Bad Gas Mileage: What To Fix?
 

Okay, I Know these threads are frowned upon, and that our cars get bad gas mileage, but bear with me here.
I have an 88 NA vert that was maybe sitting for a year- two years before I started driving it this month. Anyways it gets crappy gas mileage, 9 city maybe 12 highway. So far I've replaced the air filter, Added some injector clearer to the gas when I filled it up(Had less than 1/4 a tank of old gas when I put the injector cleaner and new gas in) Thats really all I've done that would effect gas mileage I think.
So im thinking will putting a new fuel filter in and Some of that stuff that takes the water out of old gas(Im thinking the old gas might have been bad and could cause a problem) help? Any other suggestions? Right now im using a cheap fram air filter, would upgrading to a K&N really make that much difference?

Edit: Whem im changing the fuel filter, I just disconnect the old one and put the new one in its place right? Do I need to wait till my tanks low so fuel doesnt pour out the tubes? Sorry I've never changed one before.

Full tune up.

Check the FAQ, but My5ABaby basically said it. Your mpg is way below normal. Don't try upgrades, try repair.

EDIT, changing the fuel filter: Start the engine. Disconnect the fuel pump power connector. It's in the driver's side rear suspension tower, below the speaker. The engine should then die on it's own. Next you just remove the old fuel filter and install a new one. Then reconnect the fuel pump. When I did it, I found that a lot of fuel leaked out anyway, so I clamped a clamp onto the fuel line to prevent more from getting out.


This probably won't solve your problem, but it may help and it's good to do anyway. Grab a Haynes manual or download the FSM from the FAQ and take care of all the maintenance items. You should do them anyway, and they might solve your problem. Still also do the other things listed in the gas mileage section of the FAQ.

Yeah I have a haynes manual and I have done alot of the 60k Tune up, I guess ill have to just keep looking and go read the FAQ to see what I havent done.

Yeup. Good to hear that you're a step ahead of most. Assuming it's not an engine problem, those things should fix it.

Injector cleaner probably won't do anything. But if your vert got good mileage before storage, then that's much more likely to be the cause b/c the fuel system might have gummed up during storage. Chevron sells good quality fuel system and injector cleaners. For something a bit more thorough, a mechanic can clean your injectors for ~$80. But spending $80 on that would suck if it turned out that it wasn't the cause. I'd try other things first.

d15 swap 70hp 52mpg highway.

j/k haha definently a tune up will do some good. also depends on how you drive. if your shifting around 2500 or 3, then those numbers are awful, if your shifting around 6500 then thoe numbers should be close i think

with a Full tune up, what are they going to do with the car? put in some new things?

Click on the link in my sig below.

Surely you've used all the old gas by now? How are you measuring the fuel consumption? The only accurate way is to drive until the tank's near empty, fill it to the top, drive until the tank's near empty again, and then top it up again. Miles between fills divided by the amout of fuel to fill the tank give mpg, and all the fuel is obviously fresh.

None at all. The real-world difference in restriction between a cheap filter and a K&N filter (both clean) is so small it's insignificant. "Performance" drop-in filters are a waste of money unless you plan of keeping the same filter for over ten years.

Correct.

Have a suitably sized bolt handy to plug the hose when you disconnect it. Have rags ready to catch the fuel that will inevitably spill.

"Shift before 3500 rpm to keep the secondary injectors off"

"The Oxygen Sensor plays one of the largest roles in fuel economy... If your Oxygen Sensor is bad or warn out, your fuel millage could fall tremendously."

These two statements should be taken out because they're both very misleading.

The secondary injectors being on is not what effects fuel consumption. The staged injection system doesn't suddenly increase fuelling at that point. The engine uses more fuel cruising at 4000rpm (secondaries on) than at 3500rpm (secondaries off) because simply requires more power to do so. Exactly the same is true comparing 3000rpm and 3500rpm, even though the secondaries are off at both speeds.

The O2 sensor has only a minor effect on average fuel consumption. It's simply not used often enough, and has only a small effect when it does. For fuel usage to fall "tremendously" without it, the engine would have to run very rich while cruising, but it simply doesn't.

during closed loop it has a large roll, I have seen millage go down when an o2 sensor is bad.

I thought I already took that out a month ago, damn.

Never said it wouldn't. I have measured, over a very long period and many fills, the difference in average fuel consumption between no sensor and a brand new one, and it was ~5%. The difference would be even less if you compared an old sensor with a new one. It's worth replacing an old sensor, but it's not going to fix bad mileage or have the huge effect you claim.

I have seen otherwise. Every engine is different I suppose.

No, the O2 sensor works exactly the same on every engine.

The ECU doesn't run the engine bulk rich without an O2 sensor as some people believe, and the sensor is only used during specific driving conditions. So it makes a small difference some of the time.

You say you've seen bad milage improved. By how much and what was the test procedue? How many miles of driving either side of changing the sensor?

From what I remember it was around 400 miles back to back. Half tank to half tank back to back before and after the sensor was put in. The difference was about 20 miles to a half a tank.

That's a totally inadequate test. You have no way of knowing if the driving conditions were the same. I get that sort of variation from tank to tank all the time without changing anything expect where and how I drive. My results were based on several thousand kilometers each side of adding the sensor.

I have no way of knowing? I don't notice the road or type of driving im doing everyday?

I drive the same roads every day to and from work. Sometimes the weather is good, sometimes it's bad. Sometimes the traffic is heavy, somethimes it's light. Sometimes I take every opportunity to drive it hard, sometimes I like to just cruise. Occasionally I also have to drive a long distance at freeway speed. Somedays I might make a couple of short trips. All these things dramatically effect average fuel consumption. Are you telling me your driving conditions are identical every single day?

A tank's worth of driving it totally inadequate for this comparison. That should be obvious.

wow 9 mpg city? i think my worst was 12 and that was a full day of WOT pulls and my o2s not pluged in. i usually average 18 in the city if i dont get on it too much. and 24 highway. u sure uou dont have a fuel leek somewhere??? i would start there and if you are leaking gas then that would explane 9mpg.

aside of that basic tune up stuff or leaking injectors is a possibility.

changing the Fuel filter.If you use a pair of Neddled nosed Type vise grips,and wrap the Grooved part up with a bit of electrical tape so it doesn't Cut through the Gas line,Take the Vise grips and Clamp it BEFORE the Filter on the rubberized line on the bottom.That should cut Fuel flow Fairly nicely as to Not get Fuel all over you...I changed one out and ..Well,let's just say that I DID NOT have a Cigarette for awhile afterwards.(spilled fuel and Smelled like the EXXON Valdez!).If you know someone with a FULL TOOL BOX(like every tool made to man) there are Hose "crimpers" that will squeeze the Line Enough to Cut fuel,without Damaging the Line.

things that gave me better fuel mileage:

- making sure car is completely maintained (proper timing, TPS adjustment, etc.)
- new exhaust system (RB header, Magnaflow mufflers, no cat)
- SAFC tuned with wideband O2 sensor

as NZ said, the biggest effect on fuel consumption i have noticed is how you are driving. sometimes a day of hard driving i will only get 400km a tank. but on the highway cruising i can easily get 650km to a tank. even little things like driving technique make a difference.

the second biggest effect is how well your car is maintained and the condition of various internal and external engine components. for example, an engine with low compression and bad injectors will get much worse mileage than one with great compression and injectors.