[dtrombley]

Looking for some help on an issue I can't seem to diagnose, wondering if anyone has run into something similar. Any help or suggestions would be greatly appreciated.

2008 Nissan Xterra Offroad 6MT

Truck starts up right away, idles beautifully, no codes and no abnormal noises. AFR stays right around the commanded 14.65 on both banks.

Under load, truck misses and shows 10 AFR bank 1 and more reasonable (but still leaner than commanded) on bank 2. Only code I am getting is a P0300 random misfire, check engine light will flash when giving 'er the beans then goes away when I coast. A screenshot of an uprev data log is attached.

No vacuum leaks to be found with propane or brake clean. Vacuum reads -24ish at idle.

Parts replaced:

• Bank 1 & Bank 2 cam sensors
• Crankshaft sensor
• All 4 O2 sensors
• MAF sensor
• Throttle body (&TPS, TPS was making a weird buzzing and would not relearn)
• All 6 plugs and coils
• Intake plenum gaskets
• Throttle body gaskets

All sensors (and coils) that were replaced were OE Hitachi, spark plugs are NGK 93759, gaskets are OE Mahle. Only thing that was not "strictly" OE spec is the throttle body, that is a Cardone reman unit.


Modifications:

• Volant Cold Air Intake
• XTP IMS
• Doug Thorley Long Tube headers
• Dynomax 19443 cat-back
• Uprev base tune (have not been able to send accurate logs over to uprev due to this issue, issue pops up whether on the base tune or stock tune just doesnt throw a code under stock tune)

History:
Vehicle developed a misfire on cyl 1 on a road trip, tried to make it home and fried the catalytic converter. Had a family member towing it for me, they got in an accident, which smacked the front end of the Xterra. No frame damage, so I bought it back from insurance and replaced the grille, radiator support cross member, Passenger fender, hood, upgraded to ARB bumper, and upgraded to volant cold air intake (added XTP IMS at the same time). Went with long tube headers & removed primary cats at the same time as I did not want to deal with a failed cat again, which is apparently common on the VQ40DE. Inspected cylinders with a borescope while the exhaust manifolds were off and there was no apparent damage or catalytic converter residue.

 

[RamTest]

Try a few cans of Berrymans B12 Chemtool Fuel Injector cleaner in a full tank of gas (or 1 can in 1/2 tank). Avoid Techron, Seafoam, etc and all that other ripoff garbage.
Did you have this problem before you replaced the plugs?

AT WALMART

 

[dtrombley]

Thanks, I'll give that a shot. I had someone else mention dirty injectors too

No, I had no issues before the misfire on cyl 1 popped up.

 

[BradB]

Hi, sharing as I had the same random misfire which was enough to fry my passenger side primary cat as well, I notice you haven't replaced any vacuum lines, though have checked. For mine (2007 OR 6MT), I was able to narrow it to cylinder 1. The cause of the misfire for me turned out to be a vacuum leak on the hose that connects the two heads which runs underneath the intake plenum. It had a massive crack right where it connects to the head above cylinder 1. Oddly, this was not detected in a smoke test or with brake cleaner, though I could hear it. It crumbled apart as I took it off. I replaced the vacuum hose and 2 other large hoses in that area, new cylinder 1 coil to be sure, and eventually had to get a new OEM cat, and all is well.

 

[dtrombley]

I didn't mention replacing vacuum hoses, but I should have lol, just forgot about it.

I bought the whole pcv hose set from Z1, replaced all 3 of those and redid the small hoses from the evap stuff too. Also replaced the pcv valve at the same time

 

[RamTest]

Thanks, I'll give that a shot. I had someone else mention dirty injectors too
No, I had no issues before the misfire on cyl 1 popped up.

If the injector cleaner doesn't resolve the trouble, consider replacing that #1 plug.
New plugs can be damaged in shipping if they are dropped, or can crack internally if improperly torqued, or damaged from improper gapping methods.
Be certain the electrical connections on the coils and injectors are fully on and latched.

 

[Xtra99]

In the data file shown, that bank 1 AFR value of exactly 10 is interesting. It's not 10.12 or 10.xx, it's just 10 without decimal places. New parts aren't always good parts these days, so a faulty bank 1 AFR sensor is a possibility too.

There's no mention of mileage on the vehicle or if the new parts/mods were installed before or after the initial bank 1 misfire issue and current random misfire issue. Timeline of changes/effects helps.

