As the title states, I'm having some gas mileage issues,
i know these cars aren't the most fuel efficient out there but i know i should be getting better than 10 mpg...
Ive recently replaced all my intake gaskets, rebuilt my injectors, and have redone all my oil injection lines.
Im at a loss of what could be causing this....

forgot to ad that the gas mileage was the same before i did the aforementioned work

 

Damn! That sucks. Do you still have cats on your car? Im getting about 26 mpg's on the freeway if I stay bellow 80. Hope you figure it out!

 

Do you have any noticeable loss in power?

Did this all the sudden happen or did it suddenly start gulping down the gas?

 

maybe its your fuel and air ratio. im having the same problem. actually trying to look into that. ill let you know how it goes.

do you have a high smell of gas ?

 

Its been getting this kind of gas mileage since i bought it, in September of last year.
All the cats are still there, no power loss, no gas smell but there is strong smell
of exhaust fumes when i do a pull up to redline with the sunroof open.
Im debating pulling my cats off for a while and seeing if that helps but i dont
want the exhaust to be loud and cant afford a presilencer right now, saving for a turbo swap.
Ive been shifting at 3000 all the time and hardly ever go above that on the highway, so i
know i should be getting better mileage.
Engine is completely stock, coolant seals confirmed good, im out of ideas aha

 

could be your water thermo sensor. if it's so rich it burns your eyes when you stick your head under the hood, you can try leaning out the screw on the tb.

 

17/25 is typical. Mine drops down to at least 15/22 with "enthusiasm". Your car is broken. Possible causes include driving under 30 mph all the time, broken O2 sensor (worse than a disconnected O2 sensor btw), running way too rich, poor compression, leaky or otherwise poor fuel injectors. Strong exhaust smell sounds like running too rich or the injectors. Check fuel to air ratio as said. Probably destroying your cat too. No way you'd pass smog with that much fuel in the exhaust anyway.

 

I pulled my o2 sensor and the buildup looks as if im running lean, whiteish deposits.
Im putting in a new o2 sensor right now, I dont think the injectors leak, i can hold fuel pressure and i havent noticed any symptoms of low compression.
What would cause a rich mixture?
Would an o2 sensor really make that big of a difference?

 

The o2 sensor usually only adds 2 mpg and only while cruising at a constant throttle. It takes a really broken one to screw you, so no, it's not likely to be the o2 sensor. You could even try disconnecting it until you figure out your problem. Ya, sounds like you're running rich or otherwise getting too much fuel. Dunno why, ask one of the pros here :-P. Checking your fuel to air ratio would confirm.

 

Would a bad AFM cause bad mileage?

 

well o2 sensor changed nothing... Would plugged up cats cause this?

 

ive got 11-12mpg since i bough my n/a cats were removed and ive got a strait pipe and it smells terrible. normal? or does that mean something to me

 

Could be a faulty/unplugged/poorly grounded map sensor causing the car to go into open loop which would kill mileage. Open loop is a preset MAP in the ECU that the car runs on until it warms up to a certain degree. My AFR is around 12-13 (very rich for cruising) in open loop and 15-16 (a little lean) in closed loop.

With plugged cats you would notice a power loss if it was killing your mileage like that. Im guessing the car is running in open loop for some reason.

 

that sounds plausible, im just wondering after looking over the FSM, if im supposed to have anything beside the MAP sensor, in the book it shows a Vacuum switch or something like that, round in shape and connected to the same vac line as the MAP.
Is this a required sensor? aswell as that i noticed a single green wire with a black connector that didnt go to anything up above the water pump.

 

how many miles are on the engine? compression loss or not an older engine gets less mpg, also i believe your just having some air/fuel ratio problems check and clean us MAF, check for any small vac leaks and also, do u know if it is running an aftermarket fuel pump?

my 88 n/a had stalling problems everytime i went to drift it with below half a tank of gas, the engine was 100% strong running, i replaced my fuel pump and my mpg went up along with the stalling issue being releaved. idk maybe something to look into

 

I not sure what pump its running, ive been meaning to pull it and check. Engine has 124 k miles but may have been rebuilt im not sure. What is the best method for cleaning a MAF?

