Terrible fuel economy

[ZeroCool]

Car is an '86 325e, 203k miles.

I noticed yesterday that my econometer was behaving a little strange. It seemed that if I gave it even the tiniest bit of throttle in gears 1-3 it would just drop down to 10mpg or below, and would be just sitting at five with any more than 1/4 throttle. 4th gear seemed a little more normal at above 45mph, but giving it even enough throttle to keep up with traffic drops the econometer down to like 15.

I just got the car, so I'm not entirely sure how it should be doing on gas yet, but I got to drive one whole tank through it and got about 19mpg. Some of this is probably a lot of the kind of driving, where I live is all stop and go on 55mph roads with lights every like 200 feet and tons of traffic, so I wasn't expecting much. However, when I got 18mpg on that tank, my econometer was behaving more or less how I'd expect it to act. It never dropped down past like 15 way unless I was flooring it, and even then it sort of hovered above 5 and moved around a bit. Now it just seems like any throttle input in gears 1 and 2 especially, and also 3, totally bombs it out and it just sits at 5.

Something that may be related is that my check engine light is constantly flashing, however, I think it's the O2 sensor. When I bought the car, it would flash for about 15 seconds after I started it cold, then turn off. That time got longer and longer until it just never stopped flashing. I understand this would cause the car to run a bit rich? Otherwise it feels like it has the normal amount of power, and it runs pretty smooth throughout the power band (bit of a hickup/surge from 2k-3k though).

What are some things I should check?

[Mr Deagle]

A bad o2 sensor would cause the car to run very rich, and go into open-loop mode (motronic is just guessing at the fuel mixture)


Replace the o2 sensor and see if that fixes it.

[rThor432]

O2 sensor did nothing for mine. Mine was an 86 auto eta. The mileage used to be above 30 all the time, but then it just dropped off. Very annoying.

Edit: I also changed all filters and fluids regularly. Still had the gas consumption problem. I ran a shit load of Sea Foam, and that did help a little bit, but not much.

The car ran fine, so I just didnt worry about it because I have other (more important) cars to take care of.

[asubimmer]

A bad o2 sensor would cause the car to run very rich, and go into open-loop mode (motronic is just guessing at the fuel mixture)


Replace the o2 sensor and see if that fixes it.

or check codes so you don't waste $

[thectrlguy]

+1

or check codes so you don't waste $

[euroe30]

i get 200 miles or less to a tank. is that bad?

[lordofthewaters]

or check codes so you don't waste $

+2

It's flashing for a reason.

[lordofthewaters]

i get 200 miles or less to a tank. is that bad?

T'aint good......

[rThor432]

i get 200 miles or less to a tank. is that bad?

Well you are averaging about 14 mpg. Sounds bad to me.

That is the mileage that I was getting in my eta before we parted ways.

My E36 M3 gets about double what my eta was getting, which doesnt seem right at all. (M3= about 25-28 mpg)

My eta was sucking gas.

[ZeroCool]

or check codes so you don't waste $

Ok, this sounds kind of stupid, but is it possible to check codes on my car? Bentley manual says it only works on the 325i, and I googled a bit and it appears you can only get codes on Motronic 1.1 or newer and I have Motronic 1.0.

I tried doing the thing where you just put the key in and turn it to on, and it says it should start flashing the codes after 3 seconds, however mine just starts flashing continuously when you turn the key.

[backtrail69]

i get 200 miles or less to a tank. is that bad?

Thats horrible. I get better mileage in my turbo car.. Like 21 mixed and 26 freeway. MS FTW! My eta gets 25 mixed and 30 freeway.

[silversleeper]

yea, 320+ miles per tank. just adjusting my tire pressure raised my mpg up a few.

[ZeroCool]

I've seen on other cars that people have eliminated their O2 sensors with some kind of simulator that just feeds the computer the correct voltage the sensor would be supplying if it were there are working properly.

Is there anything like this for our cars, or would something like this be easy to make? Seems like all you'd have to do is reduce 12v down to under 1v or whatever the correct reading from the O2 sensor should be. It doesn't look like my O2 sensor has ever been removed and I really doubt I'm going to get that thing off without breaking something.

[lordofthewaters]

PB blaster, patience, time, and don't try to use an adjustable to take it off. If it's that bad I'd get a O2 sensor wrench.

Your symptoms sound to me like maybe you have a bad temp sensor. If it causes the ecu to "think" that it is then it'll spritz more fuel and cause it to run rich..........

As for the erratic econometer, stopping and such, I'm thinking that your si batteries are probably to blame......