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My 2000 Xterra had been really irritating me the last few months. It seemed I had lost a lot of power, pickup, and acceleration, and it seemed like it would get bogged down easily. I was having to downshift going up moderate hills, and my mileage had gone to crap, and by crap, I mean 12-13 mpg. Ouch. It idled just fine, and wasn't throwing any codes. It also passed its emissions test recently.

So, I start a tuneup in the driveway Saturday. I clean the MAF sensor, replace the fuel and air filter, and replace the distributor cap and rotor. I used new NGK plugs and wires not too long ago, so they should still be fine. That seemed to help a bit, but not like I was expecting. I was looking closely at the distributor adjustment bolt while doing this, and noticed it seemed to be awfully far to one side. I checked the timing and it showed 15 ATDC with the light, which is correct, but when I started watching the live feed data with my scanner, the timing advance would sometimes bounce down to 7 or 8. That seemed kind of weird, so I did some more research.

It turns out I am supposed to disable the PCM from attempting to control the timing by unplugging the throttle sensor at the intake manifold, and setting the timing first with the PCM "dumb" once the engine is warmed. When I did this and restarted and rechecked the timing again, I discovered that it had been set at ZERO degrees, right on TDC. Ouch. It ran ok there at idle, but ran terribly weak when the throttle was applied. Also had some shudders between 2000 and 3000 rpm.

I reset the timing to 15 degrees, shut it down, plugged the throttle position sensor back in, and rechecked the timing with the PCM engaged. Right on 15 degrees at idle, and stayed there, not dipping below at all. I took a test drive, and it is a HUGE improvement in performance having it properly timed. I mean, it is still a little 175HP 3.3L v6, but now it's not acting like a 90hp 4 banger. I've also noted my fuel economy has improved I'd say at least 30%.

It seems the PCM was struggling to keep the engine running vs the bad timing setting, and with the severe loss of power I was driving with a lead foot to keep up with traffic.

Just a little lesson: do a tuneup now and then, or at least check the basics.
 

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Now your going to have everyone thinking there timing is off, Fuel Economy sucks on this trucks.... but poor timing wont help anything,
advancing timing makes it lean and lean is mean.
If your timing is far off it may also mean its time to change the belt. I know thats why mines off my bet has stretched...
 

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Thanks for reminding me ayjay. I recently replaced the timing belt, water pump and tension bearing. But I have not gone back to check ignition timing. I had the same experience with loss of power and horrible mileage (11 MPG). I think my situation was more related to clogged injectors, an overdue timing belt, and a faulty fuel pressure regulator, but ignition timing is pretty important. For what it's worth, I did the SeaFoam treatment in both the oil and fuel. My power increased dramatically, and the startup lifter noise disappeared. It took about 150 miles of driving before I noticed a difference, but wow, what a difference.
 
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