2008 Nissan Altima 3.5 SE Coupe - Engine shudders after spark plug change


I am the proud owner of an '08 Altima 3.5 SE Coupe. It’s a great vehicle, and I really haven’t had any sort of mechanical problems with it up until now. It’s got around 109k miles on it.

A little background info for you - I’m an IT guy, not much of a car person. I’ve done a few oil changes, changed spark plugs on some four-bangers, and I get the basics of how an engine works. But I am nowhere close to being a mechanic, maybe just a once-a-month weekend one.

Here’s my situation. At around 105k miles, following the Schedule B maintenance list, I decided to be brave/frugal and try to tackle a spark plug change on my engine, even though the back three plugs are notoriously hard to reach. Before the spark plug change, the engine ran perfectly smooth, like butter. I followed the excellent instructions/photos on importnut.net: http://importnut.net/nissan-altima-spark-plug-replacement-4th-gen/. I’m the second owner of my vehicle, but I assume the plugs that I took out were the originals - Denso FXE22HR11’s.

I followed the instructions precisely, with the following differences:

  • I installed Autolite XP 5684 (Iridum XP) plugs, which multiple retailers confirm are compatible with this engine. I gapped them at .044 before installation.
  • I did NOT replace the intake manifold gasket or throttle body gasket
  • No anti-seize, as I have read from spark plug manufacturers that using it can lead to over-tightening of the plugs

Also, although the pliers that I used to help wiggle off the vacuum hoses roughed up the ends of them a bit, I did not notice any leaks, so I did not replace any of them.

After completing the job and putting everything back together, the engine started right up, and it seemed to idle pretty smoothly. However, over the next day or two, I noticed some ever-so-slight shuddering that traveled up through the steering wheel and the seat when driving on surface streets (25-50 mph). When I got on the interstate, it was much more noticeable, especially around 70-80 mph. With cruise control on and my foot off of the gas, I could feel the engine kick and shudder a bit. It would then seem better for a few seconds, then repeat again. The shuddering was worst if I disabled cruise control, let the speed drop for a few seconds, then gave it some gas. It wasn’t like the whole car was violently shaking, but there was definitely a moderate shudder coming from the engine bay for a couple of seconds. When I leveled off the speed, it wasn’t as bad, unless I stayed around 70-75 mph, where I would feel it kicking a bit. What’s weird is if I dropped below that, say around 60-65 mph, I didn’t really feel any kick anymore.

So, the first thing I did was take the car to the mechanic down the street. I started it up, took the engine cover off, and he put his hand on the upper intake manifold. It was idling pretty smoothly, but he said that he could “definitely feel it misfiring”, but he couldn’t tell if it was one cylinder or multiple.

I went back home, put the old Denso’s back in the front three slots since those are the only ones that are easy to get to, fired it back up, and still had the same symptoms. So I figured it was a problem with one of the three rear spark plugs or with the intake manifold gasket.

My neighbor and I performed open-heart surgery on the car again this past weekend. I used the same model of spark plugs, we DID change the manifold gasket this time (OEM replacement), and we replaced one of the vacuum hoses that we noticed had a slight leak.

I took the car out for a spin on surface streets, and it seemed pretty good. Maybe a slight lack of power, but no shuddering that I could tell. The next day I took it out on the interstate, and I had the EXACT same shuddering symptoms at 70-80 mph. I also started to notice some very slight shuddering at low speeds and at idle. I took the engine cover off, and sure enough, when I put my hand on the manifold, I could feel it shudder a bit every now and then, so it was probably the same misfire that my mechanic spoke of after the first plug change.

So that’s where I’m at now. Two not-so-easy spark plug changes within a month. No problems before, same symptoms after each change. I am looking for suggestions on how to proceed from here.