 

It's not a MAF sensor, it's an AFM, or airflow meter. It's only a flapper with a rheostat, so there's effectively nothing in there that will screw with your mileage if it gets dirty. The rheostat can go bad, and if it's giving faulty signals, then it should be replaced anyways. But these AFM's aren't like today's heated wire MAF's that can get grease, dirt, oil, etc. on them and make the engine run all funky, then a can of MAF cleaner later it's fixed. Not the same at all.

Anyways, things to look for:
Leaky injectors- The best way to check this is to remove them and send them off to get cleaned and flow matched.
FPR- Get a pressure gage and check fuel pressure with the engine running. Check the FSM for pressure tolerances, and verify that the pressure increases when you remove the vacuum from the FPR.
Exhaust leaks- Not as severe, but if air's getting sucked in around the O2 sensor, then it'll dump more fuel to "compensate"
Coolant temp sensor- DEFINITELY a known repeat offender. I had a severe problem in this area, but it wasn't the sensor itself, the wiring was bad.
Thermostat- If the car doesn't warm up all the way, well, it'll never get the fuel economy you'd expect.
Spark plugs/wires- Fouled plugs or old wires can hurt economy pretty bad. Change the plugs/wires if they're old. If the spark plugs are fairly new and getting fouled by the extra gas, clean them.
Catalytic converter- It's been discussed already, but you'd notice the car acting all weird, like not accelerating or getting above about 35mph.
Variable resistor- Oh yeah, sometimes a "tuner" will drill out the epoxy out of these things so they can "tune" the ecu to get more fuel to make more power. Wrong answer, ********. Not saying you did that, but if you bought it from someone, or it's been worked on by someone else, it's worth taking a look at.
Initial Set Coupler- If it's jumpered because the ignition was timed, then the car will supposedly stay in open loop.

There's plenty more, but that's a good start for now.

 

do you mean the variable resistor near the passenger side shock tower?
I thought that only affected mixture at idle? if not thats probably whats wrong, i have to have it cranked full rich to get a smooth idle, if it isnt itll misfire.

 

Yes the resistor only affects mixture at idle.

 

Would the coolant temp sensor cause the idle to not jump up to 3000 on a cold start if it where not working?
Because that is something that hasnt been working on my car.

 

sounds more like a bad thermo valve to me

 

10 mpg is awful. You should see 20 mpg or more.

Some quick checks you need to do:
1. Check for trouble codes. You can make a code checker for about $3; search this forum or see the writeup at teamfc3s.org
2. Check for closed loop operation. It takes only 1 LED to do this, so making that checker is about $1.50. Search this forum for my posts on 'green lamp test'.
3. Adjust idle speed, idle mixture, idle speed (again) per FSM
4. Check timing and advance per FSM procedure
5. Check for clogged cats: if your mixture is or has been running too rich (i.e. no closed loop operation), you may have cooked the cats, in which case they can be restricted and blocking your exhaust, robbing you of power. Quick way to check is to remove the O2 sensor and leave the hole open and go drive and compare the performance. If the power/gas mileage improves significantly, have the backpressure checked with a guage. If it is high, there is an exhaust restriction, probably clogged cat(s).

 

I made a a code checker but it will not work, i dont get the three seconds light at startup nor do i get anything. I think i know why tho, i built my test lights with 12 volt incandescent and after looking into it it seems that the ecu limits amperage to 30ma on those connectors , so ill go get some led's tomorrow.

 

OK, you want to use LEDs. You can either buy 12volt LEDs, or buy regular LEDs and add a current limiting resistor in series. I find they work with anything from 500 ohms to 1500 ohms in series. The higher resistances reduce the current load on the circuit powering the LED. Also, unlike the incandescents, the LEDs only work in one direction. Ususally, the longer lead goes to the + terminal. Once you build the tester, check its operation on the terminals of your battery to verify the proper polarity.

 

yeh im going to pick up some led's tomorrow