okay as of late as in pretty much the past 5-6 months or so, my 98 maxi has been getting nothing but 60 to 75miles(max on fumes pretty much) for 20 bucks. Now all I use is prem gas and that is all I am able to get. Now I understand it is is a pretty old car and I am not supposed to get the projected 130miles+ for 20 bucks like it would if it is brand new but what gives? with 60-70miles per $20 bucks of prem. I am getting approx 13miles per a gallon. That is insane... Now I narrowed it down to these things...

1.I understand I do need a tuneup as I haven't done one in 3 summers.

2. I did the resistor Knock sensor mod. Got tired of constantly replace sensors plus I put in prem anyway, so I dont' have to worry about Knock AS MUCH I am sure.

3.I do have an EVAP check engine light and other than the knock sensor that is really it as well I also got a 02 sensor a while back but I need to recheck that cause that only happened once and it kinda confused me on where the banks were.

4. Bestbuy did drill into my gas tank as the morons they are to install an audio fuse switch on the back seat a few years ago, but the hole is so tiny I didn't even bother to fix the gas tank or replace it. I wish I could of sued them or did something about it but of course I had no prove "THEY did it" as the manager said. Plus the hole is very tiny and to smell the gas you got to put your nose up to the hole on the back seat. I will fix it maybe this summer but I can't see gas evaporating out of that, affecting my mileage all that much in a 3-4 day span as the holes are tiny, I know that is why I am getting my EVAP code tho.


So you think it could be any of these? I honestly think fixing the gas tank and replace an o2 sensor would be the best, but shes so old and I can't spend like $70 bucks on all 3 sensors on a "guess" it will fix it. IS it always a guarantee that if you get an o2 sensor code that if you replace it, your gas will improve?

 

Lets say you are paying $3.70 a gallon for premium. $20/$3.70=5.4Gallons.
75 Miles/5.4 Gallons= 13MPG. Pretty bad mileage there. Start with a tune up, replace your plugs and replace clean the air filter. How is your compression, engine worn out?
Don't waste money on O2s unless you have codes.

 

I will recheck the codes. I haven't done a compression test on the engine not sure where I can, without costing me a ton of money, but I will keep that idea. The engine has 225 or 230k miles on it. Yeah the gauge didn't last and I bought one off a dude on here and even that gauge is going already. He sold me a **** gauge.

Also I am using the K and N Stillen filter but It needs a cleaning for sure. I will probably get on that sometime next week

 

I've got basically the exact issue. Recently replaced KS, rear 02 sensor, PCV valve, Plugs, air filter, fuel filter, etc. all that got me maybe an extra 30 miles on a full tank. Still sitting around 13mpg.

 

The evap makes a difference. Also quite a few other factors effect gas mileage, dirty oil, dirty air filter, old plugs, improper tire pressure. I have 2 leaky tires, and trust me, if your tire pressure is off a couple psi it takes quite a bit off the gas mileage. O2 sensors should be replaced every few years also because it effects the fuel trims, you may not get a code for them, but you can look at their voltage on an obd2 scanner and see if the voltage fluctuates quickly like it should.

 

I am having this issue as well but I hear a leak in my pre-cats which I plan to replace with O2 sensors which are still OEM and never replaced since owning the car and have a KS code which will not go away...

 

Do you have any other codes? In the absence of other codes, a KS code guarantees a bad KS. A bad KS will kill mileage as well as hamper HP.

 

Something really odd must be happening. I have a 98 with 204,000 on it and I get 27+ consistently - now 29 to 30 with the TW3C 2 stroke oil. I drove 457 miles on my l last tank before I filled it.

I understand too well how throwing money at an issue does not always work - I need that with my two previous vehicles.

What is your driving style? I don't know the proper term do describe how I accelerate, but I push the gas down to a point where pushing harder does not provide a linear increase in acceleration matching the extra effort, and I don't push down too hard to cause a jump in RPM). It I need to go WOT, I will, but I usually don't.

A good question to ask, and I am sure you have asked it to yourself already is , "where is the gas going?" Could the car be losing gas somewhere between your tank and the engine?

All my cats, O2 sensors, etc., are original, except for the ignition coil on Cylinder 5 and the knock sensor.

Do you think the car is running too rich on the fuel?

 

I will admit. I am kindaaa a heavy foot but... I don't ever shift past 2.5k unless I am trying to pass someone or blast by someone who pissed me off and almost made me hit them or doing a nice little 6k rpm engine clean going onto the highway(that I hardly do anymore cause my inputshaft bearing is getting real bad so I don't want it to get worse) but in terms of driving I am really good at driving standard and also revsync sometimes do down shift but as of late I just get lazy and throw in neutral before a stop. On a cold drive, I will double clutch cause my synchos are really crunchy and notching on a cold tranni. So that helps a little bit, but overall. I always shift at 2.5k and always amd ALWAYS in 5th gear if I am going around 35mph and up.

oh and yes, Car is def running rich. Very potent smelling out of the tailpipe. My car has always ran rich so much you can taste it.

