HELP! - Problem - Fuel Economy/Fuel System

[jlbauer23]

I have a 2000 Xterra with approx. 52K miles on it.

My gas mileage has dropped like a rock from over 17 MPG to around 13-
14MPG. I had a Fuel Injector Service done around 45K miles, but
recently over the last week during a 3,000 mile road trip, I saw the
mileage plummet to around 13-14MPG.

I have had Diagnostics done on it and the code returned was PO325 an
error for Knock Sensor #1. According to the service guy the "Check
Engine Light" is faulty and should have come on. Has anyone had a
similar problem or know enough about Knock Sensors to know if it
actually does or could effect gas mileage. The service guy said it
had something to do with how the engine gauges the load of the truck,
and thus could affect how much fuel it uses. However he wasn't sure
if this was actually the root cause. (Cost to replace - $500)

I have read the many complaints about the Fuel Sender problems, but
am unsure if that could be the problem as my vehicle has not stalled
out yet.

Any ideas? Anyone have similar expereinces?

Thanks in advance.

[evergreen]

the knock sensor "could" be the cause, but there is no way to know for sure until its replaced.
it could also be something as simple as a clogged fuel filter. if you're getting error codes on the sensor its probably wise to replace it. the fuel filter replacement is cheap and easy too.

please post the info you find on the knock sensor and if it does solve your problem.

[jlbauer23]

Thanks Evergreen.

So what does that knock sensor do exactly? Am I understanding it correctly in my original post?

And what if it is just the fuel filter? This may be an extremely stupid question, but is the knock sensor so important to my engine that it's worth dropping $500 to fix it if the problem I'm having could just be fixed by replacing the fuel filter?

Originally Posted by evergreen

the knock sensor "could" be the cause, but there is no way to know for sure until its replaced.
it could also be something as simple as a clogged fuel filter. if you're getting error codes on the sensor its probably wise to replace it. the fuel filter replacement is cheap and easy too.

please post the info you find on the knock sensor and if it does solve your problem.

[evergreen]

i believe the knock sensor has to do with the air fuel ratio and if the motor is running on the rich/lean side. knock is when fuel isn't properly being ignited. the knock sensor and the o2 sensor work together to make the engine run at its most efficient.

i only going on what i have learned trying to tune a dfi turbo buick motor so i could be off a little. i'm sure there are a few around here that can better splain stuff.

one thing i know for sure....replace it asap because it can cause other problems.

[jim dillard]

The knock sensor "hears" ping/detonation like using cheap gas and accelreating hard. when it senses these noises it retards the ignition timing to prevent engine damage (melted pistons). If it fails the engine computers sets the timing to maximum retard (for ping), hense the poor performance. The check engine light should have been on. if not and it is a computer problem a federal 8 year, 80,000 mile warranty should be in effect for the car.

[jlbauer23]

Jim,

Thanks for the reply. According to the mechanic, the knock sensor is failing and there was no Check Engine Light on.

Are you saying that despite my car being out of factory warranty, that a federal warranty (I was unaware of) could cover this problem and force Nissan to foot the bill for getting it fixed.

Where can I get more information on this warranty and will Nissan be forced to honor it???

ALSO...

I am currently using 91 octane and getting about 13.6 miles to the gallon. Could this be the result of the engine using "maximum retard" because the knock sensor is faulty???

Originally Posted by jim dillard

The knock sensor "hears" ping/detonation like using cheap gas and accelreating hard. when it senses these noises it retards the ignition timing to prevent engine damage (melted pistons). If it fails the engine computers sets the timing to maximum retard (for ping), hense the poor performance. The check engine light should have been on. if not and it is a computer problem a federal 8 year, 80,000 mile warranty should be in effect for the car.

[OffroadX]

The knock sensor WILL NOT trigger a check engine light on its own. According to the service manual: Freeze frame data will not be stored in the ECM for the knock sensor. The MIL will not light for knock sensor malfunction. The knock sensor has one trip detection logic.

Brent

[jim dillard]

there is an 8 year or 80,000 mile federal emissions warranty on the catalytic converter and the engine ecu. ; if your tech says the sensor is bad, replace it. what else did you have done at 45,000 miles beside cleaniing the fuel injectors?

[jlbauer23]

Just that...nothing else.

From what I have been told by other mechanics and the tech that looked at my car....they say having the knock sensor replaced won't fix my problem with the gas mileage. It's just an easy way to throw $500-$750 to the wind.

Originally Posted by jim dillard

there is an 8 year or 80,000 mile federal emissions warranty on the catalytic converter and the engine ecu. ; if your tech says the sensor is bad, replace it. what else did you have done at 45,000 miles beside cleaniing the fuel injectors?