Maybe someone out there has had this problem/symptoms and knows the answer...I have a 2005 Montero Ltd. 3.8L V6 engine with 136K miles, same engine as in the Diamante. About 8 months ago it started to misfire badly at any speed, it was two ignition wires that were frayed, they were changed and problem gone. But, previous to fixing it, while driving to the mechanic, one of those wire shorted against the engine block, which caused a major jolt which turned on the ABS and Traction Control lights, I have not fixed it since then, and I'm sure it is something electrical; hopefully someone knows how to diagnose or knows the solution. That I'm not much concerned, and will fix at some point.

My concern is that currently I am having issues with the engine having what seems like a misfire, at idle the tach would go nuts down to 500 RPM's, then back up to 800-1000 RPM's, then up to 1500 RPM's, and back down to normal RPM's (800-1000 RPM's). Sometimes engine will shut off at idle, then you can start it back quickly without a problem. The Service Engine Soon Light (SESL) came up and I took it to AutoZone to have it scanned; it had P0455 and P0431 codes, which relate to EVAP Vacuum Leak and Left Catalytic problems or Bank 2 Rear (Left) O2 sensor. Since, changing the Cat's is the expensive route I purchased a new Denso OEM O2 Sensor and changed it, the right one had been replaced about 3 yrs ago, the left one was still the factory unit.

I disconnected the battery to clear the codes. The misfire didn't go away, but it was less than before changing the O2 sensor. So, drove it with the misfire for two (2) days, then the SESL reappeared. Took it to AZ and their $20.00 scanner read codes P0456 and P0300; P0431 and P0455 were fixed with O2 sensor and didn't show up in the new scanner reading. P0456 is a "Very Small Leak in the EVAP System" due to Faulty Fuel Cap, or Leaking Vent or Purging Solenoid, or EVAP Hose Faulty, or Leak on Fuel Tank. Since it is inexpensive and quick, I purchased and installed a new fuel cap and threw away the old one. P0300 is somewhat more complicated, which is "Random Cylinder Misfire" due to Ignition System Faulty, or Fuel System Faulty Injectors or Pressure, or Vacuum Leak, or Engine Mechanical Condition. Just in case, AZ had Seafoam on sale, and I pour a bottle into the gas tank with approx. 13 gals. of fuel in it.

Well, the misfire is more tamed, but it is not consistent know, at times the engine can be somewhat rough at idle, but will not hesitate too much, and other times it misfires more at idle, but it rarely will shut down. I changed the fuel cap and added Seafoam to the tank at about 8:00 AM in the morning, I also checked the battery terminals and the negative was loose and could be taken off with the bare hands, so I tightened it. I have driven the SUV around town and used close to 1/4 tank of fuel (+/- 6 gals.). The SESL has not come back on since, but the misfire at idle is there.

I am leaning towards a Vacuum Leak somewhere, because of the previous P0455 (no longer an issue), and now the P0456 and P0300. But, I have driven many cars and road raced many others, and to me it seems an ignition problem. As soon as the SESL comes back on I will take the SUV to a local diagnostic scanners and car ECM's distributor who charges $40.00 to have the vehicle scanned and diagnosed by a real $10,000.00 scanner with the factory programs for most vehicles in it, including the Mits program. Hopefully that scanner can exactly pin point where the problem is.

But, in the meantime, has anyone had this specific problem, and how it was solved? Thanks in advanced.

 

Idle dropping down to 500 RPM then racing up to 1500 RPM is not caused by a misfire. This is caused by an imbalance of fuel to air mixture. Your vehicle's computer is trying to adjust the fuel/air mixture to an appropriate level, but it it not able to do so because the info its getting from the sensors (MAF, MAP, O2) does not match the actual reality what is getting into the engine. There are several possible causes - 1. Dirty throttle body and/or clogged air passages in the TB. 2. Dirty Mass Air Flow sensor. 3. Major vacuum leak somewhere in the system. What you need to do is to get the freeze frame data from the error code(s) you are reading.

 

Thanks! And you might be in the right path...See after 2 bottles of Seafoam, the problem still persists, but very subdued compare to before. The problem is that it has been 2 days of lots of driving and the SESL (CEL) will not light up again, so no codes, waiting for the darn engine light to come on again, as to read it again.