It's this engine: http://en.wikipedia.org/wiki/Nissan_VQ_engine#VQ30DE BTW

A friend has this problem, it idles really roughly, and has reduced power at cruising speeds, the computer is showing a knock error.

Is it worth it to try and change the spark plugs, or would this be a waste of time?

I took the front 3 plugs out and they looked fairly normal.

7/31/2011 3:39:15 PM

does it have the gold package?

7/31/2011 5:07:46 PM

Ha... no...

7/31/2011 5:14:31 PM

see if running a higher octane fixes the problem. could have excessive carbon build up on the piston crowns

7/31/2011 5:43:13 PM

I was going to suggest that, but i was skeptical it would make that big of a difference in such an old car. I'll let him try that see if it works...

There did seem to perhaps be a tiny bit more carbon that i would have expected on the plugs, but the electrodes seemed the right color/texture, but I really haven't seen enough of them to say.

It seems though that you get knocking in response to carbon build up, not carbon buildup as a result. And most people seem to think gas these days is clean enough where this shouldn't matter.


edit:

I just asked and he said he's tried using the 93 in there with no difference. And a fuel injector cleaner (which i've read on here is just an octane booster?) didn't make a difference.

There's kind of a "clunk" sound after you turn the car on that seems to come from the back left of the car somewhere too, not sure if this is related ( i was thinking the fuel pump firing up or something)...



[Edited on July 31, 2011 at 5:52 PM. Reason : ]

7/31/2011 5:45:31 PM

if he was running lean enough to knock, you'd see evidence of it on the plugs

7/31/2011 5:54:26 PM

Could a bad fuel pump trigger the knock sensor in this way? Would there be other symptoms if it was the fuel pump?

7/31/2011 6:05:30 PM

yeah, if your fuel pump pressure was low enough for the car to knock, you'd be running real lean, your EGTs would be much higher, and you'd be down on power but perhaps better mileage

7/31/2011 7:00:51 PM

sounds like fuel pump to me

7/31/2011 8:04:28 PM

It's funny you mention the higher temperature because me and another friend both thought the engine bay was unusually warm (it's maybe 3 miles from the car-owners house to where we were) when we opened it up.

I figure the oil temperature gauge though wouldn't reflect this excessive heat, because the thermostat would just open up to compensate, right?

7/31/2011 8:11:16 PM

oil temp gauge? on a maxima?

7/31/2011 8:38:02 PM

... Isn't that the gauge on the right? That's what i always assumed... it looks like a thermometer dipped in oil...

7/31/2011 8:45:49 PM

water temp

[Edited on July 31, 2011 at 8:56 PM. Reason : well, coolant temperature if you wanna get technical]

7/31/2011 8:56:16 PM

I guessed that from your disbelief at the existence of an oil gauge ;-)

Honestly, until this point, for the past 10 years, i've thought that was oil temp

BUT, even if the fuel was running lean, that gauge would still be nominal right?

[Edited on July 31, 2011 at 9:01 PM. Reason : ]

7/31/2011 9:00:40 PM

you probably wouldn't notice the difference on that gauge

7/31/2011 9:05:09 PM

yeah, it would affect EGT (exhaust gas temperature). The impact to coolant (and similarly, oil) temperature would be minimal, I think.

7/31/2011 9:13:00 PM

What are the chances of this just being a bad knock sensor?

And is there a sensor for egt? Seems like there would be...

7/31/2011 10:06:55 PM

not on a stock '96 Maxima.

I had 2 EGT gauges on my old turbo Miata, but that was sort of a different animal.

[Edited on July 31, 2011 at 10:15 PM. Reason : sensor could be bad, but it sounds like you have something wrong wtih the car]

7/31/2011 10:14:56 PM

you don't have to drop the tank to change the fuel pump on that thing, do you? that'd be a cheap, easy DIY fix. i would try that first.

7/31/2011 10:19:10 PM

I just googled, and apparently it's just under the back seat (on my old malibu you had to drop the tank).

I'm going to suggest that before he goes out and buys a $130 sensor (he has it in his mind this is the problem...).

7/31/2011 10:29:10 PM

no no, don't go dropping that kind of money on the sensor. i mean, it's running roughly and is down on power--at a minimum, there is something wrong besides the sensor.

also, check http://www.rockauto.com for parts. they might have it for less than $130.

7/31/2011 10:42:26 PM

knock sensor is bad.

replace sensor and harness

8/1/2011 7:25:47 AM

old nissan temp gauges are 3 position gauges (cold, normal, fucked)

also, the knock sensors on those go out all the time. I have replaced many a nissan knock sensor. I would put money on it being that.

what is the OBD2 code?

8/1/2011 4:05:55 PM

bet it's the actual knock sensor.

8/1/2011 7:06:57 PM

def the knock sensor. and why would you not run high octane fuel in a car that has timing set for it?

8/1/2011 8:05:28 PM

^^^ not sure of the actual code, something bout the knock sensor...

^ Would you recommend that? Seems like the internets doesn't care if the engine is < 10:1 compression, this one was specced at 9.8:1.

8/1/2011 10:21:16 PM

good grief, get the thing scanned FO FREE YO at any parts store and report back with the actual code(s). all this speculation based on nothing is hilarious.

[Edited on August 1, 2011 at 10:31 PM. Reason : while we're at it, lets throw out O2 SENSORS!]

[Edited on August 1, 2011 at 10:32 PM. Reason : ^ compression =/ timing]

8/1/2011 10:29:17 PM

He's got it scanned, but it was weeks ago, and I don't remember the exact code

8/1/2011 11:46:53 PM

What I'd be interested in knowing is if you got some timing chain slap going on. Seems the early VQs, especially those with high mileage, had problems with this.

8/2/2011 8:55:22 AM

So his car just refused to start one day and he had it towed to a shop.

The starter had failed and they said the knock sensor was fine. According to him, they read misfire codes and pin pointed the fuel injectors, so they replaced all the injectors too.

8/21/2011 5:31:51 PM

I had similar problems in my 96 Maxima

Replacing the knock sensor took care of it

8/21/2011 8:13:17 PM

Would really be surprise if the knock sensor didn't fix the problem, but on the early VQ's its basically maintenance at some point. Its a pain in the ass fix if you have anything besides tiny hands, but every one that I've done has yielded much better driveability and fuel economy.

8/21/2011 8:45:56 PM

I'm sure he misspoke, but he said they said a failing starter causes fuel to "back up" and causes stuttering and knock issues. Apparently one of the spark plugs had fuel on it or something.

overall i think he spent $1100... he'd be pissed if replacing the knock sensor would have fixed it...

8/21/2011 11:32:17 PM