will pickup more carb cleaner and spray around everywhere and yank on all the vac hoses I can reach. It looks fairly clean and tight under the hood. Like it was maintained ok before me and all stock parts, perhaps.

I'm wondering if the timing belt was ever changed. Is there a trick or observable trait I can look for or test that may give me tip on that?

Also, this Montero only gets like 12mpg, sometimes less, like 10 to 11. Is that typical for 3.5L? My 90, 93, 94s with 3.0L got like 15 to 17 in regular around town driving.

 

Oh forgot to previously mention that this Montero also has an exhaust leak somewhere. I can hear it and perhaps smell it. Might that cause any sensors or fuel mixture problems? If the exhaust gases are leaking out prior to getting read by the O2 sensors could that make fuel mix problems?

 

It is possible that it would affect O2 sensor readings and fuel economy. If some air getting in through the exhaust leak O2 will read lean and will be calling for more fuel demand.