  • If I change out the spark plugs again, I’m putting in the exact same plug that I originally took out, either a Denso FXE20HR11 or FXE22HR11 (or possibly NGKs).
  • My neighbor suggested that perhaps one of the spark plug boots or wiring has gone bad. I just find it strange that the engine worked perfectly before I pulled the original plugs out, and the boot/wiring is suddenly fouled up. The only explanation for this is if we somehow damaged the wiring the first time, but we were very careful, and we did not yank on any wires or anything.
  • My neighbor also guessed that it might have something to do with the fuel filter. But again, why would this symptom suddenly appear after a spark plug change?
  • My guess is that the spark plugs themselves are functioning as designed, based on the fact that we got the same results after both jobs. I just question whether these plugs are really compatible with the vehicle.
  • We triple checked all of the nuts, bolts, gaskets, and hoses and they are all connected properly.
  • We did not use a torque wrench either time, but we did the standard “extra 1/8 to 1/4 turn after hand-tightening” to ensure the plugs were installed snugly.
  • I’ve noticed a slight hissing sound (with the A/C turned off) coming from somewhere around the top-back part of the manifold. I don’t know if it was there before I pulled the plugs out the first time. My mechanic said it was a normal sound. I checked all of the hoses for leaks, made sure they were on all the way, and sprayed them with carb cleaner with the engine running; no change in idle. Can anyone confirm that they hear this sound on the same vehicle with the VQ35 engine?

At this point I’m not sure what to do other than to put in some Denso’s or NGK’s and see if that makes a difference. Has anyone else ever used the Autolite XP 5684’s (aka Iridium XP) with success?

Thanks in advance for any helpful feedback. The only thing I ask is that you don’t waste any time saying “Autolite stinks!” or “Only use Denso/NGK!” unless you’ve tried both and you can personally attest to the fact that one is superior to the other in this type of vehicle. If you check out the Ford Mustang message forums, they all swear by Autolites, and lots of them use them at the track. But I realize that may be comparing apples to oranges since we’re talking about an Asian car vs. an American car. However, I also know that they’re made by Fram, who gets a lot of flak over the quality of their products, plus they’re priced lower, so it does make me wonder how good the quality really is.

Thanks again! I love this car; please help me get it back to how it was a couple of months ago!

  • Kevin

I would definitely change the wires before going much further.

Put in Densos or NGKs. I’ve had problems in the past when trying other plugs in a Toyota.
It’d be nice to know which cylinder is misfiring too. Have you checked the codes? It is possible that one of the COP coils isn’t seated properly on the plug’s end. Come to think of it, maybe variations in the plug end physical characteristics is the reason other plugs don’t always work well in Toyotas… maybe the COP units don’t clip exactly right. I’ll have to ponder that possibility…

"The only thing I ask is that you don’t waste any time saying “Autolite stinks!” or “Only use Denso/NGK!”

Well, then I guess that I am going to waste your time by advising you to use ONLY the spark plug brands and exact model numbers that are listed in your Owner’s Manual. More than likely, this will be Denso and/or NGK.

The nature of the ignition system on modern cars is such that simply using a different brand of spark plugs than specified can frequently result in very negative performance differences.

You might want to check if any of the springs in the coil-on-plug boots were broken when the coils were removed.


If these break it causes a gap that the voltage has to jump causing rough running.


One bad sparkplug can make it run like crap. I’m not a fan of Autolite plugs. Denso and NGK make EXCELLENT plugs and are the OEM manufacturers for most of the asian vehicles.

I’d spend the time and money to put new Denso or NGK plugs back in.

From densoiridium.com:

Q. Do I need to set the gap? A. Generally, no. The DENSO Iridium Power plug comes pre-set with a protective sleeve over the firing end, to protect the gap from accidental alteration. You could accidentally break off the very hard, and therefore brittle, tip.

Great, thanks for the quick feedback everyone! Lots of good responses; this gives me lots to look into.

I could have done without the one snarky response, but I appreciate the feedback all the same. That part of my post was intended for product ‘fanboys’ who blindly pledge allegiance to a particular brand for no good reason. I agree that if the owner’s manual calls for a specific brand, that’s the way to go. I did not think to check the manual for something like that, but I’ll definitely look into it, thanks!

My vote is for a damaged plug boot.

It might be noted that often the same plug is used for many different applications and with varying gaps. The gap should be adjusted, carefully, as required and recommended by every plug manufacturer.

