Spark Plug Gap Tolerance on VQ

[colinnwn]

My truck is taking a little longer to start than it used to, but it turns over fine. I bought it at 135k miles and don't know the service history, it is now at 165k. I thought it might be the spark plugs, so I am changing them, and the fuel filter after seeing my neighbor's that could barely blow through, and the air filter for grins.

 

The service manual says the "nominal" gap is .043 inches. I carefully tested the gap of new plugs, not to break the hair thin electrode, and it is .041. My current plugs are at .049, so it is .006 from "nominal". You can see just a little bit of the thin part of the electrode left. My dad thought maximum tolerance could be as much as .015, but he has been out of the car mechanic avocation for a couple decades. These plugs seem either not worn enough for 165k miles, or much too worn for 60k miles.

 

Has anyone checked the gap of their spark plugs after the factory recommended change interval of 105k miles? If so, what was the gap? Does anyone know what should be the outside tolerance for these new platinum spark plugs with really thin electrodes?

 

Thanks.

[BowTied]

I replaced my plugs when I did the power valve screws since I had it that far apart at ~110 000km. They were .044" and I recall .043" being the correct spec.

 

As to tolerance, I could see .0015, but not .015. Modern manufacturing should not have .015 ("15 thou") for anything like this IMHO. 1.5 thou is understandable, that is less than the thickness of a peice paper.

 

For the starting, is it turning over slower or although turning over fine just taking longer to start?

Edited July 4, 2009 by BowTied

[colinnwn]

For the starting, is it turning over slower or although turning over fine just taking longer to start?

It turns over plenty fast. Used to if I bumped it, it would start 95% of the time. Now it takes a second or 2 of starting, and sometimes I have to do that 2 or 3 times.

 

Maybe tolerance is the wrong word I used. Manufacturing tolerance for sure can economically be much better than .015. But I have seen some older cars with really high powered ignition systems running gaps of .055 and greater. And my neighbor's 2004 Grand Cherokee was running ok (but not great) with spark plugs worn down to a gap of almost .060. He didn't know what the mileage on his spark plugs was. I was curious what maximum gap Nissan expects this engine to run acceptably at.

 

Since your truck had worn spark plugs by at most .003 in 68k miles, and possibly as little as .001, then it sounds like my plugs with .008-.006 wear at 165k miles may be the original spark plugs.

 

Thanks for the info.

[parrish]

It sounds, to me at least, more like a fuel problem than a spark problem. Dirty injectors maybe? You might want to do a SeaFoam treatment on it- definitely wouldn't be a bad idea with that many miles and unknown service history.

[colinnwn]

It sounds, to me at least, more like a fuel problem than a spark problem.

 

Could be. I am replacing the fuel filter after I saw that my neighbor couldn't blow through his with 120k miles on a Grand Cherokee. He didn't know the service history of it either. He cut the filter open and the element was full of this reddish dusty sludge. After he replaced it, his ECM threw the code for injector outside turndown limit. So the new filter was flowing so much more fuel that the ECM was mad it was having to undercompensate. After resetting the codes it hasn't come back.

 

I always figured fuel was pretty clean and a fuel filter could probably go 200k miles before you needed to worry about it. It's funny that the service manual doesn't have a recommendation on the change interval other than to change when the car doesn't respond well to the throttle.

 

I've tried Techron additive and Seafoam on my Scout and old Maxima. I was underwhelmed that nothing seemed to change with the running quality or fuel economy. I read an injector cleaning specialist say those additives, and even the expensive on-car injector cleaning services that some garages advertise don't do much. They are really just some detergent and the same solvents already in normal gas. I thought that it figured, but of course he also wants to sell his off-car service.

 

Anyway if the spark plugs and fuel filter don't fix it, I will probably try the SeaFoam.

[colinnwn]

Finished installing the new spark plugs and fuel filter. The fuel filter was a little hard to blow through with mouth pressure, but not near as bad as my neighbor's. The spark plugs were really easy to install, except number 4. I could have done the rest in less than an hour. Number 4 required removing the air filter box, snorkel, and throttle body from the truck. The whole process of changing number 4 probably added an hour and and a half. I hate fighting with hoses to get them off the barb, but really this could have been worse.

 

The truck is starting more quickly, and running really smooth. I didn't realize it was running a little rough before, but after feeling the difference on the first test drive, it is pretty clear it was. Looks like that solved the problem. If I discover it is still hard starting occasionally, I'll update the thread.

 

Looks like .048 is the outside limit for the spark plug gap on the VQ engine. Except if you have the engine already torn down enough to remove number 4, I would replace it at .044 (even if you don't do the others.) Don't forget to be extremely careful testing the gap, and don't ever try to change the gap on these fine electrode spark plugs. There should be virtually no drag on the gapper, and if using a wheel gapper, rotate it back to remove all resistance before pulling out of the gap. I'd also recommend using a little anti-seize on the threads of the plug, mine were binding some on removal, and a bit of dielectric grease around the top ceramic indention.

 

Also interesting that I have all original coil packs (knock on wood). None of them had the black dot signifying the revised coil pack. I was half tempted to replace the number 4 one while I was in there. But I guess it isn't the end of the world if I have to bust into it again.

Edited July 5, 2009 by colinnwn