I've been getting very low gas mileages for a long time, and its been bothering me. The problem is that it seems as if the o2 sensors and knock sensor seem to be running fine, because I have not received any type of check engine light, so I'm assuming that either my throttle body and/or fuel filter is going bad. This is a second hand car, I bought it three years back with 70k miles on it. Now it has almost 89k miles on it and I'm wondering if its time to replace my fuel filter. From the previous owner, the receipts say that the spark plugs and knock sensor was replaced at 60k, but no mention of fuel filter.

 

thottle bodies dont go bad, they just get dirty around the butterly. 5minute cleanup with a rag and brake parts cleaner

KS doesnt set off the CEL by itself. But if its bad and something ELSE sets off the CEL, then you should see w/e set it off, PLUS the KS. Chances are, if you have more than 80-85k miles on that vq30, the KS is cracked. KS is like part of the "unofficial maintenance schedule" for after 85k, cus it is VERY LIKELY its gone by then.

Also, why are you asking if its time to change the fuel filter? Fuel filters SHOULD be changed every 30k miles. So yes. Either way, you bought the car used. Not gonna bug you about a reason, thats your business, but I always tell people when they buy a used car, do ALL tuneup maintenance required right away, never trust the previous owner unless you SAW them put the parts in.

So yes, change your fuel filter, change the KS when you get a chance, and clean up that TB

 

What is the condition of the air filter?

You might as well change the PCV valve. They cost peanuts.

A little fuel injector cleaner wouldn't hurt either, eg. Chevron Techron, Redline SI-1, Gumout Regane.

Given you didn't put the bulk of the miles on the car, I would be inclined to use a stronger cleaner like BG44K.

Also, have you determined whether the plugs were replaced with NGK plugs.

I question who installed them at 60K. If the car was taken to a stealership, they would have sold him everything under the sun that goes with the major 60K service interval. That includes a fuel filter, air filter, etc.

 

You hear the guy above.

Clean the TB, replace fuel filter and check the KS.

 

just how bad is you mpg? what kind of driving do you do? these cars only get 21-25mpg city, so unless you're in the teens and not going WOT at every stop light, you might be ok.

fuel filter will not impact your mpg unless it is so clogged that it's causing hesitation. is your car hesitating under medium-hard throttle?

KS will not kill your mpg either.

O2 sensors, however, can hurt mpg, even if you don't have any O2 codes. if they are coated in carbon they will send false (albeit "in range") readings to the ECU, and you could be running rich as a result. improper closed loop control is the #1 cause of fuel economy issues. O2 sensors, temp sensors, and the MAF all play a role here.

low compression will also cause poor MPG. does your engine burn oil?

 

Um well it turns out that the throttle body isnt dirty at all. There was no dust inside what so ever. I tested the knock sensor. According to this post, I followed their direction
http://forums.maxima.org/showthread.php?t=512892

According to that post, my knock sensor was giving a resistance of about 560 kilo-ohms so its under the range. Also according to the receipts the 60k mile tuneup was done at a nissan dealership. I have a feeling now that it may be the fuel filter, but I'm also going to do the 470k resistor mod as a dummy knock sensor. I'll try the fuel injector cleanup, but I doubt that will help as I've done that before.

So I was looking at the posts on how to replace the fuel filter. Where can I find a genuine filter for cheap? Also I read some posts about people replacing fuel filters w/ a 300zx fuel filter. I know that those filters are bigger in diameter but does it fit in the bracket properly? I have a 97 model.

 

Alright I just checked the ecu for any problems. It gave me two errorrs (0304) and (1001). So basically my knock sensor is bad and front left htd o2 sensor.

 

how often do you guys change your 02 sensors? i don't think mine have been changed at all and i'm just over 90k.

and i've heard that our PCV valves are pretty much a no maintenance item, as in good for the life of the car.

 

there you go Replace those, and you should see a good improvement

 

I dunno about the o2 sensor, cuz I've heard that you can change these sensors over and over again. The knock sensor, I'm going to put a 470k ohm resistor and see if that helps.

 

forget the KS for now. you don't even need it anyway.

as I said before - the O2 is the critical one. the ECU cannot control the fuel mixture without it. and your car will not pass inspection without it.

I speak from experience.

 

Ok where can I find cheap o2 sensors? I would like to get a genuine nissan one.

 

well it will be decided once and for all if the KS has any affect on a car once i change mine out and get some 91 finally tomorrow.

get the o2 replaced and see if the KS is still there.

fuel filter doesnt affect mpg, and replace the PCV valve if u r worried about it, its only a few dollars and takes 5 minutes

 

Whenever they go bad, i.e. CEL. As for the PCV valve, the idle will be affected and other problems could arise. They're cheap and easy to check. Read the stickys for further information.

Not needed but nice to have for performance. The AFR is still controlled by out of spec O2 sensors, they just run a little richer.(~12 vs 14.7) MAF also plays a role in AFR management under light load, when o2 sensors are out of spec.

Courtesy Nissan , or www.pinnaclenissan.com, click on Buy parts.

 

One problem now, well I drove the car around. I put a 470k resistor in the socket and now I checked the ecu again and I saw that there is no error for an o2 sensor, however there is still an error for knock sensor.

 

There are a set number of starts (im not sure how many) before an error will clear itself, the only way to reset it is to erase the code, then run the car to see if it throws another code.

 

right, that's exactly what I said earlier...
without properly functioning O2 sensors the ECU can't maintain stoich and proper closed loop control can't be achieved. i.e the system is out of control relative to its closed loop design parameters.

true the ecu can rely on other sensors to "guess" where to set injector pulse duration - but that is not closed loop control (output is not feeding back to the input).

 

Ok well now I'm in the market for buying o2 sensors. Is there any difference between the aftermarket and genuine o2 sensors?

 

I've found some pretty good deals on o2 sensors particularly from ebay. Here are a couple. These come w/o the connector but I have no problem with cutting the existing connector and soldering it to these o2 since I'm an electrician.
http://cgi.ebay.com/ebaymotors/NEW-N...QQcmdZViewItem
http://cgi.ebay.com/ebaymotors/NEW-B...QQcmdZViewItem
http://cgi.ebay.com/ebaymotors/NEW-B...QQcmdZViewItem
http://cgi.ebay.com/ebaymotors/OXYGE...QQcmdZViewItem