 

[dtrombley]

If the injector cleaner doesn't resolve the trouble, consider replacing that #1 plug.
New plugs can be damaged in shipping if they are dropped, or can crack internally if improperly torqued, or damaged from improper gapping methods.
Be certain the electrical connections on the coils and injectors are fully on and latched.

I re-replaced all 6 already, as it reminded of an issue I had on a previous vehicle that ended up being a cracked spark plug ceramic (it was either cracked from new or during install). I've double and triple checked all the connections, and using a stethoscope at idle I can hear each injector & coil firing at what seems to be regular intervals, with no weird or abnormal noises. I also swapped the coil from the most fouled plug (cyl 5) over to bank 2, and no change.

I'm thinking you're onto something with the dirty/failed injector, as I feel like thats the only thing I haven't checked at this point.

 

[dtrombley]

In the data file shown, that bank 1 AFR value of exactly 10 is interesting. It's not 10.12 or 10.xx, it's just 10 without decimal places. New parts aren't always good parts these days, so a faulty bank 1 AFR sensor is a possibility too.

There's no mention of mileage on the vehicle or if the new parts/mods were installed before or after the initial bank 1 misfire issue and current random misfire issue. Timeline of changes/effects helps.

Yea I noticed that. I figured 10 was just the lower limit of the sensor, and it may actually be down around 9 or something. Not really sure what the lower limit actually is though. Regardless, I tried swapping the O2's from side to side and there was no change so I can think I can rule that out.

There's 135k on the vehicle. New parts/mods were installed after the initial misfire cy1 issue, after the accident, during the front end rebuild. Sorry, thought I made that clear in the 'history' section of my post, but looking back now it is a little ambiguous.

 

[Xtra99]

If the bank 1 AFR reading pegs to 10, regardless of the sensor installed in that position, then it sounds like an injector is sticking open too long. A bank 1 AFR of 10, compared to the bank 2 AFR of around 12-13, says too much fuel is being injected into bank 1 for a commanded pulse width.

The bank 1/bank2 injector command pulses agree, so the commanded opening time seems correct, but the bank 1 injector (or injectors?) may not be able to mechanically close in that commanded amount of time.

 

[xterratx]

I agree, and I would look for a leaky injector.

Things that stand out from the graph you shared: long term fuel trim shows 100% on bank 1 and 2, any reasonable explanation I'm missing for that?

Did you successfully do an idle air relearn after all the new parts? That MAF data never goes low enough to indicate engine idle. Was the engine at idle in that data sample?

 

[dtrombley]

@Xtra99 - That's what I was leaning towards as well. It's really the only thing I can think of that I haven't verified proper function of or replaced at this point.

@xterratx - I also saw that and was semi-concerned about it, but am not familiar enough with LTFT's to point to it as an issue. This was not long after the battery had been disconnected for a day or so, I'm not sure if that would be the cause?

• I did not include any idle data in this screen grab, as everything looked good to me at idle. AFR's are right around 14.7 on both banks, MAF gm/s was around 4.18 IIRC. I don't recall what STFT or LTFT was showing though. When I get back home I can post the entire data log spreadsheet.
• I was able to do the throttle closed position relearn, accelerator pedal released relearn, and the idle air volume relearn after replacing the throttle body.

Any good way to test injector spray pattern or for a leaky injector without an injector test bench? I really don't want to keep throwing parts at this thing, especially if I can't verify it is for sure the issue before hand. I suppose I could start switching injectors back and forth between banks and use a process of elimination type strategy, but that's a lot of removing and re-installing of the plenum, especially in my driveway in the middle of these freezing temps and blowing snow.

 

[Xtra99]

I've seen these in-vehicle fuel injector testers, but can't speak to their usefulness. The tester is around $30.
Amazon Injector Tester

As for the 100% LTFT's, that should be a high enough percentage to throw a code (SES light)...a fuel trim code. Is that 100% LTFT reading coming from the vehicle itself? Hooking up a scan tool will tell you what the actual LTFT & STFT is. Normal is around +-10%.

 

[RamTest]

If you still have the original air intake, try re-installing it.
Also try disconnecting the maf, see if anything changes. (Disconnect and then start engine.)
I can barely read the table posted above, but it looks like it might read LT fuel trims of 100%. If that is what it says, an injector is not the problem.

