Bad Gas Mileage [Merged 9-6]

[99GST]

I am still getting some pretty bad gas mileage.

I am only getting about 19.7 mpg with mostly city driving shifting at 4k.
I have already replaced my front O2 sensor and am looking to replace the fuel filter.

I have a 99gst with about 55k miles on it. Should my fuel filter already be bad?

What else should I check or change? I have NKG plugs and Accel wires are on the way.

Thanks,
Nathan

 

[disillusions]

Get and a/f gauge when you get your boost gauge, this will tell you what your a/f ratio is. If your mileage is that bad you are running rich, also check your ignition timing to see if it is with in spec.
And I agree a bad 02 can cause poor mileage.

I was thinking about doing just that actually, once the tax return comes in of course ^_^

disillusions - Fill out your vehicle profile so that we can see what mods your car has. It's helps greatly with threads like this.


Any fuel mods? What are you tuning with?

The only mods I have done are what I listed in my initial post. 1G BOV and poly bushings, thats all. I haven't tuned with anything since I never got to do anything major to the car, the BOV was a replacement since the stock 2G plastic POS snapped on the vacuum port.

 

[jwahl16]

I had the same situation when i first got my car (also a 95 gsx a/t). I replaced a number of things and went from ~10 mpg - 18 mpg or so. Things i replaced:

-front o2 sensor
-dirty air filter
-fuel filter
-vaccuum lines (do a blt)
-exhaust/intake pipes (the less restrictions the better mpg)

If i remember the o2 sensor and the exhaust helped out the most (about 2-3 mpg). All of those things are good maintenance things to do anyways.

 

[Tyeler18]

Driving style is a huge factor in MPG. Revving it up to 3-4k I could understand why you're not getting that great gas mileage. My old 2g Talon I'd hit near full boost at 1/2 throttle by 2500-3000. That's a lot of wasted fuel when you're just driving somewhere. Try shifting at a max of 2500 and see how much that WILL increase MPG.

Another thing is just general maintanence. What kind of plugs are in it? When was the fuel filter changed last? Airfilter good? Any leaks vacuum or boost? O2 sensors may be failing, but haven't thrown a code yet. When was the last oil change and what oil are you using?

 

[disillusions]

I had the same situation when i first got my car (also a 95 gsx a/t). I replaced a number of things and went from ~10 mpg - 18 mpg or so. Things i replaced:

-front o2 sensor
-dirty air filter
-fuel filter
-vaccuum lines (do a blt)
-exhaust/intake pipes (the less restrictions the better mpg)

If i remember the o2 sensor and the exhaust helped out the most (about 2-3 mpg). All of those things are good maintenance things to do anyways.

The exhaust and intake I was waiting a bit to do since I wasn't sure if I wanted to do a 6-bolt swap or not. But I'll def try that little list of fixes. Last I checked though my air filter was changed recently, and the fuel filter was during the tune up.

Driving style is a huge factor in MPG. Revving it up to 3-4k I could understand why you're not getting that great gas mileage. My old 2g Talon I'd hit near full boost at 1/2 throttle by 2500-3000. That's a lot of wasted fuel when you're just driving somewhere. Try shifting at a max of 2500 and see how much that WILL increase MPG.

Another thing is just general maintanence. What kind of plugs are in it? When was the fuel filter changed last? Airfilter good? Any leaks vacuum or boost? O2 sensors may be failing, but haven't thrown a code yet. When was the last oil change and what oil are you using?

I'm aware I can get heavy on the gas, but accelerating at 3-4k on the highway really isn't all that abusing. Trust me when I say its nowhere near even half throttle. During city driving its likely around 2k if that. Also remember, I have an A/T so I cant shift at 2500.

NGK plugs were put in, fuel filter was at the last tune up, so lets say a year ago (it hasn't really been driven since), air filter was done recently (no more than 6 months ago), no leaks that I'm aware of (had a boost leak test done). Last oil change will be within the next few weeks, . As I said earlier, it didn't get driven much in the last year..and thats mostly due to the fact that it had been in a shop for more than half of that time....but that nightmare is another story all together.

 

[cookmeup272]

Boost leak test and o2 sensor will probably be the biggest things that will affect your mpg.

 

[disillusions]

Boost leak test and o2 sensor will probably be the biggest things that will affect your mpg.

Had a feeling. Now to get this damned sensor replaced -_-

 

[Absit]

I have the same driving style and I got 25-26 MPG. What fuel are you using, what's your coolant temp when you're driving, and does your front O2 bounce or stay steady (I see you're replacing it, but check anyway, if it is bouncing properly you're just wasting money).