 

This is your problem, find the cause. Are you using larger injectors? Exhaust leaks close to the primary O2s? Headers that modify O2 placement?

 

Cover that hole and get rid of the cel and you'll be fine.

 

Get the knock sensor fixed,because the car is running in limp-safe mode. The car is not running at it's full 190 hp rate. it's lacking in power.It's running at 150hp,because the KS is about 40-50hp itself. This will fix the MPG and the fuel filter is also a must it will increase the MPG. The other recomended service is a Tune-up if not done already and the tires inflation.

 

Also the heavy foot means the car goes into open loop a lot more often and can default to an afr of almost 10 in that loop. Those events may not be long but it all adds up, especially with the fumes leaking out of the tank since it is supposed to maintain pressure. When you remove your gas cap after running low you should hear that suction sound as the system releases negative pressure.

 

Step 1
Fill fuel tank, FULL and write down mileage

Step 2
Drive normally until your at or very close to empty.

Step 3
Fill fuel tank again, FULL write down gallons displayed on pump.

Step 4
MATH 101 - divide miles by gallons and report back.

There is no way you can even come close to accurately obtaining your vehicles MPG on 20 bucks worth if fuel or 60-80 miles driven (about 2 hrs). That's just ridiculous.

 

you should replace the front O2. the computer uses that one to read air/fuel.

 

It uses BOTH upstream sensors for A/F. Front and rear in a FWD vehicle translates into right/left bank. The "front" O2 is fine, he has larger issues.

 

No exhaust leaks as far as I can tell but I do have the LSS y-pipe and just had the exhaust leaks fixed last year. and Stock injectors.

I have the resistor mod done tho. So the car thinks it is working fine. That is the whole point of the resistor mod I thought? And I would know if it is running in that mode because I felt it before it is horrible.


uhh...yes you can. first. $20 bucks is about 5 gallons for prem around here. Divide the Miles you are getting with the 20 dollar fill divided by the 5 gallons you got on that 20dollar fill until you are empty. Works out to be how many miles you are getting per a gallon.

Also with the Tiny holes I have on the top of my tank. I wouldn't fill her up the high anymore.

 

Are you currently throwing an 0304 DTC? Any other DTC's? Are you sure you properly installed the right value resistor?

 

I will check if I am throwing any codes, and I have no power loss, I would know lol and It was the same one that the guide suggested

 

possibly injector(s) stuck open?

 

Not sure, but I do get a misfire and hesitation lol

 

x 10e^10

 

Better yet was his response, love the part about the holes too

 

Trust me, I bought two OEM KS and the code still exists to this day...

 

i'd say try a junkyard fuel rail with injectors... that's what i did and it boosted my mpg to maybe 21 :/

 

A bad KS won't kill mileage. I had a bad KS in my car for 6 months and was averaging 23 mpgs.

 

yes it will^ yours was probably just not that bad because mine caused mpg loss and a decrease in hp

 

good idea

 

got 25.7mpg on my last tank with a KS and O2 sensor code. just turned over 203k today. about a 60/40 mix of highway/city.

 

i've never gotten over 23. Even without codes and with granny driving and all highway...

 

By this time on this thread you should now see that the KS is not as big an issue as some make it.

Fact is a bad KS, if that's the only problem with your car, does not hurt your car as much. With a bad KS power is down to like 89% and gas mileage is barely affected. This is mostly because the KS only really comes into play when you drive your car hard, not like a granny.

 

I can somewhat agree with that. But, IME, the "KS mod" was the most noticeable of all my mods WRT power. .

 

OP also clean MAF and run GOOD injector cleaner.

 

Sounds like you have a bad circuit. Test the subharness and clean the two grounds on the front of the intake manifold.

 

The circuit is good from KS to ECU and continuity and grounds are good. There has to be a partial malfunctioning sensor that is holding this KS code which I would not mind changing O2 sensors since I never changed them since owning car..

 

if it's throwing the KS code and the voltage are good and the harness is good than just go and replace the KS. The rare case can be the ecu that's if everything checks out good.

 

Yeah but two OEM KS and still the code though. When I first put them in, the codes did disappear for two days and then reappeared. I already know my ECU will be kind of hard to find as it is an early 95 cali ECU and the EGR system is kind of weird because I don't have a EGR vent valve on the canister. Just a big hose going into canister...

Once I swap out the y-pipe and put fresh O2 sensors, it may go away since as I stated previously, it sounds like bees in a can..