 

[FearinLoathin]

This may be completely off base, but I had a misfire in cylinder 5 that just would not go away. After doing everything from changing the plug, then the coil, then the injector, and finally just gutting the cats on that side, it turned out to be a compression issue that has the bank 1 side of my engine currently disassembled while I contemplate ( and attempt to avoid) just changing the head gasket.

I don't want to blow anything out of proportion because you seem in good hands, and I hope it's simply a fuel delivery issue, but a compression test on that cylinder would eliminate (or confirm) some things that may be worth the peace of mind as far as bigger issues go. I'm not a mechanic, let alone an expert, but if I'm not mistaken compression loss can cause a misfire that really rears its head when you get into the throttle. That's just my experience, and I came upon this thread looking for some related answers. Good luck to you!

 

[tmapres]

Late to the train here, but this sounds like exactly the problem I had last summer on a 2015 XTPro4X. My hot-rod mechanic and exhaust specialist traced it to a clogged secondary Cat, so he amputated both and put in straight-thru pipes. We had already done the primarys and O2 sensors when I bought it, to correct doggy performance and low mileage. But the new problem was as you have, check engine would flash and the vehicle would choke on acceleration.

With the amputated secondaries, this thing is a monster on the road, really moves when you floor it. It does however throw the code for Excess Exhaust flow, which the mechanic reponds, "how can there be too much exhaust flow?" That leaves the Ck Engine light on, however not flashing, but showing the code is thrown. I've chosen to live with that for the performance so far, but hoping someone can tell me how to calm down the ECU and accept the improved exhaust flow!

 

[dtrombley]

@Xtra99 @FearinLoathin So I checked fuel trims with torque pro rather than Uprev, and am showing 0% LTFT1 & LTFT2, STFT2 hovers around ±2%, and STFT1 ranges from -3% to -13%, which would support the leaky injector theory. Not sure why uprev would show +100% constantly if thats not the reality.

@tmapres When I put on the exhaust, I cored out the secondary catalytic converters. The B-pipes are now essentially just straight pipes. Thats an interesting code, I've never heard of that one. It's not a code for catalytic converter performance?

I did a compression test today, and came up with:
Cyl 1: 169
Cyl 2: 165
Cyl 3: 179
Cyl 4: 170
Cyl 5: 180
Cyl 6: 172

I know spec is 185, but this was also done on a cold motor. (Didn't remember that I should get it to operating temp before I already had half the plugs out.) What I find interesting is that the cylinder that is suspect is showing the highest compression.

@RamTest I would love to install the original intake, but it was cracked in the accident.


There's a semi-local shop that does great work, I've got the question out to them to see if they're willing to diagnose & fix it (so far, every other shop I've called has said it's too "highly modified" of a vehicle and they won't touch it ). If this other shop doesn't want to work on it, I may have to mock up an injector test bench of some sort and go down that road.

 

[xterratx]

STFT changes very often, and sometimes it can indicate a problem, but mostly we refer to LTFT to gauge what the engine is doing. 0% LTFT is perfect. This essentially means the computer is happy with the what the O2 sensors are telling it, and it's not having to compensate by pulling or adding any fuel in the long term.

Your compression numbers look great considering that's a cold engine, so that's good news.

If all you wanted to do was test for a leaky injector, you could just take out the fuel rail from the lower intake manifold, leave the fuel line attached (there's enough slack to move it around while still connected), and turn the key on. That'll turn the fuel pump on and should expose a leaky injector I believe, never had a leaky injector myself. More of a quick and dirty way of checking... I don't recommend this method since it involves open fuel source, but just saying...

 

[dtrombley]

@xterratx
- I may pull the fuel rail and do that test, I had been considering something like that. Is there enough slack to slide a thin drain pan or something in there? I've never had the fuel rail off an Xterra before.. I don't necessarily mind an open fuel source, but I do mind unburnt fuel sitting atop the engine. If I didn't see a smoking gun, that's when I would start looking into building some sort of test bench.

- Yea, I was pretty happy with those numbers, I was starting to get worried that compression was going to end up being the issue

I know STFT changes pretty frequently, but the thing that confuses me is if STFT is consistently negative, wouldn't LTFT also trend negative? I also don't really understand why UpRev would show different readings than TorquePro

 

[Just a Hunter]

Keep us updated.