Author Topic: Lousy fuel economy (Read 4058 times)

colin86325 · « **on:** November 22, 2011, 09:06:36 AM »

My girl picked up a 1991 318i in the hopes of saving some money on gas. Well, she's only getting about 16mpg. I haven't had a chance to go over the car yet, but i was wondering what could make a huge impact (~50%) on fuel economy?

The car does run great with good power, no hesitation, no real smoke that I've seen. The oil does smell like fuel which points to an overfueling problem, but if the CTS or fuel pressure regulator were bad, wouldn't there be a running problem that would be noticeable?

I think the vacuum hoses can stand to be replaced and I plan to install a new O2 sensor. The Check light is inoperative (probably has no bulb) so I can't do a stomp test.

But yes, my question is: can a car have nearly a 50% decrease in gas mileage but still idle and accelerate great?

Geoff · « **Reply #1 on:** November 22, 2011, 09:37:11 AM »

I guess it could. gas leak? or, if not, having fuel in the oil is not good. could lead to all sorts of problems, maybe a poor tune and bad cylinder rings is putting unburned fuel in there. Maybe it runs "ok" but maybe it would run a lot better once you figure out its problem. while the gas is in there, it can break down the oil and make it a less effective, allowing seal and bearing damage. look at the plugs to get a basic idea of how the car is running.
Geoff

DesktopDave · « **Reply #2 on:** November 22, 2011, 06:04:20 PM »

I had a Subaru once with poor fuel mileage. There was a pinhole leak in the high pressure hard line off the tank.

Do you ever smell raw fuel? On the M42 there are two short lengths of rubber hose under the intake that have a habit of cracking. You have to pull the upper intake to get at them, unfortunately.

victor.askew · « **Reply #3 on:** November 25, 2011, 06:51:19 PM »

I just replaced the head on my 91 318I and drove it last week, refilled last nite and and I am getting 24 mpg. This would be alot better if I were not driving up and down mountain roads hwy 17 and hwy 9 everyday 2 times a day. Vacuum leaks from the nest of hoses under the intake manifold, clogged /old fuel filter, dirty air filter,bad oxygen sensor,dirty AFM sensor, If I were you I would start with these. Good luck.

d.hitchcock · « **Reply #4 on:** November 25, 2011, 07:40:43 PM »

I don't think your mileage could be that bad without some accompanying driveability issues.

You might check your odometer against mileposts or a GPS to see if it is functioning correctly.

At one point, my e30 325i appeared to be getting 13-15 mpg and it turned out the odometer was only registering about 8/10s of a mile for every mile driven.

Wild idea, to be sure, but I thought it worth mentioning.

d.hitchcock

Ryann · « **Reply #5 on:** November 26, 2011, 01:35:31 AM »

Quote from: d.hitchcock;108331

I don't think your mileage could be that bad without some accompanying driveability issues.

My thoughts exactly.

Either your GF drives it like she stole 100% of the time or the miles travelled/fuel filled info is whack.

colin86325 · « **Reply #6 on:** November 26, 2011, 11:37:58 AM »

Haha, thanks for the ideas guys. The car is currently under the knife. It's gettign new injectors, a new O2 sensor, all new hoses (including fuel line), new spark plugs, etc. I really hope this addresses the issues, because I will be really pissed if it doesn't!!

Oh, and I have to rebuild the odometer as well!

DesktopDave · « **Reply #7 on:** November 26, 2011, 11:59:32 AM »

I'll bet it helps, and it's good preventative maintenance regardless. Post your results!

BTW, if your GF does a lot of commuting, I'll bet the car won't get better than mid-20's even if it's running perfectly...especially with cheap winter mix gas 'round here. The M42 wasn't really meant as a econo-motor.

victor.askew · « **Reply #8 on:** November 27, 2011, 07:53:23 PM »

I followed my own advice today and changed out my fuel filter and what a world of diffrence it has made in the smoothness of the engine .We will see what improvments this also makes in the MPG this week.

colin86325 · « **Reply #9 on:** March 23, 2012, 07:11:44 AM »

The car is now getting 21mpg. I'm wondering WTF is still wrong with it. I'm going to start to look for gas leaks next. The car runs just fine with no propblems whatsoever, but the fuel economy is just poor.

Here's a list of things i've replaced (both relevant and not):

new fuel injectors
new fuel pressure regulator
new coolant temp sensor (for ECU)
new spark plugs
new air filter
new O2 sensor
changed oil and filter

new lower and upper intake manifold gaskets
new throttle body gasket
new: all vacuum hoses
new: all fuel lines under the hood
new coolant hoses (two to the heater core and one from the expansion tank)
new plastic coolant junction pipe
new heater core
new heater valve
new coolant
new rear passenger axle
replaced muffler
flushed differential fluid
flush transmission fluid
new driveshaft center support bearing
clean idle control valve
new throttle cable
flushed brake fluid
flushed power steering fluid
new driver's door lock cylinder
rebuilt rear driver's door lock actuator
rebuilt rear passenger door lock actuator
new rear driver's window motor
new odometer gears and instrument cluster bulbs
replaced turn signal switch
install aftermarket antenna

keflaman · « **Reply #10 on:** March 23, 2012, 07:38:23 AM »

Have you checked your guide bolts and pins on the brake carriers to ensure your brakes aren't dragging?

colin86325 · « **Reply #11 on:** March 23, 2012, 08:20:39 AM »

Quote from: keflaman;111346

Have you checked your guide bolts and pins on the brake carriers to ensure your brakes aren't dragging?

Thanks, I need to check that. I don't really have an ideal place to test this right now, but will fix that soon!