 

[JStrateger]

This doesn't really help anything, just kind of contributes but after I replaced my o2 sensors, (both) I filled my tank up and ran it nearly dry, divided the amount of gas I put in it by miles I drove and got about 30-31mpg.

I have a baby TD05HL turbo on mine though.

 

[Voodoo262]

Like Absit said, you should also check your coolant temp sensor. Is your temp gauge going all the way to the middle once the car is fully warm? If not, your CLT sensor could be bad, causing your ECU to get stuck in open loop mode

 

[TurboATM]

another thing that affects mpg is the cold...dont know if you have taken that into consideration or not.

 

[disillusions]

I have the same driving style and I got 25-26 MPG. What fuel are you using, what's your coolant temp when you're driving, and does your front O2 bounce or stay steady (I see you're replacing it, but check anyway, if it is bouncing properly you're just wasting money).

I should probably amend my driving style. On my way home today I noticed its more like accelerating 2-3k, and 4k for very brief periods, though 4k isn't even quite half throttle yet. My coolant temp is usually midway, but does raise a notch or two then subside (I've heard this is normal of boosted eclipses). Not to sound naive, but what do you mean by bounce?

This doesn't really help anything, just kind of contributes but after I replaced my o2 sensors, (both) I filled my tank up and ran it nearly dry, divided the amount of gas I put in it by miles I drove and got about 30-31mpg.

I have a baby TD05HL turbo on mine though.

How much smaller is that turbo than the stocker? But it is sounding more and more like I need some o2 sensors in my life .

Like Absit said, you should also check your coolant temp sensor. Is your temp gauge going all the way to the middle once the car is fully warm? If not, your CLT sensor could be bad, causing your ECU to get stuck in open loop mode

As I said to Absit, it does go to the middle once warm; though it also raises then lowers during driving.

another thing that affects mpg is the cold...dont know if you have taken that into consideration or not.

I did, but this kind of gas mileage isn't anything new. I just recently started to be able to really put attention into fixing this POS.

 

[Absit]

A normally operating narrowband O2 sensor running around stoich should bounce back and forth between ~.2V and ~.8V, constantly. If yours isn't doing this it's likely bad. And regarding coolant temp, we'll need actual numbers, the gauge isn't accurate enough. See if you're below or above certain limits your ECU skips the O2 sensor, working or not, and just dumps what it was programmed to dump.

Your driving style and the weather aren't causing this, it's likely either your front O2 sensor or your coolant temp (sensor or actual temp, hard to say). I do know your temp shouldn't move around much, if at all. This whole problem could be solved by a simple log at operating temp.

 

[disillusions]

A normally operating narrowband O2 sensor running around stoich should bounce back and forth between ~.2V and ~.8V, constantly. If yours isn't doing this it's likely bad. And regarding coolant temp, we'll need actual numbers, the gauge isn't accurate enough. See if you're below or above certain limits your ECU skips the O2 sensor, working or not, and just dumps what it was programmed to dump.

Your driving style and the weather aren't causing this, it's likely either your front O2 sensor or your coolant temp (sensor or actual temp, hard to say). I do know your temp shouldn't move around much, if at all. This whole problem could be solved by a simple log at operating temp.

Oh okay, bouncing is now understood. A coolant temp gauge is on the list of things to get, though maybe my shop can do a log of it for me..I just don't know how that'd work since it only does that rise and fall deal under load.

 

[sirslex]

Like Absit said, you should also check your coolant temp sensor. Is your temp gauge going all the way to the middle once the car is fully warm? If not, your CLT sensor could be bad, causing your ECU to get stuck in open loop mode

hey! I seem to be having the same sort of problems.. 10-15 mpg and I've noticed the temp gauge never really goes up to the middle. close, but usually more like a third of the way up.

so call me stupid but what is CLT sensor?

(and I guess my next thing to invest in is a pocketlogger? I have a boost gauge on order, and further down the road I expect to get a wideband too)

 

[iguanahotsauce]

So I have been reading through quite a few of these threads to figure out why my gas milage is so bad. I have a 92 Talon Tsi and I am getting about 14mpg and I have been making sure not to have a lead foot I received this car for free about three months ago. The only catch was that it had been sitting in the previous owners front lawn for about six years. But anyways I finally got it running after changing the fuel pump, fuel filter, spark plugs, wheel bearing, brakes, oil, and accessory belts. From what I have gathered from the other posts is that I could have a bad o2 sensor, bad pcv valve, or a bad clt module. I am very new at working on cars, which is why I am posting in the newbie form, but today I am going to pick up a new air filter, some fuel injector cleaner, and possibly plug wires. I was told by a family friend who is a mechanic that the plug wires look fine but how do you tell if the plug wires are still good?. What I am wondering is can I test my o2 sensor with a cheap multimeter? From everything I have read it says I need a high impedance multimeter but I don't know what that is. Also how do I tell if my CLT module is bad? My temp gauge hasn't been getting to the middle. It will get about half way between the middle and the C mark. I drive about 20 - 25 miles at a time ( 85% Highway). But it is also pretty cold over here so I have a feeling that could be effecting it. One other question I have is that today I noticed when I had the cruise control on when I was on my way to class it was at about 3k rpm steady at 65 mph and I was wondering if that was normal or if it should be shifting up one more gear.

