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Discussion Starter · #1 ·
Hey y’all, bought a 2006 Xterra off of Copart (showed 120k actual miles) to try fix up, upgrade, and give to my sister since she likes them. Problem is, as it turns out, it runs extremely rough, obviously misfiring like hell and when it got delivered to me I was barely able to pull it into the driveway. Scanned for codes with OBD2 scanner and got P0300 (random misfires). So after thinking for a bit I just decided to replace the coils and spark plugs. Got the aftermarket coils from Amazon, but resistance is reading close to spec. After removing the plenum, noticed intake ports on cyl 1,3,5 are dark and wet with gasoline, spark plugs are all good, but cyl 1,3,5 plugs are black. Replacing all those things didn’t help. I checked for vacuum leaks, but couldn’t find any. I’m a mechanic, but I mostly work on semi trucks. I’m suspecting the timing chain and ready to tackle it, but I first must exclude any other possible causes as it’s a bit of a complex and time consuming task. This was 1 evening worth of diagnosing, so I haven’t had enough time to check compression. What would cause all cylinders on the same side of the engine to run rich? Thanks!

P.S. I did the dry compression test, shows all cylinders 145-ish PSI, a couple read about 148PSI (maybe while testing the first few cylinders I didn't crank the engine enough, hence the difference). I checked all the injectors’ resistance and got 12.1-12.5 ohms on all of them. The injectors are firing nice and misty fuel, not flooding anything. One concerning thing I found though - I assumed that on the injector connectors (2 pins) one pin would be 0V (ground) and the second one - 12V. But, what would seem to be, the ground pin shows 2.5V on all 6 connectors, if I remember correctly. Is that normal? I really doubt, but I’m limited on how much time I can spend per evening on this car. Any ideas? Thanks!
 

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Could be a plugged cat on that side. They often throw P0300 and no other codes if they implode suddenly - which does happen around here. You can pull the primary A/F sensor - aka 02 sensor and check for back pressure with a gauge - you should have none. I am told you can also pull it and it will run better - when cold at least when its in open loop.

Could be the timing chain skipped as mentioned but that would be pretty unusual around here. Even when they do skip, the run poorly but they still run.

I will mention - although I doubt this is your issue - our rigs don't like cheap sensors or coils - Hitachi, Denso, OEM etc only.

I presume you in fact checked you had good spark? Since the ECU controls all that - If you don't on that side its wiring likely not control.
 

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Discussion Starter · #3 ·
Could be a plugged cat on that side. They often throw P0300 and no other codes if they implode suddenly - which does happen around here. You can pull the primary A/F sensor - aka 02 sensor and check for back pressure with a gauge - you should have none. I am told you can also pull it and it will run better - when cold at least when its in open loop.

Could be the timing chain skipped as mentioned but that would be pretty unusual around here. Even when they do skip, the run poorly but they still run.

I will mention - although I doubt this is your issue - our rigs don't like cheap sensors or coils - Hitachi, Denso, OEM etc only.

I presume you in fact checked you had good spark? Since the ECU controls all that - If you don't on that side its wiring likely not control.
Hi, thanks for a fast informative reply. I haven’t checked the spark per se, but I have tested the injectors and wiring for them and the coil wiring, I seem to be getting proper voltage. But that was gonna be multi next step today, to actually pull the coils and spark plus and checking the spark. I’ll let you know how that goes once the rain is over.
 

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You're not getting combustion obviously. U have compression so you're holding good spark, good fuel (all based off what you have mentioned) so what about the air? If the air can't flow in and out cleanly, that's just about the only thing left.

Can rule out no air coming in since other head is running fine. So unless the plenum on that head specifically is blocked, all 3 ports would be very very unlikely and I'd assume u checked that already. So that leaves air isn't moving out of the combustion chambers. But it is moving out of the exhaust pipes after the Y from the head that operates fine. Like mentioned above, it almost has to be a blocked cat. Nothing else makes sense. If the timing was off there wouldn't be good compression (I'm assuming u check all 6 for a baseline and not just the 3?)

