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

 

[JwUoNoG]

This question doesn't necessarily apply to DSMs, but just manual transmission cars in general.

Is gas mileage worse when you're going WOT in the highest gear but at a low RPM?
Or is it worse when you're giving partial throttle in a lower gear at a high RPM?

 

[saw wave analog]

well wot at a low rpm only exists for about a second.. wot means the rpm needle is going to go up and quickly, so my vote goes there.

 

[Defiant]

Engine load will make the difference. If you're cruising down the highway at a constant speed, you want as little throttle opening as possible, and the highest vacuum. The engine needs to make about 15hp to hold 60mph.

If you're climbing, a higher RPM with a lower throttle opening will be more efficient than a low RPM with a more open throttle. Downhill, you want as high a gear as possible.

 

[jasoncitrano]

What could be wrong with my car here! I WENT THROUGH A FULL TANK IN ABOUT 3 DAYS. normal driving not even beating on it. i get about 105 miles per full tank. list of mods are as follows for a 1991 eclipse gsx automatic on 91 octane premium

evo III ported 16g with bigger flapper door
evo III exhaust housing
evo III o2 housing
stock exhaust
ams 680cc injectors
ams mbc
apex'i safc II
walbro 255lph fuel pump
and yes it's automatic!
have an etg hooked up and it's reading a little above 700 degrees celcius.
i'm clueless now...thought it could possible be the fuel pressure, but i was told the stock can handle up to about 500 horse and i'm no where near that.

Question is, what the hell is going on with the gas mileage!! i can't get it to give me more then 105 miles per tank one of you guys have had to have had this problems before, and i already changed out the o2 sensor!! please help
-jason

 

[Eastwood95]

Hey man, you might have a boost leak somewhere........that will cause your dsm to slurp up some hella fuel. it will also cause a lack of power, that is something you should definitely check out.

 

[Shifty]

Have you tuned your car? I'm guessing with the s-afc you have set that completely up?

Also checked for fuel leaks, just throw a match under your car and run. Then you'll know if any leaks

700 degrees celsius? Convert that to Farenheit and then we'll really know the culprit.

 

[jasoncitrano]

afc is somewhat tuned not 100% though, no boost leaks and no fuel leaks

 

[DSM90AWD]

I thought you were going to get a datalogger and/or contact AMS
http://www.dsmtuners.com/forums/showthread.php?t=148478

Yes the stock fuel rail/FPR can support 500HP... but if you are injecting 500HP worth of fuel via those 75% larger than stock injectors and only 250HP worth of air.. you are going to run rich as hell and get crappy gas milage.

 

[Shifty]

You had it tuned after the 680cc injectors?

 

[jasoncitrano]

Yeah, i called ams, they said, yeah it IS possible to run the 680's on an automatic ecu but the fuel would have to be cut almost 50% which is alot and could throw the timing map outta wack, so they said they can wideband street tune it for 150.00 which i think is a huge rip off but no body else can do it, i had my buddy doing it for a while, and i think he only fixed WOT and not part throttle, so that could aslo be why, becuase i do alot of cruizing. Anyways i'm not doing anything until i get some more responses, let me know what you guys think. also , i'm holding off on the data logger until my friend is selling his, soon.
-jason

 

[DSM90AWD]

I'll bet your Low-Trims are at or near 60% (i.e. lower limit of the ECU's fuel compensation based on 02 feedback).

Take a look at RRE's AFC Primer http://www.roadraceengineering.com/newafc.htm

IMHO.. $150 for someone that has all the tools and the knowhow to get your car running right may be worth it (especially when Premium gas costs ~$2.20/gal )

 

[jasoncitrano]

your exactly right, i mean it may just be worth it to get the damn thing running top shape ya know...i mean yeah it hurts me to spend more money to get the ####in thing tuned espiecaly when i spent 2000 on parts but what the hell can't do everything

 

[DSM90AWD]

Put it this way.. I guarantee spending $150 for professional tuning will get you 20+WHP from where you are now.

Not many bolt-ons that would give you more bank-per-buck

 

[leet]

What's your fuel pressure at?

 

[jott5555]

you need a afpr, the large injectors and 255 are over running the stock fpr, time to get a adjustable one.

 

[jasoncitrano]

actually that wasn't the problem, the problem was my injectors are way to big for my setup but we got them to work what we had to do was adjust the afc so that it was correcting 42% of the fuel almost half...then we had to retard the timing about a degree to make up for all the adjustments the ecu was doing...anywho she's running better now and were even boosting to 15 now!! yey!!! anyways a afpr would'nt hurt! hey by the way what does the "A" stand for in afpr??

 

[jott5555]

"ADJUSTABLE" fpr. hence afpr

 

[DSM90AWD]

actually that wasn't the problem, the problem was my injectors are way to big for my setup but we got them to work what we had to do was adjust the afc so that it was correcting 42% of the fuel almost half...then we had to retard the timing about a degree to make up for all the adjustments the ecu was doing...

Nice! Good to see you've got it tuned.

As I mentioned above.. the DSMChips EPROM is the way to go. Jeff can whip you up one that will correct your fuel at the ECU level (not the S-AFC).. so you would'nt have to mess with base timing.

I've got one calibrated for my 2G MAS and 550s and my S-AFC HI settings are at 0

 

[leet]

You're running a 255 fuel pump, go get an adjustable FPR and get it set correctly.

 

[mattspyder]

I filled my tank up the other day and set my tach back to zero. i used up a half tank but have only gone 100 miles. wtf is up with that shit. i noticed it does smell like its running rich. i also noticed when i step on the gas i hear a very loud pshhhhhhhhhh like all the pressure is just leaking out. my blow off valve is stripped on the adjuster so i dont know if that would cause it to lose power or what?? help me out guys......

 

[98eclipseRS]

well these cars do run rich from the factory, and second you own a dsm don't expect good gas milleage just cause its a four banger doen's mean shit. Man good gas milleage haven't experienced that in quite some time.

 

[mattspyder]

but come on man 100 miles for half a tank. i think thats alittle un heard of????

 

[Grimis]

something is up. i get about 280-305 per tank, depending on the driving. Now if your on it all the time etc, yeah your going to burn it up quick. Have you filled if from 1/2 to see if your gauge is off?? Mine is off 1/8th tank, but still 100miles in 1/2 not good.. just my .02

 

[MattnGsx]

Yeah it is common to go 200 miles a tank! Is this normal driving? Wait whats your normal driving?

 

[Grimis]

well, my NORMAL driving intown i dont build boost, and I dont get on it all the time. My avg is about 280 a tank, my highest was 305, and that was i think from out of state gas, so i dont know. but it is diffrent from the way you drive and where your at...