It might also be noted that on the denso site they also reference in the last paragraph the gap adjustment procedure (carefully of course) and by doing so with a plug gapper or needle nose pliers

They also recommend changing the Iridium plugs at 30k miles. That’s going to get pricy; especially if someone is having to pony up and have it done by a shop.
To me 30k is a bit young but I would definitely never allow plugs to remain in place for 100k miles. The risk of seizing is too great and a 100k miles plug is simply not as efficient as a 30k miles plug even if on the surface the engine appears to run fine.

FYI no engine codes are being thrown.

Also, I checked my owner’s manual, and it does not list any specific spark plug brands. It just says “Always replace spark plugs with recommended or equivalent ones.”

Is there an under-hood sticker with emissions info? Usually so, and it will specify the correct plugs. Anything else - unless the carmaker has updated its recommendation for your engine - is a gamble.

That said, the problem is more likely something that was disrupted in the removing/replacing process… a misrouted or damaged wire perhaps. Good luck and please keep us informed.


This neighborhood mechanic . . . does his scan tool have decent software?

Even though your car has no codes, he could watch the misfire monitor at idle, to determine which specific cylinder(s) are missing

He could also monitor the fuel trims at idle and under load, cold and warmed up

I mentioned this, because I’m assuming your neighbor knows what I’m talking about and is set up to watch that stuff

Just throwing some ideas out there

There are two types of iridium plugs, those with an iron anode and those with a platinum pad on the anode. The ones with the platinum pad last for 120k miles, without the pad, 30k.

NGK and Denso plugs have an anti-seize plating on them so you do not use an anti-seize compound. If the Autolite plugs were black, then they do not have the plating and you have to use the anti-seize compound or they may seize to the head when it is time to replace them again. If they were silver colored, then they are probably plated.

If you are using the torque angle method of tightening instead of a torque wrench, you do not have to worry about over tightening the plugs whether you are using the anti-seize compound or not. Torque angle is snugging the plug or torquing to a very small value, enough to know the plug has seated, then adding a specific angle such as 1/8th turn.

If you can tell where the hiss is coming from, spray something like WD40 or a carb cleaner on the area, if the idle changes at all, you have a vacuum leak there and it needs to be fixed. Other than that, reread Testers post, I think he is right.

I can say without a doubt the Autolite spark plugs in Asian cars stink out loud. I tried them in my wife’s Mazda. The car was instantly sluggish and sounded more muted. Replaced the Autolites with NGKs, and the car’s acceleration was back to crisp and sound returned to normal. This was back in the mid '90s, but I doubt much has changed.

I agree with Busted Knuckles. I buy many aftermarket parts for my Hondas and Toyotas but NEVER ignition parts. Plugs (and wires, caps and rotors where applicable) always come from the dealer. Every time I have tried to feed a Honda or Toyota a non-OEM plug, cap, rotor or spark plug wire I have had problems or poor performance.

Dont let the running condition of the vehicle PRIOR to this repair throw you off. You MOST CERTAINLY could have caused this problem yourself while doing the plug change. SO YES…It was running perfectly prior to your plug change… HOWEVER you disturbed A LOT of items in order to get this job done. In doing this job you removed ALL of the coils…and could have EASILY damaged one of them during the job. SO dont focus on how it ran before…its how it runs now thats important.

I believe that you need to remove the intake manifold in order to change the rear 3 plugs.

As USUAL…Tester is right on the money with his info… You could have damaged one or more of the coils during the job.

The problem you are having can reside in one of only a few places.

It can be:

  1. A FAULTY NEW PLUG (Rare but Happens)
  2. A broken/cracked intake manifold gasket, not allowing the cylinder to properly fuel itself
  3. A damaged coil…or the coils “Spark Tube/Spring” that touches the plug…
  4. A coil pack that is not plugged in all the way…

Thats about it really… Your vehicle will tell YOU what plug or cylinder is not firing HOWEVER it can only throw a code when the coil pack is NOT firing or plugged in…If the issue is a gasket leak or the Spark Tube of the coil pack the computer sometimes cannot throw a code on that issue because as far as the computer is concerned, its systems ARE working… I.E … A faulty Spark Tube/Spring causing the plug not to fire…the computer knows the coil DID fire…whether that energy got to the plug or NOT is entirely a different story and one which the ECU may not be able to detect and tell you about.

