Gas Mileage in HALF!

[bmwstephen]

and it would help to post your most recent smog numbers to see if you are running to rich

[aTron12]

Hey Stephen,

Before my initial post I ran my tank down, took down my odometer reading and compared that to my tripometer & both calculated the same mileage by the time I reached the bottom of the tank. I'll try this again to be certain.

Henry,

Thx for the tipoff on the valves. Since mileage was great with recent valve adjustments (about a year ago and then 3 months past) I might check that variable out last. The fuel pump is most likely original...

[bmwstephen]

adam, remember to run it down to the yellow light. The yellow light only occurs after you EXCEED the red mark on the gas gauage unlike the modern BMW's that have the light come on once you are in the red.

Let us know how many gallons it takes you to fill up after that. Seriously when this happened to me, my yellow light came on when I had 3-4 gallons of gas left. I filled up in 12 gallons which perplexed the shit out of me before I narrowed it down to the fuel level sender and fuel pump

[aTron12]

adam, remember to run it down to the yellow light. The yellow light only occurs after you EXCEED the red mark on the gas gauage unlike the modern BMW's that have the light come on once you are in the red.

Let us know how many gallons it takes you to fill up after that. Seriously when this happened to me, my yellow light came on when I had 3-4 gallons of gas left. I filled up in 12 gallons which perplexed the shit out of me before I narrowed it down to the fuel level sender and fuel pump

I have actually been running it down to the amber light. When I refill I am slightly more than 14 gallons for the fill up. Can't hurt to try this again.

So as I said before I have been subtracting my trip miles from Odo miles and have come up perfect BUT I did notice on my way from Fremont to Walnut Creek this morning that I clocked in 15 miles. I believe that is at least a 30 mile trip. I know that the odometer plugs into the differential. Wonder if something is wonky there? I'm going to keep comparing mileage to known distances a few more times to verify this.

[MTechnikII]

Hey Stephen,

Before my initial post I ran my tank down, took down my odometer reading and compared that to my tripometer & both calculated the same mileage by the time I reached the bottom of the tank. I'll try this again to be certain.

Henry,

Thx for the tipoff on the valves. Since mileage was great with recent valve adjustments (about a year ago and then 3 months past) I might check that variable out last. The fuel pump is most likely original...

[MTechnikII]

you're welcome , I hope it will solve your problem, keep us posted
Henry

[M-technik-3]

Surprised this has not been mentioned, Temperature sensor, water
13 62 1 357 414
20CEL 2500 OHM


This will cause havoc with fuel mileage.

[MTechnikII]

It will , I changed it on my car when I had gasmilage problem but didn't change anything, Good point.
Also I always wondered if it's a good idea to have a flip switch inside to short the temp sensor and make the ECU dump more fuel and get some more Hp while driving, isn't this what a larger injector is doing fulltime? any opinion?
Henry

[kevdeno]

Just read through this entire post. The one thing that stuck out to me most?

So as I said before I have been subtracting my trip miles from Odo miles and have come up perfect BUT I did notice on my way from Fremont to Walnut Creek this morning that I clocked in 15 miles. I believe that is at least a 30 mile trip. I know that the odometer plugs into the differential. Wonder if something is wonky there? I'm going to keep comparing mileage to known distances a few more times to verify this.

Bad diff speed sensor?
Wiring damaged?
Wrong part(s) in diff?

Given the immediate change in fuel econ after having the rear end worked on, this would be my first check.

If you can't trust the odometer, grab a GPS that has trip mileage and run it every time you drive. Or an iPhone/Android app if you have one of those. Or just go for a drive predetermined on Google maps beforehand.

[MTechnikII]

Bad diff sensor will also make the speedo meter niddle jump around,I've seen a bad sensor make the milage add up really fast but it was a digital odometer.
Henry

[jlevie]

It will , I changed it on my car when I had gasmilage problem but didn't change anything, Good point.
Also I always wondered if it's a good idea to have a flip switch inside to short the temp sensor and make the ECU dump more fuel and get some more Hp while driving, isn't this what a larger injector is doing fulltime? any opinion?
Henry

The ECT sensor is a negative coefficient device, meaning that it is high resistance when cold and low resistance when hot. So you'd want to add resistance to make the DME think the engine is colder than it is. I have only done a little playing around with this on a dyno. But what I saw suggests that you don't want to make the DME think that the engine is stone cold. But rather that you want to make it think that the engine is just above the open to closed loop transition point. It looks like that occurs at 60C (140F).

[MTechnikII]

thanks for the info, maybe a potentiometer can be used to have variable hp , how much improvment did you see on the dyno?
Henry

[jlevie]

thanks for the info, maybe a potentiometer can be used to have variable hp , how much improvment did you see on the dyno?
Henry

Not much. My real goal in fiddling with the ECT signal was to richen up the mixture above 4500rpm, which it did (slightly). The peak numbers didn't move around much, but the area under the curve (which is what really counts) got a touch better. The dyno plot seemed to say that making it seem like the engine was colder also advanced timing slightly (which it should). Since the head on this engine is milled down to the legal limit, the engine would like a bit more timing advance.

This car is a Spec E30, so I'm limited by the rules as to what I can do to the engine management system. Once I'd proved that a lower temperature signal from the ECT was beneficial I sorted through a box of ECT sensors and found one that was "off spec" and indicating a bit colder than it should be. Using that in the engine is legal.

[aTron12]

Just read through this entire post. The one thing that stuck out to me most?



Bad diff speed sensor?
Wiring damaged?
Wrong part(s) in diff?

Given the immediate change in fuel econ after having the rear end worked on, this would be my first check.

If you can't trust the odometer, grab a GPS that has trip mileage and run it every time you drive. Or an iPhone/Android app if you have one of those. Or just go for a drive predetermined on Google maps beforehand.

So after a few days of calculating trip miles on my iPhone, I have verified that my actual mileage is just fine. The ODO/TRIP is not responding accurately. I have access to a known good cluster. Going to swap that out for a week and see what happens. My car has 241,000 on the body so this could be a worn out cluster. If no change then I'll look into the diff sensor.

[MTechnikII]

well that's good to know and glad you figured it out what was the cause,
Henry