Okay the engine is fairly new, and the ignition system is rather new as well. It has a quality Petris opti, and new plugs. But the ICM and plug wires were re-used from the last motor.

Here is the issue. The motor will be running fine then all of a sudden it will start lightly bucking under very light throttle/acceleration. There is also an unusual sound from the exhaust tone too. Then in a few minutes, the issue will be totally gone. Then it may or may not return at any random time.

I cant see how this can be anything mechanical or it would do it all of the time. Any ideas?

 

Is there still an EGR on the engine? If so, diaphragm could be bad and will act up at cruising speeds.

 

EGR is blocked off. It does have the EVAP system though, and I think the line from the canister to the intake may be clogged. Even if it is clogged, cant see that being the issue. Could be wrong.

 

Anyone?

 

Double check porcelain on spark plugs. I dropped one during install one time, LOOKED fine but must have been a hairline crack causing me "under load" misfire.

 

My go to is the opti if everything else checks out. It can only be three things- fuel, spark and/or timing.

 

Got any more details? Speed? Highway cruising? In a parking lot? Can you data log it and note about what time in the log it happens?

I don't generally associate bucking with a misfire. I consider them two different things. Like with my CC503 it can start bucking at tip in a parking lot. However, it does not have a misfire problem. A misfire will make the engine feel unbalanced, feeling like it missing, and you might feel some power loss.

Last time I had a misfire with my LT1 it was damaged plug wires. It would misfire under heavy load(6th gear, flooring the throttle).

Monitoring current draw on the primary side of the ignition could help show you what cylinder it is....or scoping each plug wire to view what the firing line of each plug looks like during a snap throttle. Search for some Scanner Danner misfire vids on youtube to learn more.

This is of my pickup when it had a constant misfire. The top graph is a current clamp on the ground line for all the coil packs, the bottom graph is of an inductive pickup on the #5 coil pack. You can see a missing current ramp in the top graph. I knew what cylinder I had the inductive pickup on(bottom graph), so with the graphs on top of one another, knowing the firing order (1,2,3,4,5,6 oddly enough).....you can see cylinder #3 is the missing current ramp. The PCM had already flagged #3 with misfires, but it started doing it all the time, so I decided to scope it. It was a bad coil pack.


Constant misfire cylinder #3 '05 Nissan Frontier V6

 

Hey guys. Thanks for chiming in.

Again this issue comes and goes. If I am cruising in 4th at 55 MPH and just give it some light gas, there is a shaking/bucking feeling in the seat of the pants. Other times I can perform the same operation, and the issue is just not there. When it happens, it just feels like the motor is crapping out. I am also using the same tune from the last motor I blew, but the current motor has the exact same LE heads and cam. Same spec motor through and through.

I also want to rule out the Opti. The motor currently has a high-quality Petra opti, but it previously had an Advance Auto opti that did this as well. I replaced the ICM, the problem remains. I will likely try plugs next, and I feel good the MSD Super Conductor wires are good.

Yes I can get a datalog if I can find my old Datamaster. Pretty sure I still have it.

 

Replace plugs and wires since they are cheap and easy.

Next step, convert to TorqHead 24X

 

If it is only happening when you roll onto the throttle check the TPS and idle air valves too.

 

Did the old motor with this tune do the same thing? Trying to rule out the tune and/or cam choice. Also, did you mean 6th@55mph instead of 4th@55mph?

 

Or at least ohm them out. MSD specs their 8.5mm wires at 40-50 ohms/ft. Measuring tape, a decent meter and a calculator...

 

I had an intermittent like this - turned out to be a failing coil. Gradually got worse and finally acted up in the driveway so I could diagnose it.
An old shool timing light on the coil wire as a spark tester showed erratic spark, and I confirmed it as the coil by swapping on my spare ICM.
You can run a known good coil as a quick check.

Hope this helps,
Jim

 

Is this the most ideal coil to get?

 

...or this:

 

I tried the StreetFire coil in 2009 after my brand new Accel (don't get me started) coil left me on the side of the road. The SF was all the parts store had other than a white box OE equivalent at the time so I gave it a try. Weak spark under heavy load. I ordered the 8226 from Summit a few days later, still running it with zero issues 12 years on. On my truck, I'm running a Pertronix D3002 and it's been great since 2017. I'd stay away from any of MSD's budget line SF stuff.

 

Put the new coil on, same problem.

But I found something interesting today. Because it's now cold out, it takes longer for the engine to warm up and go into closed-loop. I drove it today while the motor was very cold, and the miss was totally non-existent UNTIL the computer went into closed-loop. Once in closed-loop, the misfire was back. I think this might be a smoking gun.

I also noticed from watching the injector pulses on my Scanmaster, when lightly accelerating and the misfire starts, the injector pulses will suddenly start to drop. Obviously, this should not happen when accelerating. So the PCM is clearly pulling fuel during this condition, and lots of it.

So here is the new question. Why is this only happening in closed-loop? What components are possibly unhappy and need to be looked at? All I can think of is the O2 sensors. One more question. What connector on the engine can be pulled to force the engine to remain in open-loop for further diagnosing?

 

So I think I am going to retract my statement regarding it only missing in closed-loop. I think the random miss is there in open-loop too, but its just not as obvious.

So I replaced the o2 sensors - the problem remains. I bought a new set of plugs to install when time permits, and I will certainly gap tighter than the current ones which I think are at .050" IIRC. But as of today, it's running like a Raped-Ape with absolutely no signs of the miss, and I have done absolutely nothing to the motor. Could be that way for a week, or a month - who the Hell knows? I also cant quite get the data-log when its running like ****. Fun times in LT1 world.

 

I think you're seeing a lean misfire on tip-in. The drop in fuel delivery is telling. You said the PCM is "pulling fuel, lots of it." Consider this- at idle/ cruise the FPR has pressure around 37-39psi. As the throttle opens, pressure increases slightly, moving toward 43.5psi at WOT. As the TPS changes, the PCM is going to try to best-guess how to trim fuel, based on what MAP sensor input, MAF input and IAT input are telling it about air mass change. The stock FPR pressure curve isn't mentioned much, but it's a factor in any vcuum-referenced system. I'd bet GM figured that curve out (duh) and based the BPC value changes accordingly in the VE and MAF tables.

So... it's possible the PCM expects or assumes more fuel as the throttle is cracked and tries to trim it to allow increased delivery without going overly rich. That would make sense if you saw the issue ONLY during closed loop. Now that you're seeing it in open loop, where everything the PCM commands is pretty much static from the VE vs MAP, it begs the question what's gong on with your fuel pressure. Also, I'd hope you don't have some aftermarket drilled-out injectors that act wonky and confuse the PCM. Static KOEO fuel pressure check, FPR diaphragm check and maybe a 'tape the gauge to the windshield' exercise would shed some light?

 

The current thing I am closely looking at is possible come-and -go exhaust leaks at the header collectors. A couple of days ago, I hit a very large rock in the road and it knocked my 3" to 2.5" reducer off the driver side header. I found that there was about 1/2" of insertion at this joint, and it was likely not sealing well. When I put the pipes back together, I found that the reducer was a bit mangled from the clamp, and there was no getting it to seal to the collector as it should. I have new reducers on the way, and it will be back together next week.

I am wondering if there has been a minor exhaust leak all along that was jacking up the O2 sensor, and thus the A/F mixture, and maybe it was so slight that it could seal itself at times, and at other times there was a tiny leak. Does that seem plausible for a come-and-go misfire?