I have an idea... If he's cali spec with a clogged front cat, and a bad knock sensor, that would send his mpgs into the teens easily, a member here had that combination and wouldn't see anything higher than 15 if he was lucky on the highway.

 

IIRC, only the 99 Cali-spec had front pre-cats, the OP's is a 97.

 

If you think o2's don't effect gas mileage disconnect them and see. It may not be a big deal on the highway trips because most of the time you may be in open loop but you will get killed on in city driving/closed loop. Unless you have a piggyback like emanage, etc you cannot manipulate the maf voltage to pull fuel, you will lose mpgs. I'm pretty sure the ecu defaults to a rich fuel map when there is no o2 or improper o2 feedback. I wasn't saying the o2 are the end all but may be part of the problem.

 

i changed out a ks and front o2 and noticed an approx 30% mpg improved on a 500 mile trip. The default with a bad ks or bad 02 is the car burns a safe but very expensive rich or virtual straight fuel. The rear o2 behind the cat apparently only tells you if the cat is working or not, it doesn't affect rich to lean mixture setting like the front 2. Sometimes the 02 code with also throw a ks code which may errant, so you wait that one out to replace the ks. Changing the ks is not a big deal with a double universal socket set up.

 

Technically they are... Like i said before I have a code for all 3 now (pop'd up during the trip), and I was in closed loop during the whole trip (6th gear keeps me under 3k until 80+mph).

I'll unlpug the front two since that's easy, but I only have work tomorrow (no school) so it'll only be a 50 mile mixed driving day.

My knock sensor really helped though, I got about 33mpg on the way up and 35-36 after I replaced it (it could also be the fact that this leg of the whole trip was downhill... But I have never noticed an MPG drop/increase north or southbound before so I doubt it's that).

BUT! Before with my bad knock sensor, City MPG was abominable, like 15-19mpg, instead of my old 25-26mpg in the city.

 

I wonder where the OP is ....

 

right here, will be filling up today, so 70$ dollars or so later today and I report back with how much gas it takes and will reset my trip and begin the hunt for my issue.

 

hahaha very funny, no long idles or borrowed car and no remote start, basically all city driving and some spirit mixed in. might be my knock. will soon find out

 

ok so car took . 15.43 gallons and trip has been reset.

 

Lols! The muscle guys are laughing at you

 

Any updates?

I just plugged my o2 sensors back in this week, we'll see if i can get better than 30mpg in the city.

 

30 mpg on the highway, at normal speeds, more realistic.

 

Eh???? For a stock auto sure.

 

s same results after a couple tanks. 15.5 gallons gets me 200 miles. wtf. thats like 13.3 mpg and almost costing me 35 cents a mile to drive

 

I'm in the same dilemma, calculated my mileage. I get roughly no more than 16 mpg on city driving with a light foot. Cleaned EGR, TB, MAF, filter, premium gas. Next to be done is my fuel filter this week, then my 02 sensors when I have the $$$. On the highway, I can get close to 26-27 MPG.


No one mentioned fuel filter? Or did I miss it...

 

I'm having a very similar problem and I changed my knock sensor (along with a bunch of other thing) which did not make a bit of difference. My car runs great with no cel (yes when I first start the car) and has great responsiveness and acceleration. However I can't seem to get better than 15 or 16 mpg when I usually get around 20 (with my lead foot).

 

I took a trip to Houston today, was on the light for a few miles in San Antonio right before I filled up. 15.11 gallons of 93 octane.



I drove 243 miles, my gas indicator is just above half. Pretty decent highway MPG if you ask me. That was me going 80 mph, strong winds, full a/c's. Few pulls here and there when I was feeling sleepy. My mods are y-pipe, hfc, catback, and short ram.

I just need to improve my city MPG, I'm really thinking it is my 02 sensors

 

Sounds just like what i was gettin in the good ol days. 27-28mpg


OP what tires are you running??

 

my 96 auto is also getting s****y mpg only codes are p0300 and p1445 but i changed maf and not the problem not a vacuum leak i checked by spraying carb choke cleaner over vacuum and no change in idle it also said air/fuel heated o2 sensor? i know what an o2 sensor is but is that what it is saying the problem is?

 

Please tell me everything about your setup! Thats civic territory on mpg. I'm getting 25 mixed with a heavy foot from time to time. Do you have mods? Lighter rims? What do you attribute to your mileage?

 

According to FSM - P0300 is multiple cylinder misfires; P1445 is EVAP canister purge volume control valve. Misfires could be spark plugs, coils or injectors and would ipmact mileage. EVAP system is generally not mileage affecting unless that valve is causing a vacuum leak. What "said" o2 sensor?? Neither of these codes indicate O2 sensor.

 

i had it checked at autozone they printed out a paper for the p0300 and it said the cause of the random miss could be the "af/ho2s (air-fuel/heated oxygen sensor) fault" as it is printed on the receipt i got a new purge volume control valve i runs a little better but i believe it is still missing im going to try plugs this weekend it also sputters when i use a little pedal i have to give it a lot of gas to get it to stop but its not throwing any more codes

 

The FSM doesn't show air/fuel/O2 sensor as a resolution for P0300. It showes the following as possible causes:
- improper spark plug
- insufficient compression
- incorrect fuel pressure
- EGR valve
- injector circuit open/shorted
- injectors
- intake air leaks
- ignition secondary circuit open or shorted
- lack of fuel
- magnetized signal plate (flywheel)

My experience is that if air-fuel/O2 sensors are bad you get codes specific to those sensors.

 

that is what had me confused but the receipt was printed like this:
Troubleshooting P0300
OEM Brand: Nissan/datsun
definition
-random cylinder misfire detected
probable cause
-ignition system fault
-fuel system fault
-large vacuum leak
-maf sensor fault
-af/h02s (air-fuel/heated oxygen sensor) fault

so im not sure if it is something that might fix the problem or not
and i was wondering were this is located, and how could i tell if my egr tube is leaking the plenum was removed by the previous owner it appears to be tight but is there any way to tell without taking it apart?

 

see the signature.

 

Uh...am I missing something? I don't see a signature.

 

he has a 6 speed and bolt-ons

 

post #41, default settings show the sig on the user's first post on each page.

OP, so you repeated your results, what actions have you taken from the wealth of suggestions we've given you?

 

I see. Thanks for the lesson.
I take it that the majority of your exceptional mpg is attributed to the sixth gear?

 

^ Correct.

 

Once again i am pulling the p0300 an i have cleaned my maf, tb,and changed the the plugs then the problem stopped for about 25-30 miles the miss does not seem as bad as before but it is still frustrating. i also have to change my plug well seals my #3,and #5plug had a little oil on them, could this be my problem?

 

if oil gettin into the cylinder i wd def say thats the reason for the random misfires.

 

thanks thats what i figured i just ordered a valve cover gasket kit with the plug well seals for like $52 so i think that should solve the problem my valve cover gasket needs to be changed any way