To preface, I have already done these things, oil change, air filter, plugs, coil pack, can of sea foam.


I dont usually pay attention to my mph because I fill up here and there, but lately i had a suspicion that I may be getting poor mph. So i ran a full tank through and got 246 miles out of it.

Im no rocket scientist but wtf is going on. Other than the gas gage and a calculator, you would never know that Im getting poor mpg, car sounds, runs and drives fine, no lack of power, i have no mods.

Any ideas, I read about the knock sensor or EGR tube???

Also I do not have any check engine lights or thrown codes

thanks

 

The upstream O2 sensor(s) that are starting to wear out can cause this. Once it gets out of it's operating range it will eventually trip the engine light.

Cats will do this too once they start getting excessive buildup in them and your exhaust flow gets reduced.

 

Does your car accelerate normally, idle normally and start normally? It could be bad injectors stuck open. If thick smoke comes out of your muffler then your are running very rich fuel wise. You need to narrow down your possibilities because unless you are a mechanic with years and years of experience, which most of us aren't, you cant tell what is causing a problem by looking at it or listening to it. I have a feeling its something in your fuel delivery system though.

 

He already stated that it runs and drives fine.

Older cars with reduced fuel economy (no engine codes) could be caused by several factors. Reduced compression, excessive carbon buildup, aging sensors, reduced flowing cats, etc.

 

where did he say that?

 

His first post.

 

Ive read it 3 times now and I still don't see it lol, im not being an ***. Saying he had a suspicion he was getting bad mileage I assumed something was off other than the quick decent of the fuel needle. Im surprised the ole Seafoam didnt throw a CEL for the o2 sensors though.

 

It's there... third paragraph from the top. You're probably tired since it's getting late and past your bedtime.

 

wow I suppose your right. I swear that was an edit....Even still. Bad injectors seem a likely cause for a quick drop in gas. Maybe not so drastic though. More info please. Obv you need to repeat yourself for me lol

 

I can see injectors causing a problem, but I wouldn't go as far as saying that their bad. They can get gummed up, or get a small piece of debris in it to cause it not to spray properly. A fuel filter change and a bottle of fuel system cleaner would be a good start.

 

Probably your KS.

 

If the fuel filter was clogged though it would be the opposite. Drop in power and probably an INCREASE in MPG. If it sounds and runs "fine" which in reality it couldnt be, its tough to diagnose. I might be stating the obvious but I think its a fuel system issue. Honestly it could have a cut line. OP, look under your car check for a cut line. and look at the exhaust to see if abnormally thick. More info is necessary though either way

 

You do have a good point about the fuel filter. I wouldn't say it's clogged, but if it's dirty enough, it won't be able to do it's filtering job properly and it could be degrading and letting larger materials (including degrading filtering media) to pass through it and it can end up getting into the injectors.

But yeah, as you said, more info is necessary.

 

Time to do the ole 300zx fuel filter swap. I was going to do this but in reality your injectors flow at the same rate so whats a bigger fuel filter going to do anyways. I always think on an older car(150k+) no matter how well youve taken care of it and no matter what brand of premium you use, your injectors arent operating at 100%. It would be nice if Deatchwerks could post data as to the flow rates of injectors they get in. THen again you dont know how well they were cared for so I guess thats stupid. It could be on the form you send in though. fuel used, miles driven, etc

 

The Z32 FF just lasts longer. I have one sitting in my room that I've been too lazy to install along with a new PCV valve. Oh well.

 

http://www.fullybolted.com/catalog/p...u=313038343535 these are awesome but not worth that price at all. Injectors are expensive yes but that much for 270cc?

 

Interesting... Never heard of a knock sensor being able to cause a reduction in fuel economy. Can you fill us in on the details?

 

That's under $65 per injector, which isn't too bad... I've seen injectors sell for much more than that.

 

ive heard of the KS throwing off ignition timing but wouldnt that throw a CEL that he said didnt exist?

 

A bad KS will retard timing, giving you a decrease in power and MPG. It doesn't always throw a CEL either. I just have a resistor in mine to bypass it and I've had it like that for years with no problems.

