Author Topic: Horrible gas mileage out of my M42 (Read 5122 times)

mikey-rj · « **on:** January 01, 2008, 09:36:44 PM »

Just looking for a little help as to where to start on my diagnosis with my current problem. I suspected that I might have been getting pretty bad mileage out of my 318is but wasn't sure becuase my speedo doesn't work 100% of the time but on this last fill I decided to test it as best as I could. What I discovered is that for me to drive approx 220km (140miles) I used approx 35 liters (9.2 Gallons) of 91 octane. This seems to me to be completely ridiculous since in my summer car with the same engine I was getting almost double that.

I don't drive this car any different then my summer M42 but yet I'm loosing almost half my mileage.

I'm going to be getting the car into the garage hopefully within the next week here so I'm looking for some suggestions as to where to start. As a given I plan on going straight to the air filter to see if it's clogged up, but I'm wondering if there's anywhere else I should be looking? Could it be the oxygen sensor?

Thanks for any help guys.

Mike

hoevesruperd · « **Reply #1 on:** January 01, 2008, 10:14:48 PM »

i've heard a bad O2 sensor would destroy your fuel economy. i have a bad fuel economy as well but its from aggressive driving and a hole on top of the tank + probably an old O2 sensor as well

i'd look for leaks just about anywhere. it could also be a fuel management failure. then you can swap pieces from the other M42 to help you diagnose. i'd also check for drag and friction ex.: brakes calipers being clogged on disk or wheels not aligned or tires not well inflated (i doubt that would be it tho lol)

honestly i'm not a mechanic genius yet but maybe this can help you see further
good luck with it.

johna · « **Reply #2 on:** January 02, 2008, 01:20:18 AM »

The workshop that replaced my clutch forgot to reconnect my O2 sensor and my mileage went from 9.x l/100km to 12.x l/100km so I would definately check the sensor first.

gearheadE30 · « **Reply #3 on:** January 02, 2008, 09:44:59 AM »

If it isn't the sensor, check the fuel system for leaks. Seeing as our cars run at idle and light throttle off of the fumes in the gas tank, if those fumes are escaping, then you will be using gas much more quickly. This is why the gas cap is sealed and most newer cars have idiot lights that tell you if you don't have the gas cap on all the way.

JHZR2 · « **Reply #4 on:** January 02, 2008, 10:00:44 AM »

The evaporative system piping does tend to rust out faster than other undercar stuff... you may be evaporating fuel if youve got a hole in the pipe - check it. That said, in Alberta in the winter, youre not going to be evaporating a LOT of gasoline.

Likely you are buying 10% ethanol doped gas, or some other oxygenated blend. This will decrease your economy quite a bit - up to 10%. If you were getting 25 MPG in the summer (an OK city mileage figure), 10% would put you at roughly 22 MPG, not 15, which is about where you are at right now.

Assuming that you dont have any CELs, and that your O2 sensor is OK, Id also look for vacuum leaks that allow a lot of unmetered air into the systerm, causing your sensor to burn more fuel to try to get the ratios correct. There is such a tangle of breather hoses under the intake, that it is easy for one of them to be disconnected. Sometimes the car will still run, and if not idled much, wont even pull a CEL, yet youll be flowing unmetered air. Been there, done that... those hoses crack and dont stay together after a while...

When did you do a tune up last? When did you check the air in your tires??? You might be at 10 psi of air, and have a ridiculous rolling resistance.

Last but not least, and quite likely in fact, is that you got a batch of bad gasoline. This happens sometimes, and can cause all sorts of problems... I prefer Amsoil PI as the besst fuel system cleaner these days. It is quite strong and works really well.
http://www.amsoil.com/storefront/api.aspx?zo=1556971

Also, what oil are you running? If running a conventional 20w-50 in your ambient temperatures, you are going to have a LOT of pumping losses, which will sacrifice your fuel economy quite a bit, especially at cold startup.

Have you noticed any real changes in overall drivability of your car? Have you found it necessary to be much harder on the pedal to get the car to go?

JMH

mikey-rj · « **Reply #5 on:** January 02, 2008, 05:08:57 PM »

Car still drives nice and strong. I'm not sure when the last time a full tune up was done as I just got the car at the end of October, but I plan on doing one myself ASAP.

Tires PSI is fine. Checked that first.
Not a bad batch of gas either, this has been happening for a while with many different fills from many different stations including the premium "winter" gas.

I know that I have a pretty bad alignment problem that I'm looking to get fixed soon, but I can't see that decreasing my mileage to this extent. The brakes are all brand new. Replaced pads and rotors in November. No drag issues there.

As for Oil I'm running recently changed full synthetic 5w40.

I also can't use the other M42 to diagnose because it's stored for the winter at a different garage and Is being taken out of storage to go get some paint fixed tomorrow.

anisotropy · « **Reply #6 on:** January 07, 2008, 04:30:15 AM »

Try the blue temperature sensor just about accessible under the inlet manifold. Test it with the engine cold and hot with a multimeter -should be about 300ohms cold and 3-4 k hot IIRC. If its gone engine could think its cold all the time and overfuel. Most common thing here in the UK but I don't see it mentioned much on this forum.

cheesebox · « **Reply #7 on:** January 26, 2008, 05:12:01 AM »

Quote from: anisotropy;40628

Try the blue temperature sensor just about accessible under the inlet manifold. Test it with the engine cold and hot with a multimeter -should be about 300ohms cold and 3-4 k hot IIRC. If its gone engine could think its cold all the time and overfuel. Most common thing here in the UK but I don't see it mentioned much on this forum.

Is that the coolant temperature sensor? I've heard it can affect fuel economy.

I've also heard an incorrectly adjusted AFM can cause poor MPG, and also a damaged throttle position switch. However I don't know how to test these. Can anyone advise?

Choking Hazard · « **Reply #8 on:** February 01, 2008, 09:38:40 PM »

My sudden mileage change turned out to be a nice steady leak at the fuel filter at engine speeds ABOVE IDLE. I only noticed after I got out immediately after revving the engine in the driveway. I smelled gas when I got out and saw a little dark spot on the concrete right behind the door where the filter lives. When the car was idling, it di not leak, but if I ran it up for a bit and checked right away, the pattern became evident.