Any less than all 3 cylinders on the same head having a problem takes us down the rabbit hole, but this is pretty telling.

First poster was very likely right. That's where I'd be putting my money.
 

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Discussion Starter · #5 ·
You're not getting combustion obviously. U have compression so you're holding good spark, good fuel (all based off what you have mentioned) so what about the air? If the air can't flow in and out cleanly, that's just about the only thing left.

Can rule out no air coming in since other head is running fine. So unless the plenum on that head specifically is blocked, all 3 ports would be very very unlikely and I'd assume u checked that already. So that leaves air isn't moving out of the combustion chambers. But it is moving out of the exhaust pipes after the Y from the head that operates fine. Like mentioned above, it almost has to be a blocked cat. Nothing else makes sense. If the timing was off there wouldn't be good compression (I'm assuming u check all 6 for a baseline and not just the 3?)

Any less than all 3 cylinders on the same head having a problem takes us down the rabbit hole, but this is pretty telling.

First poster was very likely right. That's where I'd be putting my money.
Thanks so much, I’m beginning to think that now as well, it’s very weird how it’s only the 3 intake ports on just that one side. And yes, I checked the compression on all 6, it’s the same on all, maybe 1-3 PSI deviation, but it’s nothing concerning obviously. I’ll be checking the spark either tomorrow or some time on the weekend, just to make sure, and then it’s that cat. I really appreciate the help!
 

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Hi, thanks for a fast informative reply. I haven’t checked the spark per se, but I have tested the injectors and wiring for them and the coil wiring, I seem to be getting proper voltage. But that was gonna be multi next step today, to actually pull the coils and spark plus and checking the spark. I’ll let you know how that goes once the rain is over.
There is a Fuel Injection Pressure Regulator on the back alley of the left of the engine at the firewall about 30-100$ depending on brand.
 

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Do 2006 Xterras (with vq40de engines) have a removable pressure regulators? Aren’t they built into the fuel rails on these engines?
You are right! I have a 2000 so I only have one. On the 2005 up it is called the fuel pressure dampener. there are 1 per bank so one of your 2 may be bad.
2005 still had alot of sensor grounding issues look up the grounding diagram for the top of the intake and the grounds near the O2 sensors sit on the pass side head should be extra clean if not auxillary grounded to where the negative batt cable meets the head with some speaker wire or similar guage.
 

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Discussion Starter · #9 ·
You are right! I have a 2000 so I only have one. On the 2005 up it is called the fuel pressure dampener. there are 1 per bank so one of your 2 may be bad.
2005 still had alot of sensor grounding issues look up the grounding diagram for the top of the intake and the grounds near the O2 sensors sit on the pass side head should be extra clean if not auxillary grounded to where the negative batt cable meets the head with some speaker wire or similar guage.
I have checked all the injectors and they all spray in the same pattern and all spray nice and misty, looks to be the same amount as well. Do you think one of the dampeners could still be bad?
 

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I have checked all the injectors and they all spray in the same pattern and all spray nice and misty, looks to be the same amount as well. Do you think one of the dampeners could still be bad?
Are your spark plugs on that side black? that could mean a bad dampner. alot of variables when it comes to a bank running rich but just speaking from my experience 02 and grounds,(cam shaft sensor too!) if you download the .pdf of your factory shop manual you can go into all the proper diagnostics but when it comes down to it I'm more of a guess check refine guy
 

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Discussion Starter · #11 ·
Are your spark plugs on that side black? that could mean a bad dampner. alot of variables when it comes to a bank running rich but just speaking from my experience 02 and grounds,(cam shaft sensor too!) if you download the .pdf of your factory shop manual you can go into all the proper diagnostics but when it comes down to it I'm more of a guess check refine guy
Yeah, the old ones on that side were a lot darker than the ones on the other side. I guess that’s make sense. Thanks for the guess!
 

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Even compression on all 6 cylinders, cam timing isn't it. Not saying it couldn't be an actuator (valve timing) or cam sensor, but those usually throw different codes. Side to side injector swap will prove they are fine. The pulse damper will affect all cylinders, not just one bank.

