Hi Everyone,

My wife's G35 (2005 Sedan, 110k miles) is having an issue similar to this one:

https://g35driver.com/forums/g35-sed...s-hiccups.html

Background: The car was running great until a few days ago when cylinder 6 suddenly started misfiring badly. I replaced the spark plugs (NKG Iridium), following the DIY from QuadCam and G35Mass, and was very careful to torque the new plugs to 20 ft-lb. Plug #6 was very badly worn (gap was 1.9 - 2.0 mm); the others were not great, but were less extreme (some had gaps around 1.5 mm).

The car is running better now, but after warmup, it "thumps" or "hiccups" once every few seconds when idling. After reading the thread linked above, I'm thinking the throttle body could need a cleaning, but the problem was not there before, so I'm skeptical that it would be something unrelated to the failing plug or the replacement.

Given that I was careful with the torque wrench, debris, etc., I'm considering the following:

- Could the one very bad plug have caused a failing ignition coil? Or is there any other obvious issue that might have been triggered by the failure?
- Could it just be an improperly seated plug or coil? I tried to avoid this, and hate to pull plugs out and put them back unnecessarily, since it just increases the chance of screwing up the threads or getting debris in there.
- Could it be a vacuum issue? I did remove the PCV hose from the valve to get at one of the plugs, but it seemed to go back on properly.
- Could I have damaged the PCV valve somehow? Does it have any tendency to break?

Where would you start fishing with these or other ideas? I noticed that the OP for the thread linked above also had new spark plugs when the issue appeared, so perhaps it could be related?

Thank you in advance for any advice!

 

What about the other five spark plugs? Did you replace those as well? That one in the picture is absolutely cooked. The entire tip is completely cooked off of the spark plug.

If one plug is that bad then I can guarantee The other five are that bad.

That plug also appears to have a significant amount of oil on it? Was there a bunch of oil in the spark plug tube well?

 

Hi Cleric,

Yes, I replaced all six plugs. That one was by far the worst, but they all were done. Yes, they all had oil on the threads. :/

The well looked surprisingly clean, though the lighting was poor...

 

Here is a picture of all six plugs that were replaced - 1 on top, 6 on bottom. All of them are clearly bad, but 6 is the worst and was triggering the code (a bit hard to see in the pic, but the gap is notably larger by about 0.2 mm than the next worst). All six have oil on the threads, though the ceramic side is clean, other than what looks like heat damage.(?)

Thanks for the input. I have a good success rate with car care, but more because of caution and tenacity than any depth of knowledge, so help is appreciated.

From top to bottom, plugs 1 through 6.

 

How does it run when you do a full throttle pull up to highway speed?

I would clean out the MAF first because it's quick and easy, just to make sure everything is all clean.

I would do all the pedal dance resets just to rule out an algorithm issue. I can almost guarantee LTFT has changed because the burn on those cylinders will be drastically different with new plugs. Idle air calibration gets out of whack after the throttle body builds up a little accumulation.

Idle air reset requires the transmission to be up to temperature, follow all the steps EXACTLY as shown, or use a bluetooth OBD2 like the BAFX OBD2 adapter and a phone and the app Nissan Datascan 2. Then you can just press a button to do the idle air recalibration.

Steps for using the pedal dance method for the resets are in the FSM section EC - Engine Controls at the start of the document under "basic service procedures".

https://www.nicoclub.com/infiniti-service-manuals

 

Thanks. That makes a lot of sense - will go for it.

I haven't taken it on the highway yet, but tried to give it a good punch on the surface roads, and it has done great for everything so far, except warmed up idle.

 

Thank you for pointing me to the correct place in the service manual. It would have taken me a while to find it - that saved me a lot of time!

I cleaned the MAF sensor and also did some very light cleaning on the throttle body (did not take it off, but just used a little throttle body cleaner and gently cleaned on the side that is exposed when the air intake is removed.

Just finished the pedal dance. I did it twice actually - the first time, the idle seemed a bit low afterward (under 600). Repeated it, and the idle seems right (slightly over 600).

I think the little sputters are happening less frequently, but the issue is not gone. It actually appears pretty quickly after turning the car on - the car does not need to be fully warmed up - it appears when the idle drops below 700. Listening at the exhaust pipe, they sound like little pops of a bass drum, and the idle drops just a touch (maybe 20 RPM, barely visible on the gauge).

If I gently touch the accelerator and bring the tachometer up to about 800, the pops go away. So I'm thinking it probably does have something to do with the engine controlling the idle, and perhaps not misfires.(?)

Thoughts?

 

Cleric670,

Thanks for your help. At this point I've cleaned the MAF sensor and the easy-access part of the throttle body, replaced the PCV valve, and reset the idle air flow multiple times. I don't have any obvious vacuum leaks (though I haven't tried the stethoscope method). I'm thinking of replacing ignition coils, at this point. What do you think?

 

I think you should clean a little deeper on the throttle body. Key ON, engine off, and you can set a heavy object on the throttle pedal to hold the butterfly valve open while you clean it more thoroughly. Pay particular attention right at the edge where the plate sits shut, also clean the edges of the plate. Back is difficult to reach with it bolted to the plenum.

I wouldn't automatically assume anything was wrong with the coils though. Issues with them typically present while the engine is under load not at idle but you should remove and inspect the contact spring inside the well tube shaft, might have corrosion on it? I don't think they're your problem though.

Use your TB cleaner and spray around the intake plenum while the engine is running to see if you can find a vacuum leak.

The PCV lines are common areas to peak, particularly the one that attaches to the valve cover directly below the throttle body.

Also do you hear a sort of "thumping" sound inside the cabin near the glove box? Might be a fuel damper starting to wear out.

Last thing I would check is cleaning up all the chassis ground system for the engine. 5 total points to clean (6 with auto tranny).

Center top of the timing chain cover, 1 green bolt in the middle for 6MT, 2 if you have the 5AT.

Right side (passenger for USDM) of the timing cover larger gauge wire that lands on the chassis below the coolant reservoir on a 2 hole lug. Single 10mm holds down the coolant reservoir and you can lift it straight up without disconnecting the hose or pouring out the coolant.

Battery negative terminal
OThere end of the battery negative bolts to the chassis next to the battery on the same style 2 hole lug as the engine primary bonding jumper. Remove the plastic cover over the battery, not just the little access door. It's got a few of those push-pin-center body fasteners. Lift the center pin with a screwdriver and the pull the whole fastener out.

Each of those bolted lugs, unbolt, use a metal brush to clean up the lug, the bolt, and the mating surface. Apply a very light coat of electrical anti-oxidation grease, bolt it back together.

You can get a little packet of anti-oxidation grease usually at an auto parts house in a little $2 packet at the counter. This is NOT dielectric grease, you want anti-oxidation grease. If the parts house doesn't have it Home Depot will have ILSCO De-Ox or Ideal Noalox in the electrical section near the wire nuts. Or any electrical supply house.

These cars are highly notorious about grounding system problems. The De-Ox will ensure it doesn't ever have the issue again in the future and a slightly corroded ground system can definitely cause idle weirdness like you're experiencing.

Plus it's super cheap to fix if you do it yourself, metal brush and some De-Ox


​​​​​

 

Ohh and make sure you deox the threads of the bolts. Some of those land on painted surfaces so the contact point is actually the threads.