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

 

[spyderdrifter]

I've been assuming it's running rich for a long time. However my spark plug electrodes are white and indicate a lean fuel mix. The PCV is new, but I did buy it from Advance Auto instead of getting an OEM unit like usual.

 

[91-GS]

Does the exhaust smell like fuel? What air/fuel ratio does the gauge show?

 

[spyderdrifter]

I don't have an AFR gauge so I don't know how rich or lean it is. The exhaust smoke does have a faint fuel smell at times.

 

[91-GS]

No vacuum leaks anywhere, right?

 

[spyderdrifter]

Right.

 

[Deadspool]

Thats worse gas milage than my jeep with 33's and stock gearing.


If your electrodes are white, your burning lean, if they are tan, your burning a good mix, black, you get the idea.

Where yours are white, that would suggest all that extra fuel is either not making it into the chamber, or you are matching the extra fuel with air. meaning, the fun pedal spends a great deal of time depressed.

The other option is, have you felt any decrease in power, or anything else out of the ordinary?

A fouled cat can cause power loss and excess fuel consumption.

 

[spyderdrifter]

I was expecting my plugs to be black due to the symptoms. I have no cat anymore, just a full Apexi exhaust. However this fuel problem existed before I pulled the old exhaust off. I had guessed that cat was bad. I replaced my fuel filter a couple months ago as well. I really haven't noticed any power drops either. Replaced my piston rings and upstream o2 sensor 1500 miles ago. Thought the o2 would fix this but was wrong.

 

[Deadspool]

Sounds to me that either the fuel isnt getting where its supposed to be, White plugs, or youre into the boost.

For the engine to burn that much fuel on a factory turbo at factory boost (suspect this is what you have, and not a 6762), Black plugs hands down.

There is no place else the fuel can go other than the ground. Try running a tank with the wastegate arm off the flapper.

 

[spyderdrifter]

Why run the wastegate like that? Isn't that bad for the turbo? The turbo is a big T28 (almost stock) and I'm only running factory boost and my gauge reads 12psi. I do get on the throttle here and there, but not enough to drain my fuel so fast. This is my 3rd turbo DSM and have had more upgrades on those than this one with way better fuel mileage. And that's with the same driving habits.

 

[Deadspool]

Why run the wastegate like that? Isn't that bad for the turbo? The turbo is a big T28 (almost stock) and I'm only running factory boost and my gauge reads 12psi. I do get on the throttle here and there, but not enough to drain my fuel so fast. This is my 3rd turbo DSM and have had more upgrades on those than this one with way better fuel mileage. And that's with the same driving habits.

If you run with the arm off, the flapper will be open, and the turbo will not spool. It will tell you if your fuel issues are in, or out of boost.

Its not bad for the turbo.

 

[spyderdrifter]

Ok, if I do that, how do I tell if it's in or out of boost?

 

[spyderdrifter]

Guess I'll give it a shot. Now if it does go away, what would be causing it from the turbo? I had this turbo rebuilt earlier this year by Justin (from here) and it hasn't given me problems, that I'm aware of anyway.

 

[Deadspool]

A boost gauge.

If you take the wastegate arm off the flapper you will not go into boost, it will feel like a complete slug. This will keep you from entering any kind of positive pressure situation that your maf sensor will request to the ecu for more fuel.

run one day with, one day without, take a rough guage on how much gas you used between scenarios. If you use that much fuel NA, something magical is happening here.

 

[spyderdrifter]

Magical... ... always the right thing with car functions I'll give it a shot starting today. Can't say I'm fond of the no boost idea, I have driven this car with with the wastegate open on a different turbo and I hated the sluggish power. But for the sake of figuring this out, I'll deal with it.

 

[Deadspool]

Guess I'll give it a shot. Now if it does go away, what would be causing it from the turbo? I had this turbo rebuilt earlier this year by Justin (from here) and it hasn't given me problems, that I'm aware of anyway.

The turbo cant be the problem, it only feeds air to the motor.

There are only several things a turbo can do to negatively effect your engine performance.

1) Burn oil, most common, cause by a break or wear in the oil seal

2) Burn coolant, crack in the housing causing coolant to leak into the exhaust or intake stream.

3) High EGT's litterally burn away the turbine, not the turbo's fault, but it can happen, leading to a low or no boost scenario.

4) Eat something, a rock, or small childeren (pending size of the turbo and inlet). This will again effect the pressure development of the turbo.

3) Explode. bad day is bad.

At the end of the day, MAF see's incoming air, meters it, and tells the ECU whats up. ECU is all, cool bro, and calculates on a set table where the RPM is in relation to the engine load, using the o2 sensor as back up information. then proceeding to dump the perscribed gas into the chamber.

you have evidence of lean mixture for a majority of your driving, Two really good reasons to invest into a wideband, three when you think of it.

1) Turbo motor, unpredictable when silly little things like vacuum leaks happen. you have 0 idea where your AFR's are at WOT. It really is important information for anything outside of a factory tune / or modifications.

2) Youre lean, but you have shitty gas milage. Well thats just strange.

3) For you to fully enjoy the T28, you should tune it, really. Its not some preachy mass consensus thet "tuning iz gud" you'll pick up power, safety and have a much more in depth knowledge on how your motor is operating. To do this, you need a wideband, or access to a wideband during the tuning phase.

