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2001 Nissan Xterra SE, 3.3L V6, automatic, 2WD, towing package, 150k miles

Some history, perhaps useful:

I bought this vehicle from a friend a few months ago. Vehicle was in good shape for its age. Made a recent 200 mile trip with the only issue being a fairly noticeable oil leak from the valve covers (V6 engine).

My new insurance company sent me a device that plugs into the OBDII port to track my driving for 90 days to compute a discount. I got an email saying that since I had "unplugged" the device, I was not eligible for the discount.

I never unplugged the device. It just quit working.

Wife was out of state so the car was sitting in driveway for a month.

Battery remained charged and car cranked and started. I started it about every 3 to 5 days to check the charge and keep it fresh.

Wife returned and drove to work. Called to say car was barely running. She made it home and I checked and found out that the ECM was running in "limp mode."

I scanned the system and saw 7 codes set before the battery was disconnected. These included fuel mixture, O2 sensor, knock sensor, MAF sensor, and similar fuel mixture codes. Nothing about overheating or TPS.

Visual inspection: no harness damage, all connectors solid, several sturdy engine ground connections, plug wires good, distributor cap has a small crack near coil post, O2 sensor wires look new (recently replaced?), air filter clean, all fluids full, antifreeze full and green, oil smells a little dirty but not burned, oil leaking from valve covers.

I unplugged the battery, removed the insurance company's OBDII device, and plugged the battery back in to reset the computer.

As of now, the car constantly goes into "limp mode," limiting the RPMs to 2000 and limiting performance (especially when cold). Codes that set immediately and every time now are MAF, and 2 fuel mixture warnings. I don't have the actual codes with me, but I think it was P0100 and 2 of the fuel mixture codes like P0170, P0173, etc.

I replaced the MAF sensor and saw no change. I checked the wiring harness and saw no damage.

I'm stuck. Not sure what else it could be. I don't have schematics or the knowledge to test the individual circuits, but I would like to find out and try. I'm going to replace the valve cover and plenum gaskets this weekend and hopefully see something while I'm in there.

Any suggestions would be appreciated!

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    Is the car really in limp mode? or is there not enough fuel volume flowing to the rail? What are the fuel trims and what kind of maf voltage are you seeing at the pcm? Did you document the original codes? Any sub codes on the p0100? How soon does the maf code come back? – Ben Oct 3 '18 at 17:31
  • Wow ... that's a lot more than I knew how to check =) I'll see what I can do: * It is in limp mode. A few times I tried to drive it to the store after disconnecting the battery. The RPMs went up fine and I was able to get moving ok. The drivability was rough, but I could get up to speed. (I live right off of a 70 mph road a bit out of town.) – Skeptycal Oct 9 '18 at 20:30
  • * About halfway to the store, it got rougher and the RPM maxed at 2000. I cannot get above 45 mph in this truck under those conditions. I made it to the store and nothing changed until I got back and reset the battery again. – Skeptycal Oct 9 '18 at 20:36
  • * I borrowed a simple code reader to check it. I do not have equipment to get exact measurements for some of this. I could probably check the MAF voltage with my multimeter, but I have not yet. – Skeptycal Oct 9 '18 at 20:37
  • * It is our second car, so it has been sitting lately since we have been busy. The MAF code comes back within a minute or so and idle is very rough. It stalls out unless I 'feather' the pedal. – Skeptycal Oct 9 '18 at 20:39

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