What's up with my gas mileage?

[bstarr3]

What are possible causes of low gas mileage on a 91 318i? Fixed all vacuum leaks/mess under intake, new air filter, new plugs. Idles and pulls smooth. Tire pressure OK. Last fill, got 260 miles/13 gal =20mpg. Probably about 60/40 city/hwy. 20 miles commuting per day. Stock 4.10 open diff.

Any ideas on where to search first and get my mileage better? O2 sensor? Wires? (plugs are new) Thanks.

[vpilarrt]

My 318is started getting bad mileage. New O2 sensor fixed it right up. Back to 28 city/34 hwy with a 3.73 LSD.

[///digitalme]

Afm good? I have a similar problem with mine. I have a new o2 sensor,mess under the intake, good Afm it turns out to be the cat is clogged.

But check your Afm, o2 sensor

[bmwm42]

Where does your temp sit

[bmwman91]

Bad O2 sensor, most likely.

[AndrewBird]

What did the old plugs look like?

[bstarr3]

Old plugs: #2 was oily. Fixed that with a new plug seal when I changed vc gasket. #3 smelled like gas, but the wire was loose, so I think that explains that. Haven't checked it since I replaced them. #1 and #4 were fine.

[bstarr3]

Re: engine temp. I know my mileage is going to be a bit lower because my commute is fairly short, so I don't get up to temp sometimes before I'm at work. Once it's up and running, it sometimes runs a little hot. I've completely bled the cooling system (took me about a week to get all the bubbles out). Running w/o a fan right now, but have an e fan that I'm going to swap in eventually. It's not hot in OR yet. When I get around to my timing chain I'm going to replace tstat and water pump, too. If all of that doesn't solve my overheating, then idk what to look at next.

[bstarr3]

Next ?: I know you can swap a Ford O2 sensor in, but from what model? There's some wire splicing to be done to make this work properly, right?

[vpilarrt]

You need a 4 wire sensor. Search to find the correct Mustang part. You have to cut off your wires near the old sensor and splice them into the new sensor since the new sensor's wire length and connector won't be correct.

[bstarr3]

do butt splices work for this or should I solder them? never done any wiring work in the engine compartment.

[vpilarrt]

I used butt splices but I stagered them so no 2 were on top off each other which allowed me to pull the outer white high temperature cover (from the old sensor) over the splices to help protect them.

[bmwman91]

I used butt splices but I stagered them so no 2 were on top off each other which allowed me to pull the outer white high temperature cover (from the old sensor) over the splices to help protect them.

+1, good call.

[bstarr3]

I'm looking in yards today for an O2 sensor from a Mustang. Might pull some injectors too, while I'm there. A few people mentioned a bad AFM. How would I know if this was working properly or not?