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2006 Xterra P0300 and Cyl 1,3,5 Run Rich

4K views 27 replies 5 participants last post by  NIKITOS260496 
#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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#2 ·
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.
 
#3 ·
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.
 
#8 ·
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.
 
#4 ·
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.
 
#5 ·
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!
 
#12 ·
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.
 
#26 ·
I really appreciate all your guys’ advice! I found the problem, my passenger side upstream cat got completely obliterated and turned into dust and clogged up the downstream cat. I’ve ordered a new set of cats and they should be here in a week! That was my first problem ever with cats, and the thing has like 120k on it so that made it weird for me. My 350z has like 140k and smokes a bit, but 0 problems with cats lol. Cheers!
 
#27 ·
Change the oil. When the cat goes like that some of the dust goes back and eats up rings in the engine. Nothing you can do at this point about it, except keep chaning the oil way too often and hope you can get as much of that grit out as possible.
 
#28 ·
Yeah man, I’m changing every single fluid in this truck: transmission, engine, PS, brake fluid, diff oil. I’m actually fixing the SMOD right now. I got a brownish trans fluid that is not see through at all and a bit of trans oil in the radiator. After that’s done I’m getting to everything else.
 
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