 

[spyderdrifter]

Getting DSMLink and a wideband are both on my list of mods to get done soon, but unfortunately life likes to dictate other things. Hopefully that'll change real soon.

 

[Deadspool]

Getting DSMLink and a wideband are both on my list of mods to get done soon, but unfortunately life likes to dictate other things. Hopefully that'll change real soon.

For sure, Wideband first, if you can find a good looking used one, saves you like 100-150 bucks initially, really, unless the controller got wet or trampled by a raging triceritops, the sensor would be the only thing in question. Which i jelously see the US have replacements for $50. ($90 in Canada).

Its a lot of money for someone who is broke, but it pales in comparrison to what a new turbo, or motor, or anything that can cause you downtime and $$ that a wideband and the information it feeds to you could have prevented.


Peace of mind VS peice of block.

 

[Ghost_92TSi]

No leaks. Even realigned it a couple weeks ago. The o2 does work. Have verified that with my scanner. I do have sporadic smoke from my exhaust that to me looks white, but others think its a very light blue. Not burning coolant, but my oil level has gone down some.

Oil does lower octane. So you could be running rich in order to compensate.

 

[DSM1G90]

Its not bad for the turbo

When a turbo blows, this is a godsend to keep you on the road by wire open the wastegate, which disables the turbo since it's allowing exh gasses to bypass the wheel.

Course, that Spyder is a bit of a pig and a half weighing in about 3200 LBS due to the extra metal used to beef up the side structures due to the disappearance of the top-which is the death for all ragtops: you take off that top, it's like a large cardboard box with the top cut off and the sides are suddenly super weak with nothing on top to hold it all together.

That's a lot of grunt for a 4cyl to get moving.

-DSM

 

[spyderdrifter]

That is a lot for the engine to do, and sadly I have experienced that with this very car. 0-40mph in 3 minutes... Was a sad day.

Anyway, I haven't yet had the chance to unhook the wastegate to test out the mileage without boost due to everything I've had going on. However, I did let my car go for scanning tonight to get a new set of eyes looking over all the diagnostic readings and nothing was found.... yet. I'm not surprised though as I had expected nothing to be found. No one there knows my car quite the way I do anyhow. I did talk to a friend about my transmission since it isn't the right one. My car is by VIN a real 5-spd GST. However, I have the F5M31 trans from a GS Spyder in it. This is how I bought the car. I know the gearing is different between the GST and GS, but could it be so off to mess up the fuel mileage? I know this is a long shot, but figured I'd search for answers anyway. I never had this fuel problem before my old 7 bolt bite the dust, so I'm assuming there's no real impact on mileage. But, I could be wrong.

 

[Tyeler18]

What were your fuel trim readings? That'll be your biggest key to what's going on right now.

What do you cruise at RPM wise? That and engine load are real keys as well, although I do believe you're correct in that something is physically wrong with the car and it isn't your driving style.

I've pulled 45mpg out of my spyder driving through the rockies at 75mph. Your altitude will effect it SOME, but even when I lived at 8000Ft in Wyoming, I still managed 25-27mpg in town, and 35+ on the highways.

 

[spyderdrifter]

What were your fuel trim readings? That'll be your biggest key to what's going on right now.

I do believe you're correct in that something is physically wrong with the car and it isn't your driving style.

I'm glad to see someone agreeing that it's not my driving style doing this to my car. Everyone I talk to about it here, thinks it is my driving manners. I know it's not, so thanks.

I don't remember what the fuel trim numbers were the last time I scanned my car, and no one wrote them down when I had it scanned the other night (that did me some good ). I'll plug my scanner in for the drive to school tonight and post them later as well. However, I decided to visually check how much fuel is in my fuel tank, and it seems to be more than my gauge indicates. So I may have a faulty level sender unit causing all this "poor fuel mileage". Here are a couple pics of the current fuel level vs the gauge reading. Anyone have experiences with this? Oh, and the miles on the trip ODO, are the miles since filling up full.

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so I went for a little drive and now my fuel gauge reads a little higher than when I was in the garage with everything pulled apart. I've been on level ground with each reading so I know it wasn't reading less due to steep parking or anything.

I also took pics of the scanner readings so I'll post those tonight. My phone won't upload pics to the forum just like my computer at home. Only got these last pics posted because of photobucket.

 

[Tyeler18]

How are you calculating your gas mileage? Filling up full, driving, then filling up full again and dividing gallons it took to fill into your trip mileage? If you're just basing it off of when the needle hits "empty" or around there that isn't accurate at all, especially if you've ever taken your needles off your cluster.

For example, when I did the LED's in my cluster I removed my needles. When my needle hits "empty" now I actually still have 5 gallons left in my tank. It hasn't bothered me enough yet to fix it, but it sure looks like shit gas mileage when I've only gone like 80 miles and have half a tank left.

 

[spyderdrifter]

I usually go by how many miles I get per tank. I have LEDs in my cluster too, but they're just replacement LED bulbs, never removed the needles to do it. Before I installed the current 6 bolt short block, I never had this issue. But had a 7 bolt back then.

 

[Tyeler18]

Definitely need to start calculating by taking the gallons filled/miles driven. The floats for the level get eventually saturate in gas and will begin to sink causing gauge issues. This sounds like it's quite possibly your problem