One bank rich, either air can't get in (don't see a way for that to happen with this intake) or air can't get out. I'm suspecting clogged cat. It fits your problems. Even if you just throw a test pipe in and see if that runs right (and ignore the cat codes it will throw). You can narrow down the issue.
 

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Discussion Starter · #13 ·
Even compression on all 6 cylinders, cam timing isn't it. Not saying it couldn't be an actuator (valve timing) or cam sensor, but those usually throw different codes. Side to side injector swap will prove they are fine. The pulse damper will affect all cylinders, not just one bank.

One bank rich, either air can't get in (don't see a way for that to happen with this intake) or air can't get out. I'm suspecting clogged cat. It fits your problems. Even if you just throw a test pipe in and see if that runs right (and ignore the cat codes it will throw). You can narrow down the issue.
Yeah, that’s my only guess at this point cuz I just removed the dampeners and they look perfectly fine. I’m trying right now to remove the cat closer to the exhaust manifold on that side to see if that makes the engine run better.
 

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Even compression on all 6 cylinders, cam timing isn't it. Not saying it couldn't be an actuator (valve timing) or cam sensor, but those usually throw different codes. Side to side injector swap will prove they are fine. The pulse damper will affect all cylinders, not just one bank.

One bank rich, either air can't get in (don't see a way for that to happen with this intake) or air can't get out. I'm suspecting clogged cat. It fits your problems. Even if you just throw a test pipe in and see if that runs right (and ignore the cat codes it will throw). You can narrow down the issue.
He could take out the o2 sensors on the cat closest to the engine to relieve back pressure and unplug all the o2 sensors on the pass head so the computer will recalculate fuel to see if the symptoms get better
 

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Yeah, that’s my only guess at this point cuz I just removed the dampeners and they look perfectly fine. I’m trying right now to remove the cat closer to the exhaust manifold on that side to see if that makes the engine run better.
Just for posterity sake easy to test cam position sensors thru the wire
.
 

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Discussion Starter · #16 ·
Just for posterity sake easy to test cam position sensors thru the wire
.
Well problem is, there’s no code for cam/crank position sensors and the engine turns over and starts without any problems. Problem is with idling.
 

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Discussion Starter · #17 ·
He could take out the o2 sensors on the cat closest to the engine to relieve back pressure and unplug all the o2 sensors on the pass head so the computer will recalculate fuel to see if the symptoms get better
I unbolted the passenger cat yesterday, but couldn’t remove completely, so I jammed a screwdriver in between the exhaust manifold and the cat, and it doesn’t seem to have helped. It works a bit better, and it revs past 4000rpms now, but it boggs down for like 5 sec (feels like it’s about to die), then goes back to regular crappy idle for like 20-25sec. Also, during those 5 sec of bogging it backfires, sounds like. And when I rev it and let go of the gas there’s heavy backfiring.
 

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then goes back to regular crappy idle for like 20-25sec
The FSM section EC - engine control - has helped me a lot through the years. At Idle the ECU does all kinds of special stuff, like adjusts timing and injector pulse width. I would read through that section, there are a bunch of tests you can perform - idle air relearn, etc. Maybe you will stumble into your problem - I have in the past.
 

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Discussion Starter · #19 ·
The FSM section EC - engine control - has helped me a lot through the years. At Idle the ECU does all kinds of special stuff, like adjusts timing and injector pulse width. I would read through that section, there are a bunch of tests you can perform - idle air relearn, etc. Maybe you will stumble into your problem - I have in the past.
Thanks for that advice! I’m assumingEC section you’re talking about is the one in the repair manual?
 

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Well problem is, there’s no code for cam/crank position sensors and the engine turns over and starts without any problems. Problem is with idling.
P0300 is a code for multiple things, camshaft sensors is one of those. Not saying it is but you need to cancel things out as you troubleshoot, easy enough to test. otherwise did you unplug the o2 sensors when you opened up the exhaust? If you did not the computer is still using those sensors and fuel demand will be whack. Sounds like Cats to me since you reving higher..
 
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