 

[Tyeler18]

Have you flushed the coolant? Your temp not raising all the way is either from air in the coolant lines, or a faulty sensor/gauge. If the ecu doesn't see the car getting up to operating temp it will stay in open loop which will cause bad gas mileage.

You can pull the pcv and test it by blowing in both sides. Only one side should allow you to blow through it.

O2 sensor can be tested with a datalogger which can be found for relatively cheap on here or ebay etc.

What fuel are you running? Have you done a boost leak test or a compression test? Both can cause poor gas mileage. Any exhaust leaks? And I would shift up a gear, might as well keep your cruising RPM's down.

Looks like you've got a great start!

 

[iguanahotsauce]

I havve not flushed the coolant. I am not exactly sure how to do that . Is it somethiing I could do in my driveway? I haven't gotten to a boost leak test or compression test yet either. I used pretty much all of the 4 I had available for this car just getting it to run. But a boost leak gauge and a compression gauge are at the top of my list to get next. I am getting about 19 in hg ad idle so I don't think I am leaking but like I said I'm new to this Also is there any other way to test the o2 sensor without a data logger? I don't have the $ to get one now and I am trying to do what I can as I am driving from my house in Vancouver, Washington to Couer d'Alene, Idaho on the 16th of march and if I don't find a way to improve the gas milage it's going to cost me a fortune in gas I'm running 92 octane pump gas. Made sure when I started driving it to fill it with the highest octane I can get around here. I was wondering about the cruising rpm's on the freeway becasue my car is an automatic not a 5 speed so I wasn't sure if it should be shifting up one more gear or if 3k rpm is normal for 65 mph

 

[Tyeler18]

Coolant flush you can do in the driveway, search around on here and there should be a writeup in the tech section on how to do it. You just need a garden hose and an adapter (~$5 at walmart) to hook the garden hose up into the cooling system. It'll take about 30-60 minutes of your time to do.

19in/hg is about right, but doesn't mean you don't have a boost leak. Do you have overdrive on? I wasn't sure if you were auto or manual and owning all manual dsms I don't know where the auto's cruise at.

 

[iguanahotsauce]

Yeah I leave the overdrive on all of the time. Which that was another thing I was wondering about was is it bad to leave the od on all of the time? I will search the forums and figure out how to do the coolant flush. I'm not sure if you saw in my last post but is there another way to test the o2 sensor? So should I invest in a boost leak gauge first or a compression gauge first? Also how much coolant does a 1g dam hold?

 

[wranglerunner22]

If none of the above suggestions help you out, Check the ECU for leaky caps, I would replace the ECU with a known good one. Since you have a 1g one symptom of a ecu going bad is horrible gas mileage.

 

[iguanahotsauce]

Ok I will read up on how to check the ecu and check it tonight or tomorrow. How do I tell if my plug wires need to be replaced?

 

[Tyeler18]

I'm not sure how else to test the o2 but you can build a boost leak tester for $10 and rent a compression tester for free.

Oh and it holds 2 gallons, so 1 gallon water and 1 gallon coolant. 1 big jug from walmart will suffice.

 

[thomcasey]

Also, did you happen to drain the gas tank? Gas sitting in the tank for 6 years turns to turpentine and will mess your tank up, and kill the gas added to it. Speaking from experience on this one.

 

[iguanahotsauce]

Ok I was just wondering how much it held because I want to make sure my catch pan will hold it all. Should I buy the pre mixed coolant from walmart or should I get 100% coolant and mix it with water?

Also, did you happen to drain the gas tank? Gas sitting in the tank for 6 years turns to turpentine and will mess your tank up, and kill the gas added to it. Speaking from experience on this one.

Yep I made sure to drain the tank. My first fill of the tank I only got about 80 miles but I figured it was a combo of bad gas and letting it run a lot while I was working on it. But now I'm on my second full tank and I'm at half a tank and I have gone 100 miles

 

[Tyeler18]

Try running some fuel system cleaner, that should help.

Also NEVER buy the mixed stuff. You're paying $5 for water. When you do a coolant flush it HAS to be straight coolant because when you flush it out, some water is still left in the block.