318is Fuel Mileage Problem

[NickMav]

Hello, I have a 91 318is M42 with 146,500 miles a supersprint catback exhaust and a 3.73 lsd. I averaged my mpg to around 16ish mpg. This should be way higher shouldnt it? Im going around 260 miles on a 16 gallon tank. I dont even really beat on it, I daily it to school and work. Im always shifting under 3k too and occasionally ill get on it to pass someone sometimes. Its not like im bounching off rev limiter and dumping the clutch either ill just take it up to around 6k no more. My car even smells rich sometimes so I know its getting to much fuel. I recently replaced the fuel filter, thermostat and coolant flush. Air filter is clean. Didnt have a chance to pull the plugs but was going to do that soon and most likely change 02s. Any ideas that I could be missing on why my mpg is so poor? Thanks guys

[Dj Buttchug]

change o2 and check for vac leaks. I can get well over 400 miles to the tank on the freeway, also the tank is 12.6 gallons pretty sure

[e30Chris]

Get the plugs changed (avoid Bosch Platinum at all costs, NGK worked great for me) and change O2 sensor. Is it getting up to operating temp with the new thermostat? How's it idle and accelerate? Could be a vac leak too. And did you swap in a 16 gallon tank? The stock tank is 14.5.

[gearheadE30]

Stock tank on 318is's is 12.6 gallons. 14.5 was for some of the 6-cylinder cars. I typically get between 28 and 32 mpg.

[NickMav]

Mitchell1 Says its 16.4 gallons or 62.1 liters. I was going to use the ngk spark plugs anyway. With new thermostat it is running right at the halfway mark before it would never even go past the quarter mark so i fixed that. Im going to change the 02 when I do the spark plugs and see what happens. Thier may be a vac leak to because under the intake where that mess is the p.o tapped up some hoses.

[NickMav]

I just checked realoem and put in my vin and it said i have a 55 liter tank or a 14.5 gallon tank. Im confused on what i have no im pretty sure the p.o didnt change the tank but my fuel gauge does read as f is full and 1/2 would be empty. I think its the sending unit that is causing that. I probably have the 14.5 gallon tank but thats still only like 17mpg

[eur04lif3]

Mitchell1 Says its 16.4 gallons or 62.1 liters. I was going to use the ngk spark plugs anyway. With new thermostat it is running right at the halfway mark before it would never even go past the quarter mark so i fixed that. Im going to change the 02 when I do the spark plugs and see what happens. Thier may be a vac leak to because under the intake where that mess is the p.o tapped up some hoses.

Ya, that's most likely your issue.

I get between 28-31, depending on whether i just cruise or sprint through corners. :up:
Worst i ever got was 27.8 and that was mostly city and old sparks and O2 sensor.

[eur04lif3]

I just checked realoem and put in my vin and it said i have a 55 liter tank or a 14.5 gallon tank. Im confused on what i have no im pretty sure the p.o didnt change the tank but my fuel gauge does read as f is full and 1/2 would be empty. I think its the sending unit that is causing that. I probably have the 14.5 gallon tank but thats still only like 17mpg

Wait, how are you getting your MPGs?

You should be doing the old-school way, every fill up check mileage and gallons filled up.

[NickMav]

Wait, how are you getting your MPGs?

You should be doing the old-school way, every fill up check mileage and gallons filled up.

Thats what I did, then just divided how many miles by 14.5 gallons its around 250ish thats when my light comes on. Ill guess ill be doing the mess under the intake mod. Does anyone know how much this will cost? Can i just but hose from parts store i havrnt really checked to see if theres any weird hoses but i did go under there when i got home and moved the one hose that goes to the airbox and my engine would idle diff. at idle its around 800 but sounds like theres misfires randomly not all the time and acceleration is good but atound corners sometimes the car wants to cut off. Thought it could be sending unit since gauage is messed up too but maybe the engine is moving and that vac hose is loosining up?

[Vivek]

change o2 and check for vac leaks. I can get well over 400 miles to the tank on the freeway, also the tank is 12.6 gallons pretty sure

My tank is I believe 16 gallons because I've filled 14.5 many times soon after the light comes on and almost 15 once. Maybe it's different on a vert?

Thats what I did, then just divided how many miles by 14.5 gallons its around 250ish thats when my light comes on. Ill guess ill be doing the mess under the intake mod. Does anyone know how much this will cost? Can i just but hose from parts store i havrnt really checked to see if theres any weird hoses but i did go under there when i got home and moved the one hose that goes to the airbox and my engine would idle diff. at idle its around 800 but sounds like theres misfires randomly not all the time and acceleration is good but atound corners sometimes the car wants to cut off. Thought it could be sending unit since gauage is messed up too but maybe the engine is moving and that vac hose is loosining up?

There's an excellent thread on this here (Simon's) and another okay one (mine). It doesn't cost much at all, off the top of my head I think everything important can be done with parts locally. I opted to not replace some of the coolant things that Simon suggested (like the plastic hose) because I don't mind too much if I have to take it apart and fix it.

[irish44j]

Early cars had smaller tanks too. My '85 318 is the 12.6 gallon tank, IIRC.

[Dj Buttchug]

interesting because I cant ever get more than 12 gallons in my car. My 318i was a late model 1991

[gearheadE30]

For whatever it's worth, mine is a march of 1990 car, so pretty early, and it's got the 12.6 gallon tank...the smallest a 6-cyl car ever got was the 14.5, but I'm not sure which got the biggest tank.

[TimKninja]

My 318is had a 11 gallon tank.

My 325is has a 14 gallon tank.

[jlevie]

If you check the IPB (realoem.com or similar) I believe you will find that all US 318 cars had a 55L tank. All 325 cars had the same tank up to 8/87, when the tank was changed to a 63L type. The easy way to tell difference is that the 55L tank has a single opening on the right side for the pump/sensor and the 63L tank has an opening on both sides. I believe that some non-US 318 cars had tank smaller than 55L.

The only valid method of obtaining fuel mileage is to record the mileage (or zero the trip meter), fill the tank, drive the car until it needs fuel, note the amount of fuel needed to fill the tank, and divide the that by the miles driven. You will get better numbers by recording starting mileage with a full tank, recording fuel added for each of several refills, compute the total fuel usage and divide by total mileage.