I didn’t see that you had any engine codes thrown… If you DO and it tells you which cylinder is misfiring then you have a nice head start

If you have NO CODES…then if I had to GUESS at this issue… I would guess that you damage or dislodged one of the intake manifold gaskets… The Spark plug change is fairly easy and straight forward…so unless you noticed any issues with the coils themselves while doing the swap…they are probably OK… There IS a spring inside each Spark Tube leading to the plug…so check to see if all of those are present… Check the boot itself for any Tears or Cracks… make sure the coils are ALL plugged in properly…

THEN…I would check that intake manifold gasket… It can easily cause this issue.

If you take this problem methodically…you will find the culprit… I have been in your position so many times I cannot even count the times I’ve had to solve an issue just like this. Take your time and let us know what you get


Thanks again for all of the feedback, everyone. Much appreciated.

So here’s the latest: We swapped out the Autolite iridium plugs for Denso iridiums on Saturday. The Densos definitely make the engine feel a bit smoother, more responsive, and the quality of the the plugs themselves seemed better (more uniformly manufactured), but unfortunately I still have the same symptoms as before. So the Autolite plugs were probably fine all along. Just like before, the symptoms were more noticeable after a few days, and also at higher speeds.

FYI we also replaced another vacuum tube that had some wear on it, just to be safe. I have not yet checked the coils/springs, but since the engine is now back in one piece for the third time, I’d rather not take it apart to inspect the rear ones unless I have no other choice. I’m instead trying to find a way to pinpoint which cylinder/s has the misfire issue using a scan tool, then go from there. I have a bad feeling it’s one of the rear plugs though, because when I put my hand on the manifold that covers only the rear ones, I can feel a shudder every now and then.

My nearby Auto Zone has a good little scan tool, an Actron Autoscanner Plus CP9580A. They’ve let me sit in the parking lot and use it a couple of times. I’ve been able to pull up raw data, but I have not seen anything that is cylinder specific. However, I did see fuel trim readings, which someone mentioned earlier. I just don’t know how to interpret what I’m looking at.

So if the scan tool idea doesn’t work out and I don’t see anything wrong with the coil springs, I guess I’ll give up and let the local mechanic take a crack at it.

Oh, and I’ve sprayed carb cleaner around all of the connections near the hissing noise, but I did not find any leaks. And if the intake manifold is cracked, it’s not in an obvious spot.

Question - whenever I get this resolved, will the new plugs be OK to leave in there, or is it possible that one was damaged by the misfires and should be replaced?


Ugh… Sorry that everything in my last post got crammed together. Must be because I used my phone to post the comment.

My obsession with original equipment plugs is strictly a matter of self survival. A shop can go broke chasing down problems like this which can crop up after a repair and must be corrected for free. Many years ago it became obvious that installing anything other than the OE brand and part number was Ru$$ian Roulette.

Now that you have the propper plugs installed I will suggest that you inspect the Air Flow Sensor and all the plumbing from the intake to that sensor.

Now it the time to take some type of volatile spray like carb cleaner or throttle body cleaner and spray it all around the intake manifold gasket that you replaced. If the idle changes in any way as you spray across a section, then there is a leak there. If there is no change in the idle as you go all around the manifold gasket and all around the throttle body gasket. then you can rule that out.

If that is ruled out, then you are down to a bad coil, but a bad coil should give you a P030x code with the x being the cylinder with the bad coil. Have you checked all the fuel injectors to make sure you didn’t knock loose the connector on one of them. A little tug should tell.

If you still haven’t found the issue, then I have another very remote theory, but I have never actually seen this happen. The new plugs could have affected the engine enough that the computer has to relearn and correct the fuel trim. It should do this in a couple of days on its own but you might speed this up by disconnecting the battery for awhile, at least ten minutes. That used to be the way to force the computer to reset all its parameters, but you have to be careful with some of these newer cars because they my shut down and require the dealer to reprogram them so they run again. As far as I know, that is mostly Fords though.