 

now thats a idea. So even if you had a knock it wouldnt throw a CEL? excuse my ignorance ive never heard of this.

 

If I have a knock or ping, I'm in trouble lol But I always run premium so I should be fine. Like I said, I've ran this set up for years with no problems so far.

 

I bought a cheapie ks a while back thinking it was bad. Fixed all my other codes and now I'm getting the 0505( all good). But when my o2 sensor was bad, I was also getting the ks code (since fixing o2 no more ks code) . Given what you are saying Chris, should I swap out that ks anyway or is it safe to assume the ks is good without testing it?

 

How's your MPG looking? How's the power feeling? It could be good or bad. After the IACV problem is solved, reset the ECU and see if any codes come back.

 

Chris, just to be clear, I have no cel right now and when I check the codes using the screw on the ecu, my cel flashes 5 slow and 5 fast and I think that means all Is good In max world. Correct me If Im wrong though. Power Is good, mpg I think is around 23 city but Ive been letting it Idle alot due to the weather so obviously its around 20 right now.

 

Isn't 96+ cars should be OBD2? Anyway, how's your driving style?

 

If you're asking me...my driving style is i guess average. I drive 12 miles to work each day mostly at a constant speed with 2-3 miles of that being 25 mph speed zones.

 

Ohhh I thought you were saying P0505.. my bad. 0505 is all good.

Sounds like your KS is all good!

 

Ok, I thought there might have been some confusion on the code. Guess i'll keep my hands in one piece for now. Good thing i only paid $10 on ebay for the ks and not $176 through the dealer

 

OP, what are you coming up with for numbers on your MPG to specify what "Bad MPG" is to you? You say 246 miles to a full tank which would be, give or take 18.5 gallons which equals 13.2 MPG? If you really are that low, I think there has to be some major issues with the car. Do you idle alot? (Not sure where you are from but maybe you warm up your car for winter??) I typically get 23 during the summer and right now am getting 20 letting it warm up for 15 minutes before and after work.

 

First off, we don't know your actual MPG. How many gallons was on the pump when you filled it up after driving 246 miles? Divide miles driven by gallons and report back.
What type of driving do you do, city hwy or a combo of the two?
What's the weather like? Very cold? Do you warm the engine up before you drive it and if so how long do you let it idle? Wheels tires - are you runny heavy wheels? What psi are your tires set to? Are you an auto or 5 speed?
Lots of variables effect MPG. Go on a long trip at highway speeds, drive smooth, cruise along at a nice pace and do some math to figure out your actual MPG. You might be pleasantly surprised.

 

I put almost 17 gallons in.

My tires are fine,heavy wheels?

Yeah it's pretty cold in stl right now, but I have had this car fire many Winters and have never seen below 22 mpg.

I'm have a spare knock sensor from my dads maxima that I will throw in some time this week and see what happens.


Heavy wheels....lol

 

Auto transmission

 

clean your maf and TB. change fuel filter. change PCV

 

I concur that when my ks was bad I was having a bad gas mileage. As almost all of us know it does not flash the yellow light so you have to do the screwdriver test to check if there is a code in the computa. Don't change it if you get 505.

 

mind pointing me in the direction of instructions on the screw driver test?

thanks

 

search for 4th gen CEL self diagnosis or something to that effect and youll find the .org thread showing how or just scour the 4th gen threads

 

Turn ignition on. Don't start the car. The check engine light should be on at this moment. Then using a flat screwdriver turn clockwise the screw on the PCM (ECU) all the way. Wait for at least 2 seconds. Turn it all the way back counterclockwise. Now the check engine light will start blinking with slow and fast blinks. The slow ones are the hundreds ad the fast ones are the ones. If you get 304 then it is the KS. If you get 505 you are in business.

 

here ya go

How to get the code:
http://vbxmaxima.8m.com/ecu.html

What the code means:
http://vbxmaxima.8m.com/ecudecoder.html

 

thanks guys, will report back when its not 10 degrees outside